Process of producing metal powders

Abstract

The invention provides an industrially efficient, low-cost, mass-production system for the process of producing fine solder powders using an in-oil atomization method wherein solder is melted in a heated dispersion medium for fine granulation. Molten solder melted in a solder melting tank and a mixture of a particle dispersion medium and a particle coalescence-preventing agent, prepared in a dispersion medium heating tank, are fed to a fine-granulation machine, in which dispersion energy is applied to obtain a dispersion of molten solder particles. The dispersion is processed in a solidifier-by-cooling to obtain a dispersion of solid solder particles, which is processed in a solid-liquid separator to separate the solid solder particles. The solid solder particles are washed and dried to obtain fine powders. The respective devices at these steps are connected together by way of piping, so that fine solder powders can continuously be produced.

Description

[0001] This application claims priority to Japanese Patent Application No. 2002-075487, filed Mar. 19, 2002.

TECHNICAL FIELD

[0002] The present invention relates generally to a process of producing metal powders such as powders of low-melting metals or alloys represented by solders, and more particularly to a production process that enables spherical solder powders used for solder pastes and having an average particle diameter in the range of 0.1 to 100 &mgr;m, inter alia, an average particle diameter of up to 10 &mgr;m to be mass-produced in an industrially efficient fashion.

BACKGROUND OF THE INVENTION

[0003] In recent years, surface mounting techniques have grown rapidly with the advent of multifunctional, miniaturized wiring boards. For high-density packaging such as surface mounting of electronic parts, solder pastes, other soldering materials and soldering processes which do not only make metal mask printing of fine patterns feasible but also ensure satisfactory solder-ability are now in increasing demand.

[0004] For solder pastes it is preferable to use spherical solder particles rather than aspheric solder particles in view of fine-patterned metal mask printing. On the other hand, solders are now required to be finely granulated so as to meet current demands for accommodation to minuscule wiring areas that enable LSI chips to be directly on wiring boards.

[0005] To obtain solder powders in a spherical particle form, it is common to use an atomization method wherein molten solder is atomized and solidified in an inert gas atmosphere having a low oxygen concentration. Depending on how to atomize molten solder, the atomization method is generally broken down into a centrifugal atomization method of the type that uses the centrifugal force of a rotary disk and a gas atomization method of the type wherein gas is jetted onto the molten solder to scatter the molten solder for atomization. There is also known an ultrasonic atomization method of the type wherein ultrasonic vibrations are applied to molten solder for fine granulation.

[0006] In the centrifugal atomization method, molten solder is cast onto a rotary disk while it is formed by centrifugal force into a thin film. Then, this film is released out of the edge of the disk in the form of droplets, which are solidified by cooling in an inert gas atmosphere having a low oxygen concentration for fine granulation. With this method, the average particle diameter of the resulting solder powders may be reduced by increasing the rpm of the disk. However, there are practically some restrictions on the rpm of motors for driving the disk, and so it is industrially difficult to retain spherical solder particles while the average particle diameter of solder powders is reduced down to 10 &mgr;m or less.

[0007] In the gas atomization method, on the other hand, an inert gas having a low oxygen concentration is jetted to molten solder for scattering and atomization. The resulting solder powders have a broad particle size distribution or, in some cases, they include a large proportion of so-called satellite particles that are large particles with small particles deposited to them. Thus, the gas atomization method is just only less than satisfactory in terms of fine granulation efficiency but also renders it difficult to obtain spherical particles. With this method, the average particle diameter of the resulting powders may be reduced by increasing the pressure of the gas to be jetted. In that case, however, the “satellite particle” problem becomes more noticeable and graver, and so makes it more difficult to obtain spherical particles.

[0008] In the ultrasonic atomization method, the higher the frequency of the ultrasonic oscillator, the smaller the average particle diameter of the resulting powders becomes. To make that frequency high, however, the size of the ultrasonic oscillator must be decreased and, accordingly, fine granulation efficiency becomes worse. It is thus very difficult to industrially produce solder powders having an average particle diameter of up to 10 &mgr;m.

[0009] Moreover, there is available an in-oil atomization method that does not rely on jetting-in-gas, wherein a solder lump is melted by heating at a temperature higher than the melting point of the solder in a high-boiling dispersion medium, the melt is then agitated into droplets, and the droplets are finally solidified by cooling for fine granulation.

[0010] With this in-oil atomization method, substantially spherical, fine solder particles can be obtained, because solder is melted in a heated dispersion medium in the form of an oily liquid, the melt is agitated into finely particulate droplets, and the droplets are solidified by cooling. The fine particles are substantially free from the aforesaid satellite particles or deformed particles, and fine particles having an average particle diameter of up to 10 &mgr;m can be obtained, with relative ease, by increasing the number of agitations. This in-oil atomization method is a wet process wherein the fine granulation of solder is carried out in the dispersion medium such as an oily liquid, and so is advantageous in terms of handling during production operation, because it does not have most of problems unique to the aforesaid centrifugal atomization or other dry methods, for instance, deposition of solder powders to the machine used, oxidization of solder, deterioration in the flowability of solder powders, dusting, and inconvenience caused by decreases in powder particle diameters.

[0011] Thus, the centrifugal, gas and ultrasonic atomization methods may address the production of solder powders having an average particle diameter of greater than 10 &mgr;m; however, they have problems with the production of solder powders having an average particle diameter of 10 &mgr;m or less, and the production of spherical solder powders as well. On the other hand, the in-oil atomization method has a merit of providing a solution to those problems.

[0012] A problem with the in-oil atomization method is, however, that as the concentration of dispersed droplets (i.e., the volumetric ratio of molten solder droplets to the dispersion medium) increases, the so-called coalescence occurs, wherein upon scission and division of the molten solder droplet by an agitator into small droplet pieces, those droplet pieces immediately go back to the initial droplet, and so fine granulation of the molten solder droplets does not proceed any longer. Another problem is that even when the fine granulation proceeds somehow, the droplets contact one another due to settling or the like, and become coarse due to their coalescence. Especially when vegetable oils having a high acid number, oily materials having high viscosity, etc. are used for the dispersion medium so as to obtain solder particles with less oxidized surfaces, that problem becomes more perceptible.

[0013] To avoid such problems, it has been required to reduces the concentration of dispersed droplets as much as possible so that the molten solder droplets that are finely granulated by agitation are unlikely to contact one another. To this end, however, a large amount of dispersion medium is needed, and to obtain the end solder powder product, that dispersion medium must be removed. Disposal of much dispersion medium results in an increased consumption of dispersion medium, ending up with added production costs.

[0014] These problems could be solved by use of a specific coalescence-preventing agent or the like, as set forth in applicant's co-pending Japanese Patent Application No. 2001-395566; never until now, however, is any systematization for industrially efficient mass production figured out. In the conventional in-oil atomization method, a solder lump is melted by heating at a temperature higher than the melting point of the solder in a high-boiling dispersion medium, the molten solder is agitated into droplets, and the droplets are solidified by cooling for fine granulation. Even so, carrying out these steps in one batch tank is not well fit for mass production, and is less efficient as well. To develop a mass-production process that can be carried out with high efficiency yet at low costs, nothing is suggested at all about how a series of steps from the step of feeding raw materials to the step of obtaining the end dry metal powder product are separated and how the individual steps are set up. Thus, such a mass-production process is an unheard-of challenge.

[0015] One object of the invention is to provide a metal powder production process for mass-producing fine metal particles in industrially efficient fashions.

[0016] Another object of the invention is to provide a metal powder production process for mass-producing spherical fine metal particles in industrially efficient fashions.

[0017] Yet another object of the invention is to provide a metal powder production process for mass-producing fine metal particles in industrially efficient fashions while components consumed during the production process are reduced.

[0018] A further object of the invention is to provide a metal powder production process for mass-producing fine metal particles in industrially efficient fashions yet at reduced production costs.

[0019] A further object of the invention is to provide a metal powder production process for mass-producing fine metal particles applicable even to fine soldering areas on wiring boards in industrially efficient fashions.

SUMMARY OF THE INVENTION

[0020] The inventors have made intensive studies for the purpose of accomplishing the aforesaid objects and consequently found that the step for fine granulation of a molten metal and the solidification-by-cooling step, solid-liquid separation step, washing step, drying step or the like added thereto can be controlled in the form of a series of mutually correlative steps, thereby mass-producing spherical fine metal particles. On the basis of this finding, the inventors have figured out the present invention according to which there can be provided a production system that enables spherical, fine metal particles to be mass-produced.

[0021] The present invention is embodied as follows.

[0022] (1) A process of producing metal powders, comprising steps of:

[0023] (a) melting a raw low-melting metal to obtain a metal melt,

[0024] (b) mixing a particle dispersion medium and a particle coalescence-preventing agent to obtain a dispersion medium that may or may not be heated,

[0025] (c) supplying the melt of said low-melting metal from said step (a) and supplying said dispersion medium from said step (b) with application of dispersion energy that disperses the melt of said low-melting metal in the form of fine particles, thereby obtaining a molten metal particle dispersion wherein molten metal particles are dispersed in said dispersion medium,

[0026] (d) cooling said molten metal particle dispersion to solidify said molten metal particles into solid metal particles,

[0027] (e) separating said solid metal particles from a liquid residue,

[0028] (f) washing said separated solid metal particles with a detergent to remove depositions onto said solid metal particles, and

[0029] (g) drying said washed metal particles, wherein:

[0030] in said step (c), said particle coalescence-preventing agent adsorbs to and/or reacts with said molten metal particles to prevent coalescence of at least said molten metal particles so that said solid metal particles can be finely granulated in said steps (c) to (g), and said steps (a) to (g) are controlled in the form of a series of mutually correlative steps.

[0031] (2) The process of (1) above, wherein there is provided a dispersion medium recycle step in which the liquid residue separated in said step (e) is directly used as a part or the whole of the dispersion medium in said step (b) or a dispersion medium regenerated from said liquid residue in a dispersion medium regeneration step (h) is recycled as a part or the whole of the dispersion in said step (b), wherein said dispersion medium recycling step is continuously controlled in correlation to said step (b).

[0032] (3) The process of (1) or (2) above, wherein there is provided a detergent recycle step in which a spent detergent that may contain the depositions removed in said step (f) is directly used as a part or the whole of the detergent used in said step (f) or a detergent regenerated from said spent detergent in a detergent regeneration step (i) is recycled as a part or the whole of the detergent used in said step (f), wherein said detergent recycle step is continuously controlled in correlation to said step (f).

[0033] (4) The process of any one of (1) to (3) above, wherein the detergent used in said step (f) has a vapor pressure of at least 15 kPa at 40° C. and a latent heat of vaporization of at most 100 kJ/kg.

[0034] (5) The process of any one of (1) to (3), wherein in said step (g), said washed solid metal particles are dried such that the liquid residue deposited to said solid metal particles accounts for 0.01 to 1% of said solid metal particles, thereby reducing oxidization and dusting of powders of said solid metal particles.

[0035] (6) The process of any one of (1) to (5) above, wherein said step (d) of cooling the molten metal particle dispersion to solidify said molten metal particles into solid metal particles is carried out while said molten metal particles dispersion is passed through an inner pipe of a double pipe structure and a coolant is passed through an outer pipe of the double pipe structure.

[0036] (7) The process of any one of (1) to (6) above, wherein said double pipe structure is set at an angle of 45 to 90 degrees with respect to horizontal.

[0037] (8) The process of any one of (1) to (7) above, wherein in said step (b), the dispersion medium that is a mixture of the particle dispersion medium with the particle coalescence-preventing agent is heated in such a way that said dispersion medium is preheated in a preheating tank, and then passed through a heated delivery pipe for a residence time that does not exceed 10 minutes at most.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] FIG. 1 is a flowchart illustrative of one embodiment of the metal powder production process according to the invention.

[0039] FIG. 2 is a flowchart illustrative of another embodiment according to the invention.

[0040] FIG. 3 is illustrative of the front and bottom of the generator used in one embodiment of the invention.

[0041] FIG. 4 is illustrative in longitudinal section of that front.

[0042] FIG. 5 is a sectional schematic illustrative of the system using that generator.

DETAILED EXPLANATION OF THE PREFERRED EMBODIMENTS

[0043] By way of example but not exclusively, details of the invention are now explained specifically with reference to the in-oil atomization method wherein molten solder is dispersed by means of an agitator comprising a stator and a rotator. In particular, the fine granulation method used herein includes other method for dispersing molten metals in liquids.

[0044] As shown in the flowchart of FIG. 1, fine powders of a metal having a low melting point are produced through the following steps. For instance, “solder” (the starting solder such as solder metal) is fed into a temperature-controllable “solder melting tank” wherein molten solder is prepared. Apart from this, a “particle dispersion medium” and a “particle coalescence-preventing agent” are heated-and mixed together in a “dispersion medium heating tank” to prepare a dispersion medium. While the present invention is explained specifically with reference to solder working as the low-melting metal, it is understood that the invention is applicable to other low-melting metals.

[0045] The molten solder in the solder melting tank and the dispersion medium heated in the dispersion medium heating tank are passed by a constant-feeding gear pump or the like for the former and a constant-feeding pump or the like for the latter through the respective lines, and preferably a heating controllable line (designed to prevent coagulation of solder by temperature drops and to melt previously solidified solder on restart-up thereby creating a melt flow) for the former, to a “fine-granulation machine”. Preferably in the fine granulation machine, the mixture is heated, so that, while cooled if whenever required, dispersion energy for dispersing molten solder particles is applied to disperse the molten metal particles in the dispersion medium, thereby obtaining a molten metal particle dispersion.

[0046] Then, the molten metal particle dispersion is passed through a line to a “solidifier-by-cooling”, wherein the molten metal particles are cooled by a coolant passed between a “coolant-in (introduction of coolant)” and a “coolant-out (discharging of coolant)”, whereby the molten metal particles are solidified into solid metal particles. Subsequently, the liquid material that contains the solid metal particles is fed to a “solid-liquid separator” wherein the solid metal particles are separated from the liquid reside (“spent dispersion medium”). The thus separated solid metal particles are fed to a “washer” wherein they are washed with a “detergent” to remove depositions to the solid metal particles and separate the solid metal particles from the detergent containing such depositions (“spent detergent”). The thus separated solid metal particles are dried in a “drier” to obtain a “solder powder product” (fine solder powders).

[0047] As shown in FIG. 2, it is acceptable to add two circulation circuits to the flowchart of FIG. 1. One circuit is used as a dispersion medium recycle step wherein the remaining liquid, from which the solid metal particles have been removed, is regenerated by a “dispersion medium regenerator” as the spent dispersion medium to remove solid matters entrained therein, so that the spent dispersion medium can be used as a part or the whole of the dispersion medium in the dispersion medium heating tank (a deficiency, if any, is made up by a fresh dispersion medium).

[0048] Another circuit is used as a detergent recycle step wherein the spent detergent in the washer is regenerated by distillation or the like in a “detergent regenerator”, so that the spent detergent is used as a part or the whole of the detergent used in the washer (a deficiency, if any, is made up by a fresh detergent).

[0049] Otherwise, FIG. 2 is the same as FIG. 1.

[0050] Whether in the case of FIG. 1 or in the case of FIG. 2, a series of steps using the respective devices can be mutually connected together by lines so that they can continuously be carried out under control. For instance, the heating temperature in the solder melting tank, the mixing ratio and heating temperature of the particle dispersion medium and particle coalescence-preventing agent in the dispersion medium heating tank, the ratio and heating temperature of the molten solder and dispersion medium in the fine-granulation machine, the magnitude of dispersion energy, the traveling speed and temperature of the coolant in the solidifier-by-cooling, regulation of the solid-liquid separation rate in the solid-liquid separator, the degree of washing in the washer, the degree of drying in the drier, the degree of regeneration in the dispersion medium regenerator, the degree of regeneration in the detergent regenerator, the interior temperature of the lines, etc. may be controlled by use of computer processing. In this case, some or all of such factors may be provided as numerical data, and those data may be checked against actual data obtained from sensors attached as accessories to the respective devices and lines. Thus, control can be implemented by computer processing or the like.

[0051] That is, the term “control” used herein is understood to include “automatic control” and “control including automatic control”.

[0052] The aforesaid solder melting tank should preferably be formed of materials less susceptible to erosion by molten solder, for example, ceramic materials and carbonaceous materials. Although it is acceptable to use metals relatively less susceptible to erosion by molten solder, e.g., SUS 316 and titanium, it is preferable that these metals should be coated with an oxide film, a nitride film, a titanium nitride film, etc., thereby making erosion resistance much higher. The solder melting tank includes therein a heater for melting solder, for which a graphite heater, a ceramic heater, a quartz heater, a heater comprising a heat generator covered with a metal or the like may be used. For the heater comprising a heat generator covered with a metal, that metal should preferably be coated with an oxide film, a nitride film, a titanium nitride film, etc., thereby enhancing the resistance to erosion as is the case where the metal is selected for the material of the solder melting tank. To feed the molten solder to the fine-granulation machine through the line, for instance, a constant-feeding gear pump may be used. In this case, however, it is acceptable to immerse the constant-feeding gear pump in the molten solder in its entirety. The amount of the molten solder to be fed to the fine-granulation machine may be determined in proportion to the amount of the dispersion medium to be fed to the fine-granulation machine. For instance, the former-to-latter ratio should be in the range of 10 to 1,000 (by volume).

[0053] While the production of fine solder powders (fine powders of the metal having a low-melting point) has been described with reference to FIGS. 1 and 2, it is understood that the term “low-melting metal” used herein refers to at least one of low-melting metals and low-melting alloys and, in some cases, a low-melting metal alone or a low-melting alloy alone or both. These fine metal powders, too, may be produced according to FIGS. 1 and 2.

[0054] Set out below are examples of the low-melting pure metal together with their melting points.

[0055] Ga (29.8° C.), In (156° C.), Li (186° C.), Se (217° C.), Sn (232° C.), Bi (271° C.), Tl (302° C.), Pb (327° C.), Zn (419° C.), and Te (452° C.).

[0056] Besides, Cd, Cs, Rb, K and Na may be used.

[0057] Exemplary low-melting alloys include 67Ag/33Te (351° C.), 97.2Ag/2.8Tl (291° C.), 45.6Ag/54.4Zn (258° C.), 95.3Ag/4.7Bi (262° C.), 52.7Bi/47.3In (110° C.), 47.2In/52.8Sn (117° C.), 95.3Ag/4.7Pb (304° C.), 86.6Ag/3.4Li (154° C.), and 8.1Bi/91.9Zn (254.5° C.).

[0058] Solder is well known as a low-melting metal, and Pb/Sn eutectic solder in particular is used as joining materials or the like in the electronic and other industries. Specific mention is made of not only 100% Sn (232° C.), but also Pb—Sn base solders such as 37Pb/63Sn (183° C.), 40Pb/60Sn(183° C.), 50Pb/50Sn (212° C.) and 44Pb/56Sn (125° C.); Pb—In base solders such as 50Pb/50In (198° C.); Sn—In base solders such as 49Sn/51In (120° C.), 48Sn/52In (117-120° C.) and 65Sn/35In (162° C.); Sn—Bi base solders such as 43Sn/57Bi (139° C.) and 42Sn/58Bi(138° C.); Sn—Ag base solders such as 98Sn/2Ag (221-226° C.), 96.5Sn/3.5Ag (221° C.), 96Sn/4Ag (232° C.) and 95Sn/5Ag (232° C.); Sn—Zn base solders such as 91Sn/9Zn (199-203° C.) and 30Sn/70Zn; Sn—Cu base solders such as 99.3Sn/0.7Cu (227° C.): Cd—Zn base solder such as 60Cd/30Zn; Sn—Sb base solder such as 95Sn/5Sb (238° C.) Ag—In base solders such as 3Ag/97In (141° C.); Au—Sn base solders such as 80Au/20Sn (283° C.); Sn—Cd—Ag base solders such as 10Sn/85Cd/5Ag; Sn—Ag—In base solders such as 95.5Sn/3.5Ag/1In; Sn—Zn—In base solders such as 86Sn/9Zn/5In (192° C.) and 81Sn/9Zn/10In (178° C.); Sn—Ag—Cu base solders such as 95.5Sn/0.5Ag/4Cu (216° C.) and 96.5Sn/3.0Ag/0.5Cu; Sn—Pb—Bi base solders such as 16Sn/32Pb/52Bi (99.5° C.), 19Sn/31Pb/50Bi (96° C.), 34Sn/20Pb/46Bi (100° C.) and 43Sn/43Pb/14Bi (136-166° C.); Sn—Pb—Sb base solders such as 35Sn/64.5Pb/0.5Sb and 32Sn/66Pb/2Sb; Sn—Bi—In base solders such as 17Sn/57Bi/26In; Pb—Ag base solders such as 97.5Pb/2.5Ag; Sn—Bi—Ag base solders such as 90.5Sn/7.5Bi/2Ag (207-212° C.), 41.0Sn/58Bi/1.0Ag; and Sn—Zn—Bi base solders such as 89.0Sn/8.0Zn/3.0Bi.

[0059] As shown in FIGS. 1 and 2, the particle dispersion medium and the particle coalescence-preventing agent are mixed together in the dispersion medium heating tank, so that the latter is dissolved in the former, resulting in the dispersion medium. The term “particle dispersion medium” used herein means an oily liquid that provides a base for the dispersion of particles. Although not shown, it is acceptable to store this oily liquid in the required amount in a separately provided tank, from which the required amount of the oily liquid is fed to the dispersion medium heating tank. For the particle dispersion medium in the invention, use may be made of an organic compound having a boiling point that is greater than (or not lower than) the melting point (temperature) of the low-melting metal or the highest possible decomposition temperature, and in which the particle coalescence-preventing agent can be dissolved.

[0060] Exemplary organic compounds include silicone oil; mineral oils obtained by petroleum refining; industrial lubricating oils such as engine oil, spindle oil, machine oil, cylinder oil and gear oil; or synthetic lubricating oils prepared by chemical synthesis wherein the chemical components used are hydrocarbon components such as polyolefin, e.g., polybutene and alkyl aromatics, e.g., alkylbenzene, and non-hydrocarbon components such as polyethers such as polyglycol and phenyl ethers, e.g., polyphenyl ether and alkyl diphenyl ether, diesters, polyol esters, complex polyol esters, esters such as natural fats and oils (tryglyceride), phosphoric compounds such as phosphate esters, and fluorinated polyethers of the aforesaid compounds. Vegetable oils such as coconut oil, palm oil, olive oil, sunflower oil, castor oil, soybean oil, linseed oil, tung oil and cottonseed oil and animal oils such as whale oil and beef tallow may be used. Further, use may be made of liquid paraffin; higher hydrocarbon compounds such as decane, dodecane, tetradecane, hexadecane, octadecane and undecane; glycols such as glycerin, ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol and polypropylene glycol (which may be called polyalkylene glycols of the aforesaid triol and diol types; the glycols of the monol type may be used such as MB-7, MB-11 and MB-22, all being Nissan Uniloop MB series (of the non-water soluble type) as well as Nissan Uniloop 50MB series (of the water soluble type); derivatives of these glycols; phosphates such as trimethyl phosphate, triethyl phosphate and tributyl phosphate; substituted phenols such as octylphenol, trichlorophenol and nonylphenol; trichloroaniline; organic heat media such as those based on diphenyls and triphenyls; phenylimidazoles; undecylimidazoles; and heptadecylimidazoles. It is then preferable to use the organic compound having no flammability because there is no risk of fires. It is here noted that the particle dispersion medium may be composed of two or more of the aforesaid compounds. In this case, two or more reservoirs may separately be provided for storing separate dispersion medium compounds, so that on mixing, they are separately fed therefrom.

[0061] The aforesaid particle dispersion medium should preferably be used at a temperature that is not higher than the highest possible temperature and is higher than the melting temperature of the low-melting metal, and heated in an inert gas atmosphere. The threshold temperature at which the particle dispersion medium can be used is usually selected from the range of 120 to 470° C., and that threshold temperature is usually set at a temperature that is lower than the decomposition temperature of the organic compound.

[0062] It is preferable to add an antioxidant to the particle dispersion medium for the purpose of preventing its oxidization on heating. With such an antioxidant, it is possible to prevent oxidization by oxygen that may possibly be contained in a slight amount even in the inert gas atmosphere. The antioxidant used herein may be selected from those used with oils and fats, rubbers, synthetic resins or the like. For instance, phenol antioxidants, bisphenol antioxidants, polymer type phenol antioxidants, sulfur antioxidants and phosphoric acid antioxidants may be used. The antioxidant may be used alone or in combination with imidazoles having an oxidization inhibition effect. Exemplary compounds are set forth in Japanese Patent Unexamined Publication No. 09-49007.

[0063] In the present invention, the particle coalescence-preventing agent is used to prevent coalescence of molten metal particles due to fusion. This preventing agent may also prevent coalescence of metal particles resulting from solidification of molten metal particles and coalescence of the molten metal particles with the solidified metal particles.

[0064] Generally, when a dispersoid (particles) is dispersed in a dispersion medium such as an emulsion, the process in which the dispersion system becomes instable proceeds from creaming (a phenomenon that the particles float up or settle down due to a specific gravity difference between the particles and the dispersion medium) via agglomeration (a phenomenon that the particles come close to one another and agglomerate together due to attractive force) to coalescence (a phenomenon that the particles come together). As surfactants or polymers are adsorbed onto the surfaces of the particles, the particles come into contact with one another through the resulting adsorbed films at the agglomeration stage. It is then of importance to create on the surfaces of the particles layers having adsorbability strong enough to prevent the adsorbed films from peeling off or being forced out due to the surface slip stresses of the particles. Otherwise, the particles may come into direct contact with one another and come together. For this purpose, it is considered important that a substance having a high affinity for both the dispersoid particles and the dispersion medium is used and, at the same time, the adsorbed layers have large surface adhesion and surface elasticity (see “CHMESITRY OF INTERFACIAL PHENOMENONS”, pp. 16-18, Sankyo Shuppan, Publishers).

[0065] As shown in FIGS. 1 and 2, the particle coalescence-preventing agent is introduced together with the particle dispersion medium in the dispersion medium heating tank. Preferably in this case, the former is mixed under agitation with the latter at a constant proportion (to be described later). However, when the particle coalescence-preventing agent is solid, it should be fed in a metered amount by a screw feeder or the like, and when it is liquid, it should be fed in a constant amount by a constant-feeding pump or the like.

[0066] The dispersion medium that is a mixture of the particle coalescence-preventing agent and the particle dispersion medium is then fed at high temperature to the next step “fine-granulation machine”. However, when this dispersion medium is thermally instable, it is preferable to place the dispersion medium under such precise temperature control that the dispersion medium is preheated at a temperature that is 50-100° C. lower than the temperature at which it is fed to the fine-granulation machine, and then heated by a separately provided line heater (a heated delivery pipe) having a short residence time to the temperature at which it is fed to the fine-granulation machine. This is because when the thermally instable dispersion medium is retained at that high temperature for a long period of time, the dispersion medium is often modified by heat, reducing the effect on prevention of coalescence of the particles. The residence time of the dispersion medium in this case is found by the following formula; however, the residence time should preferably be 10 minutes at the longest (or 10 minutes or shorter).

Residence Time (min.)=the volume of feed in the line heating furnace (L)÷the flow rate of feed (L/min.)

[0067] For the particle coalescence-preventing agent, compounds adsorbing onto and/or reacting with the surfaces of metal particles, especially molten metal particles are used. Exemplary such compounds are given just below.

[0068] (i) Rosin and/or Its Derivatives (Rosins)

[0069] (a) The rosin is exemplified by tall oil rosin, gum rosin, and wood rosin.

[0070] (b) The rosin derivatives are exemplified by hydrogenated rosin, polymerized rosin, unhomogenized rosin, acrylic acid-modified rosin, maleic acid-modified rosin, rosin alcohol, rosin amine, and rosined soap.

[0071] Tall oil rosin, gum rosin and wood rosin are composed primarily of abietic acids (abietic acid, dehydroabietic acid and neoabietic acid) and contain as subordinate components pimaric acid, Palustric acid, isopimaric acid, and other resin acids, with the components contained at different ratios.

[0072] The rosined soap is a metal salt of rosin derivatives containing rosin or carboxyl groups. The metal in this case is exemplified by Na, K, Li, Ca, Mg, Al, Zn, Sn, Pb, Ni, Cu, Co, Mn, Fe, In, Bi and Ag. In view of solder material, however, salts of Sn are preferred. Preferably in view of the action of the particle coalescence-preventing agent on prevention of coalescence of particles, on the other hand, the number of carboxyl groups in the rosin derivatives having rosin or carboxyl groups should be as large as possible. More preferably, use should be made of metal salts of monobasic acid-modified rosin, and especially metal salts of dibasic acid-modified rosin.

[0073] (c) Colorless rosin derivatives (see Japanese Patent Unexamined Publication No. 05-86334) may also be used, which are obtained by hydrogenation of addition reaction products of &agr;,&bgr;-unsaturated monocarboxylic acids and/or &agr;,&bgr;-unsaturated dicarboxylic acids to refined rosin.

[0074] At least one of (a) rosin or rosin derivatives, (b) rosined soaps, and (c) colorless rosin derivatives is used. However, particular preference is given to those having affinity (adsorbability and/or reactivity) for the surfaces of metal particles, especially molten metal particles. Among others, rosins modified by monobasic acids (e.g., acrylic acid, methacrylic acid and crotonic acid), rosins modified by glycol, and rosins modified by dibasic acids (e.g., maleic acid, maleic anhydride and fumaric acid).

[0075] (ii) Triazoles

[0076] Benzotriazole and/or its derivatives may be used to this end.

[0077] (iii) Imidazole and/or Its Derivatives

[0078] (iv) Amine Compounds

[0079] Aromatic amines (aniline, o-toluidine, m-toluidine and p-toluidine), aliphatic amines and cyclic ketoamines may be used to this end.

[0080] (v) Organic Acids such as Fatty Acids Having Carboxyl Groups and/or Their Metal Salts

[0081] To this end, use may be made of dicarboxylic acids, polycarboxylic acids, hydroxycarboxylic acids (e.g., 12-hydroxystearic acid and ricinolic acid), aromatic carboxylic acids, aminocarboxylic acids, fatty acids having at least 8 carbon atoms such as higher fatty acids (e.g., oleic acid and stearic acid), acrylic acid and polyacrylic acid as well as their metal salts.

[0082] The metal salt of each of the aforesaid organic acids is generally called a metal soap wherein the metal may be Na, K, Li, Ca, Mg, Al, Zn, Sn, Pb, Ni, Cu, Co, Mn, Fe, In, Bi, Ag or the like. In view of solder material, however, metal salts of Sn are preferred. In view of the action of the particle coalescence-preventing agent on prevention of coalescence of particles, on the other hand, it is preferable to use as metal salts of organic acids (carboxylic acids) having carboxyl groups those of straight-chain or hydroxy fatty acids having at least 8 carbon atoms, especially metal salts of stearic acid, metal salts of 12-hydroxstearic acid and metal salts of ricinolic acid. It is here noted that the metal salts of derivatives of fatty acids or 12-hydroxystearic acid may be called fatty acid soaps.

[0083] (vi) Hydrazines

[0084] To this end, use may be made of hydrated hydrazine, and alkylhydrazine compounds (benzylhydrazine, tert-butylhydrazine hydrochlorate, isopropylhydrazine sulfate, and hydrazinomethyl acetate hydrochlorate).

[0085] (vii) Pyrazoles

[0086] (viii) Azo Compounds

[0087] (ix) Thermoplastic Resins such as Acrylic Resin and Phenol Resin

[0088] (x) Alcohols such as Propargyl Alcohol, Butinediol, Hexynol, Ethylaxynol

[0089] (xi) Isocynates

[0090] (xii) Sulfur-Containing Compounds

[0091] To this end, use may be made of thiourea, thioureas such as N-substituted alkylthiourea, and heterocyclic compounds containing —SH groups per molecule (e.g., 2-mercaptobenzothiazole, and 2-mercaptobenzoimidazole).

[0092] (xiii) Polyamine Compounds

[0093] Poly 4-vinylpyridine or the like may be used to this end.

[0094] The particle coalescence-preventing agent may be composed of not only the compound belonging to each of the classes (i) to (xiii) but also the compounds belonging to two or more of these classes.

[0095] Referring to the action of the carboxyl group (—COOH) on a metal, there are chemical adsorption onto the surface of the metal as represented by —COO—Me (metal) —OOC— and physical adsorption exerted by the attractive forces of charges as expressed by —O−—H+/Me−+/—O−—H+ as in the case of —OH. The chemical adsorption, because of involving chemical reactions, generates much heat of adsorption, and so requires high activation energy. However, the chemical adsorption has energy higher than the physical adsorption, making particles less susceptible to disengagement at high temperature.

[0096] The molten solder (the molten metal having a low-melting point) prepared in the solder melting tank and the dispersion medium prepared in the dispersion medium heating tank, etc. are fed to the fine-granulation machine, wherein they are dispersed. The wording “dispersing energy for dispersing particles is applied to the dispersion medium” used in the present disclosure means that when a lump or a coarse particle is finely divided into fine particles, mechanical energy is applied to those divided articles so as to prevent agglomeration or coalescence thereof. In this sense, it is preferable to divide the low-melting molten metal into particles and then disperse those particles in the aforesaid dispersion medium. However, it is acceptable to disperse a lump form of low-melting metal or a powder form of low-melting metal or both in the dispersion medium. If heated during or after dispersion, the latter can then be dispersed in the form of molten metal particles.

[0097] The fine-granulation machine may be of the batch type that is used with a buffer tank provided on its upstream side, from which a constant amount of feed is supplied to the fine-granulation machine. Thereafter, the same operation is repeated as often as necessary. If, in this case, the fine-granulation machine is operated in operative association with the buffer tank for automatic running, then some mass-production can be achieved. When mass-production is carried out on a full scale with improved productivity or when the machine used is simplified, however, it is preferable to use a continuous type fine-granulation machine to which the molten solder and the dispersion medium are continuously supplied for continuous running.

[0098] For such a continuous type fine-granulation machine, for instance, use may be made of a combined agitation and dispersion machine including a generator comprising a rotator and a stator, an ultrasonic machine, a high-pressure homogenizer, and a high-speed agitator as set forth in Japanese Patent Unexamined Publications 9-75698, 10-161667 and 11-347388.

[0099] One exemplary combined agitation and dispersion machine comprising a rotator and a stator is shown in FIGS. 3 and 4. As shown in FIGS. 3 and 4, a stator 1 is built up of a concave (a deep dish form of) member and slots 4, 4, . . . , provided radially on a peripheral wall thereof and open at their ends, and a rotator 2 is made up of a two-wing blade rotatable around the axis of rotation. By rotating the rotator 2 at high speed relative to the stator 1, a liquid mixture to which a low-melting metal melt is added is sucked in the dispersion medium obtained by mixing and dissolving the particle coalescence-preventing agent in the particle dispersion medium, so that the low-melting metal melt in the liquid mixture is divided by high shear action exerting between the stator 1 and the rotator 2 into particles, and a dispersion containing the metal melt particles is discharged from the slots 4, 4, . . . . Reference numeral 5 is the axis of rotation.

[0100] The machine of FIG. 3 is spaced away from the bottom of a processing tank 6, as shown in FIG. 5, and a shaft of rotation 5 (that is not seen in FIG. 5) is inserted through a cylindrical member extending hermetically through an upper end lid sheet 7, so that the rotating force of a motor 8 (the number of rotation of which is controlled by a rpm controller 8a) can be transmitted to the shaft 5. Through an inlet hole and an outlet hole (shown by arrows) formed in the closeably and detachably provided lid sheet 7, an inert gas can be circulated in the interior of the processing tank 6 so that the interior of the processing tank 6 can be placed in an inert gas atmosphere. Heaters 10, 10 and 10 are located on the outside of the bottom and both sides of the processing tank 6. As the bottom and both sides of the processing tank 6 are heated by the heaters 10, 10 and 10, a liquid mixture 6a that is a liquid mixture of the aforesaid particle dispersion medium and the particle coalescence-preventing agent with the low-melting metal melt is agitated at high speed by the high-shear machine immersed therein, while that liquid mixture 6a is heated to a proper processing temperature, thereby obtaining a slurry. In this case, the temperature of the slurry is sensed by a thermocouple 11 to control the amount of heat generated from the heaters 10, 10 and 10 by a temperature controller 12, so that the temperature of the slurry can properly be controlled. Between the processing tank 6 and the heaters 10, 10 and 10 a copper pipe is provided in such a way as to surround the processing tank 6 so that, by passing cooling water through the copper pipe, the temperature of the slurry under agitation can be prevented from becoming higher than a certain threshold. A turning blade 9 is provided to prevent entrainment of gas at the time the middle of the liquid surface is lowered by generation of vortexes. Reference numeral 13 is a processing tank-supporting base lined with a heat-resistant material with the heaters 10, 10 and 10 embedded therein.

[0101] Exemplary agitators comprising a stator and a rotator are an agitator manufactured by Kinematica (Switzerland), an agitator manufactured by IKA (Germany), an agitator manufactured by Silverson (Great Britain), Cavitron manufactured by Pacific Machinery & Engineering Co., Ltd., and Clearmix manufactured by M Technics. Preferably in this respect, the rotator should have a diameter of at least 80 mm with a peripheral speed of at least 15 m/sec., and the clearance between the rotator and the stator is 1 mm at most. The aforesaid slurry should preferably be supplied to the vicinity of the suction side on which the flow created by the rotator and stator is sucked.

[0102] The ultrasonic dispersion machine is designed to apply ultrasonic energy to the slurry while agitated by means of a homogenizer or other agitator. Even with this, the slurry is processed while heated, thereby dividing the low-melting metal melt into fine particles. To generate ultrasonic vibrations, for instance, use may be made of a separate type ultrasonic generator wherein the generator is separate from a vibrator, a one-piece type ultrasonic generator wherein the generator is integrated with a vibrator, an immersion type ultrasonic generator, and a horizontal type ultrasonic generator. Alternatively, a combined type ultrasonic generator designed to use ultrasonic vibrations having varying wavelengths at the same time may be used. For instance, a multiple ultrasonic dispersion machine manufactured by Nippon Seiki Seisakusho and a triple ultrasonic dispersion machine may be used.

[0103] The feature of the high-pressure homogenizer is that the slurry is passed through a narrow gap under pressure so that fine granulation takes place by way of cavitation occurring on instantaneous change from high to low pressure. Exemplary such homogenizers are Ultymizer manufactured by Sugino Machine, Micro-Fulldizer manufactured by Mizuho Industrial Co., Ltd., the H series of high-pressure homogenizers manufactured by Nihon B. E. E., Nanomizer System manufactured by Nakabishi Engineering, and a high-pressure homogenizer manufactured by Niro Soavi (Italy).

[0104] Besides, Fillmix manufactured by Tokushu Kika Kogyo, and other machines for dispersion, emulsification and fine granulation in liquids may be used.

[0105] These dispersion machines may be used in parallel or series arrangements of two or more for sharp particle size distributions, high throughput capacities or depending on other requirements.

[0106] The aforesaid commercially available dispersion machines may each be provided with devices for heating the aforesaid slurry or cooling the resulting liquid, a temperature control indicator (shown in FIG. 5), a washer and a bottom valve or, in some cases, a pressure regulator for regulation of the pressure in the processing tank.

[0107] The molten solder droplets (low-melting metal droplet) finely granulated in the dispersion machine in the fine-granulation system may then be passed through a mesh (sieve) or subjected to gravitational settling, so that not fully divided or coarse particles are removed to obtain fully divided particles alone. In this case, it is acceptable to sort out coarse particles by a liquid cyclone and allow them to go back to the dispersion machine.

[0108] Alternatively, there may be provided another passage for communicating the dispersion machine in the fine-granulation system with the dispersion medium heating tank, so that on startup of dispersion in the dispersion machine, only the dispersion medium is fed as by a constant-feeding pump from the dispersion medium heating tank to the dispersion machine until it is stabilized by heating at a given temperature. After stabilization is reached, the dispersion medium is fed back to the dispersion medium heating tank.

[0109] By increasing the number of rotations and vibrations in the dispersion machine thereby applying high dispersion energy to the molten metal particles, they can be divided into fine particles having an average particle diameter of 10 &mgr;m+a few &mgr;m at most, preferably 10 &mgr;m or less (10 &mgr;m at most).

[0110] The low-melting metal is used in an amount of 0.1 to 100 grams, preferably 1 to 50 grams, more preferably 2 to 20 grams per 100 grams of the particle dispersion medium, and the particle coalescence-preventing agent is used in an amount of 0.01 to 10 grams per 100 grams of the particle dispersion medium. When the proportion of the low-melting metal is lower than that lower limit, production efficiency drops, and when too much low-melting metal is used, there is a decrease in the effect on prevention of coalescence of the molten metal particles dispersed in the dispersion medium as is the case where the amount of the coalescence-preventing agent is less than its lower limit. When the particle coalescence-preventing agent is used in an amount greater than its upper limit, its effect remains unsaturated or is little enhanced.

[0111] The slurry obtained by dispersing the molten metal particles in the dispersion medium containing the particle coalescence-preventing agent in this way is then fed to the solidifier by cooling, if required, after coarse particles have been removed therefrom, as shown in FIGS. 1 and 2, wherein the slurry is cooled to a temperature lower than the solidifying point of the molten metal, so that the molten metal particles are solidified into solid metal particles. For the solidifier by cooling, it is preferable to use a cooling double pipe of simple construction that comprises an inner pipe through which the coarse particle-free slurry is to pass and a jacket that surrounds the inner pipe. Water or other coolant is circulated through the jacket, and the flow rate of the coolant is regulated in such a way that the temperature of the slurry is kept constant at a given position in the inner pipe. It is preferable that the cooling double pipe is set at an angle of 45 to 90° with respect to horizontal; at an angle smaller than 45°, the resulting solid metal particles are likely to build up. It is then preferable that the cooling temperature of the cooling double pipe is lower than the temperature at which the droplets of the lower-melting metal, e.g., solder are solidified and about 20 to 100° C. lower than the melting point of solder. This is required to make the processing by the next solid-liquid separator easy, and save the heat energy needed for reheating the spent dispersion medium that is fed back to the dispersion medium heating tank after regeneration in the dispersion medium regenerator.

[0112] The slurry may be cooled by water, as is the case with the system of FIG. 5 or cooled off in a vessel in the system. Alternatively, the slurry may rapidly be cooled by charging the coolant throughout a pool of the slurry under agitation, or the slurry may continuously be pored in the coolant. The coolant used may be the aforesaid dispersion medium or other medium that may or may not have volatility.

[0113] The thus obtained slurry free from any coarse particles and containing the solid metal particles is fed to the solid-liquid separator as shown in FIGS. 1 and 2, wherein the solid metal particles are separated from liquid residue. On the upstream side of the solid-liquid separator, there may be provided a slurry buffer tank capable of storing a constant amount of slurry in such a way that the slurry is fed from that buffer tank to the solid-liquid separator as occasion may arise.

[0114] By way of example but not exclusively, the solid-liquid separator used herein may appropriately be selected from a variety of desired separators depending on the properties of the solid metal particle-containing slurry, e.g., the particle size distribution of the solid metal particles, the viscosity of the liquid residue from which the solid metal particles have been separated, and concentration of the solid matter. To this end, use may be made of liquid cyclones, separators that harness spontaneous settling, and filter devices (such as Oliver filter, horizontal belt filter, rotary filter, ceramic filter, filter press and centrifugal filter). However, when the solid metal particles have a small particle diameter or the liquid residue has a high viscosity, solid-liquid separation should preferably be carried out by means of a centrifugal decanter.

[0115] In the solid-liquid separation process by the centrifugal decanter, it is preferable that the solid metal particle-containing slurry is continuously fed to the centrifugal decanter for successively separating the solid metal particles that are the solid matter from the liquid residue. In this case, it is acceptable to leave too small particles of the solid metal particles on the liquid residue side. The centrifugal decanter is also preferable in that depending on the particle diameter of the solid metal particles to be separated, the viscosity of the liquid residue, a specific gravity difference between the solid metal particles and the liquid residue, etc., the number of rotations can be regulated for easy regulation of centrifugal force. It is here noted that the solid metal particles are obtained in the form of a slurry or cake because about 10% (on a weight basis) of liquid residue deposit to the solid metal particles. The solid-liquid separator should preferably be of the type that can tightly be closed for solid-liquid separation processing in the presence of an inert gas in consideration of prevention of oxidization of the solid metal particles and liquid residue (the liquid residue is recyclable after regeneration as will be described later), and avoidance of risks of possible fires in the case where the liquid residues contain flammable components. A solid-liquid separator whose interior is easily cleanable is also preferred. For instance, preference is given to the TRV series of upright centrifugal decanters manufactured by Tomoe Kogyo.

[0116] Preferably, the solid metal particles with some liquid residue deposited to them, separated by the solid-liquid separator, should be washed in the washer as shown in FIGS. 1 and 2 so that those depositions are washed away to some degrees. In some cases, however, the solid metal particles may be used immediately or without being washed. The washer used herein should preferably comprise a tank in which the solid metal particles with the liquid residue deposited onto them are immersed in a detergent, preferably under agitation by an agitator or the like. The reasons why such agitation is preferable are that after re-pulping (that is, re-dispersion in the detergent of the solid metal particles with the liquid residue deposited to them), the liquid residue depositions to the solid metal particles are replaced by the detergent, so that the solid material particles can easily be separated from a liquid detergent residue in the next step. This particularly holds true for the case where a volatile detergent is used as the detergent, because the liquid detergent residue can easily be removed from the solid metal particles by way of volatilization. For the detergents used herein, a suitable selection may be made from the types that can be more easily separated from the solid metal particles than from the liquid residue composed mainly of the aforesaid dispersion medium. The separation means may also be selected depending on the selected detergent. The more the number of repetition of the re-pulping and subsequent solid-liquid separation process, the more the amount of depositions is reduced and the higher the degree of washing becomes, resulting in solid metal particles from which impurities are reduced as much as possible.

[0117] To separate the solid metal particles from the liquid residue such as detergent residue, a suitable selection should preferably be made from various separation devices depending on the particle size distribution of the solid metal particles, the viscosity of the liquid residue, the concentration of the solid matter, etc. To this end, use may be made of liquid cyclones, separators that harness spontaneous settling, and filter devices (such as Oliver filter, horizontal belt filter, rotary filter, ceramic filter, filter press and centrifugal filter). However, when it is desired to obtain solid metal particles having an average particle diameter of 10 &mgr;m or less, centrifugal machines, especially a centrifugal decanter should preferably be used.

[0118] In the solid-liquid separation process by the centrifugal decanter, it is preferable that the solid metal particle-containing slurry is continuously fed to the centrifugal decanter for successively separating the solid metal particles that are the solid matter from the liquid residues. In this case, it is acceptable to leave too small particles of the solid metal particles on the liquid residue side. The centrifugal decanter is also preferable in that depending on the particle diameter of the solid metal particles to be separated, the viscosity of the liquid residues, a specific gravity difference between the solid metal particles and the liquid residues, etc., the number of rotations can be regulated for easy regulation of centrifugal force. If a double pipe or the like is used between the solid and the liquid outlet of the centrifugal decanter to charge the detergent through that double pipe, then washing due to counter-current contact is achievable so that efficient washing can be carried out. The solid-liquid separator should preferably be of the type that can tightly be closed for solid-liquid separation processing in the presence of an inert gas in consideration of prevention of oxidization of the solid metal particles and liquid residues (the liquid residues are recyclable after regeneration as will be described later), and avoidance of risks of possible fires in the case where the liquid residues contain flammable components. A solid-liquid separator whose interior is easily cleanable is also preferred. For instance, preference is given to the TRV series of upright centrifugal decanters manufactured by Tomoe Kogyo.

[0119] The detergents used herein, for instance, include those of the quasi-aqueous glycol ether type, water-soluble solvent type, solvent type, terpene type and petroleum solvent type (“WELL-UNDERSTABLE EVERY ASPECT OF WASHING”, pp. 45-55, September 1999, Nikkan Kogyo Shinbun); those of the non-aqueous hydrocarbon type (normal paraffin, isoparaffin, naphthene, and aromatic detergents); alcoholic detergents (based on isopropyl alcohol, ethanol or other alcohols); silicone detergents; fluorine detergents; chlorine detergents; bromine detergents (see “WELL-UNDERSTABLE EVERY ASPECT OF WASHING”); and detergents based on acetone, methyl ethyl ketone, benzene, toluene, xylene, ethyl acetate, butyl acetate, cyclohexane, etc.

[0120] Among these detergents, it is preferable to use detergents that have low water-solubility and low hygroscopicity for the purpose of prevention of oxidization of the solid metal particles, and further have non-flammability or low flammability. However, it is acceptable to use detergents having high flammability if fire prevention means are used in combination therewith. When the detergent is distilled out of the liquid residue upon solid-liquid separation for the purpose of recycling the spent detergent, it is preferable to use a detergent that has a vapor pressure higher than those of the particle dispersion medium and particle coalescence-preventing agent contained in the dispersion medium and has a low-boiling point as measured by itself and decreased latent heat of vaporization. Even though this detergent remains in the dispersion medium or the solid metal particles, there is no problem or little or no influence.

[0121] The washing by the detergent should preferably be carried out to such a degree that some portion of the liquid residue deposited to the solid metal particles before washing remains. This is because especially when a difficult-to-volatilize substance is used for the particle dispersion medium contained in the liquid residue, it is possible to reduce the degree of powder dusting of the solid metal particles after the drying step by the drier or to prevent oxidization of the solid metal particles. In this case, the amount of the remaining hard-to-volatilize substance should preferably be in the range of 0.01 to 1% with respect to the solid metal particles. The “%” used herein is understood to mean “% by mass”.

[0122] The solid metal particles washed in the washer may be discharged from the washer with the detergent remaining deposited to them. For removal of such detergent depositions, however, it is preferable to dry the solid metal particles in the drier, as shown in FIGS. 1 and 2.

[0123] When a detergent of high volatility is used as the detergent, the drier used should be designed to remove that detergent by way of vaporization. By way of example but by way of limitation, it is preferable to use a drier that relies upon drying by heating and/or drying under reduced pressure. When the solid metal particles are solder particles, however, it is preferable to use a drier that relies on both indirect heating at about 50° C. and drying under reduced pressure. When other solid metal particles are obtained, one or both of these drying methods may be used at different drying temperatures. If indirect heating is combined with agitation of the solid metal particles, not only is efficient drying achievable, but also the hard-to-volatilize liquid residue (the liquid residue remaining upon solid-liquid separation after the processing in the fine-granulation machine can uniformly be deposited to the solid metal particles)(that liquid residue contains the particle coalescence-preventing agent). However, too strong agitation is not preferable because, for instance, solder particles are susceptible to damage.

[0124] The drier used should preferably be of the continuous type that enables the material to be dried to be continuously fed thereto and the dried material to be successively discharged therefrom. Since it is difficult to effect drying under reduced pressure in a continuous manner, however, it is acceptable to use a batch type drier. In this case, on the upstream side of the batch type drier, there is provided a buffer tank for storing the material to be dried. On demand the buffer tank feeds the material to be dried to the drier, and the dried material is then discharged therefrom. By repeating this operation, the buffer tank can be run in automatically operative association with the drier so that large amounts of the solid metal particles can be dried. To this end, preference is given to a vibrating drier commercially obtainable from Chuo Kakoki.

[0125] When the viscosity of the liquid residue on the solid-liquid separation of the slurry discharged from the fine-granulation machine is relatively low in the case where solder powders are obtained, it is acceptable to use a combined filter and drier manufactured by Tanabe Willtech or the like, so that the solid-liquid separation of the slurry discharged out of the fine-granulation machine, the washing of the solid matter of the slurry, the solid-liquid separation of the washings and the drying of the solid matter of the washings can be finished in one single unit.

[0126] Through the aforesaid respective steps, solder products are obtained from the starting solder material (metal) as shown in FIGS. 1 and 2. To cut back on the production cost of solder powders, however, it is preferable to recycle the particle dispersion medium and the detergent.

[0127] Referring to how to recycle with reference to FIG. 2, the spent dispersion medium is discharged out of the solid-liquid separator. As already described, the spent dispersion medium is composed of the liquid residue from which solid metal particles having a given particle diameter have been removed (and which, in some cases, contains solid metal particles having too small particle diameters). Although depending on purposes, this spent dispersion medium may be fed back to the dispersion medium heating tank for immediate use. When regeneration is needed, however, the spent dispersion medium is preferably fed to the dispersion medium regenerator, from which the regenerated medium is sent to the dispersion medium heating tank. For instance, for removal of solid metal particles which, because of having too small diameters, are accidentally or unavoidably entrained in the liquid residue or other accidentally formed solid matter, it is preferable to use a centrifugal decanter, Sharples centrifuge, De Laval centrifuge, or filters such as a rotary filter and a ceramic filter. The thus obtained clear liquid is allowed to go back to the dispersion medium heating tank by means of a pump, because that liquid is a dispersion medium that is a mixture of the particle dispersion medium and the particle coalescence-preventing agent (often at a ratio different from the initial ratio). Preferably, however, the clear liquid should temporarily be stored in a separately provided buffer tank from which, on demand, it is fed to such a tank. In this way, the spent dispersion medium (dispersion medium) can be recycled in a continuous manner.

[0128] As shown in FIG. 2, the spent detergent is discharged out of the washer. As already stated, this spent detergent contains the detergent that is used for wash processing in the washer and has depositions (the liquid residue occurring upon solid-liquid separation of the slurry discharged from the fine-granulation machine) adhering to the solid metal particles discharged out of the fine-granulation machine. The spent detergent may be allowed to go back as such to the washer for subsequent use. When the spent detergent must be regenerated, however, it is preferable to feed the spent detergent to the detergent regenerator for removing the liquid residue therefrom, and then return the regenerated detergent back to the washer. When the detergent has high volatility and the liquid residue has low volatility, the detergent regenerator should be designed such that the detergent is distilled out. When there is a large difference in vapor pressure between the detergent and the liquid residue, it is preferable to carry out single-stage distillation. When that vapor pressure difference is large, however, it is preferable to carry out multistage distillation in a continuous manner. In consideration of recycling, the detergent should preferably have a vapor pressure of at least 15 kPa at 40° C. and a latent heat of vaporization of up to 1,000 kJ/kg.

[0129] As explained above, solder powder products are obtained from the starting solder metal. In this case, the dispersion medium, and the detergent may be recycled. Preferably for processing, each device or unit should be run or operated while its interior is filled with an inert gas. Likewise, if required, the dispersion medium regenerator or the detergent regenerator should be run or operated in the presence of an inert gas. This is favorable just only for prevention of oxidization of the low-melting metal and its metal as well as the solid metal particles, particle dispersion medium and particle coalescence-preventing agent but also for making the whole system inclusive of the detergent resistant to flammability, thereby foreclosing the risk of possible fires. It is also preferable to provide a washer for the necessary portion of each device or unit, because it is easy to carry out washing upon alteration of the type of the metal to be finely divided or upon alteration of the particle dispersion medium, particle coalescence-preventing agent and detergent, thereby preventing contamination upon such alterations.

[0130] The shape and particle diameter of the solid metal particles in the thus obtained fine metal powder products are determined depending on the shape and particle diameter of the molten metal particles dispersed in the heated dispersion medium in the fine-granulation machine; the molten metal particles have substantially true sphericity and so have the solid particles. To reduce the particle diameter of the solid particles, on the other hand, the diameter of the molten metal particles should be reduced down to the average particle diameter of 2 to 30 &mgr;m. This, for instance, is achievable by increasing the number of rotations and vibrations of each dispersion machine depending on the application time of dispersion energy, operating conditions such as the heating temperature and processing time of the slurry, the types and proportions of the low-melting metal, particle dispersion medium and particle coalescence-preventing agent. It is here understood that the process for producing metal powders of the invention includes a process for producing spherical metal powders, a process for producing a spherical form of fine metal powders, and a process for producing a spherical form of fine metal particles.

[0131] Thus, if fine metal particles, especially a spherical form of fine metal particles are used in a reduced amount with respect to the low-melting metal in the particle dispersion medium so that the components of the particle dispersion, etc. consumed in production process steps are reduced, it is then possible to mass produce fine metal powder particles in an industrially efficient manner.

[0132] The fine metal powders obtained by the aforesaid fine metal powder production process, for instance, are used in an amount of 85 to 92% per solder paste (with the flux content of 8 to 15%). Those powders are particularly suitable for reflow soldering for recently developed printed circuit boards having soldering lands at very narrow pitches.

EXAMPLE 1

[0133] According to the flowchart of FIG. 1, the starting solder metal (Sn—Pb eutectic alloy (63Sn/37Pb) for solder is introduced into the solder melting tank (that is filled therein with nitrogen gas), where it is molten to prepare a solder melt. Meanwhile, Uniloop MB-22 (manufactured by Nippon Oils & Fats) for the particle dispersion medium and hydrogenated, acrylic acid-modified rosin (KE-604 manufactured by Arakawa Kagaku Kogyo) for the particle coalescence-preventing agent (2% with respect to the sum of the particle dispersion medium and the particle coalescence-preventing agent) are mixed together in the dispersion medium heating tank (that is filled therein with a nitrogen gas). The mixture is preheated in the dispersion medium heating tank as long as it is at a temperature 50 to 100° C. lower than the temperature at which the mixture is to be fed to the fine-granulation machine. Then, the mixture is heated to the temperature at which it is to be fed to the fine-granulation machine, using a separately provided line heater furnace (heated delivery pipe) for a short residence time of up to 10 minutes in a precisely temperature-controlled manner, thereby preparing a heated dispersion medium.

[0134] Using separate constant-feeding gear pumps, the molten solder (20 Kg/hour) and the heated dispersion medium (200 Kg/hour) are continuously fed at the same flow rate through the respective lines to the fine-granulation machine. In this case, the molten solder in particular is passed through the line whose interior can be controllably heated (to 190° C.) and the mass ratio of both is 1:10. In the fine-granulation machine, both are continuously mixed together at a constant temperature of 190° C., while dispersion energy for dispersion of particles is applied to the mixture, thereby continuously obtaining a molten solder particle dispersion having the molten solder particles dispersed in the dispersion medium. To this end, a dispersion machine comprising a stator and a rotator (the diameter of the rotator is at 80 mm or longer, the peripheral speed is 15 m/second or faster and the rotator-to-stator clearance is 1 mm or longer) is used.

[0135] Then, the molten solder particle dispersion is continuously passed through a line into the inner pipe of a cooling double pipe set at an angle of 60° for the solidifier by cooling, while cooling water is circulated through the outer jacket (cooling by coolant water passing between the coolant-in and the coolant-out in FIG. 1). Thus, the molten solder particles are cooled and solidified into solid particles.

[0136] Subsequently, the slurry that contains the solid solder particles is continuously supplied to the solid-liquid separator for which one of the TRV series of upright centrifugal decanters manufactured by Tomoe Kogyo is used, wherein the solid solder particles are continuously separated from the slurry to obtain a throughput of 19 Kg/hour as calculated on a solder basis. The thus separated solid solder particles are processed in the washer. This washer is built up of a re-pulping tank and another decanter. In the re-pulping tank, the solid solder particles are washed using ethyl acetate as the detergent, and then subjected to solid-liquid separation in the centrifugal decanter, threrby separating the solder powders from the detergent. The solid solder particles are rid of depositions while the spent detergent is recovered. The detergent was used in an amount of 38 Kg/hour in the repulping tank and in an amount of 19 Kg/hour in the centrifugal decanter. The thus washed solid solder particles are dried in a vibration drier manufactured by Chuo Kakoki (at a drying temperature of 50° C.) for the drier to obtain 19 Kg/hour of a solder powder product (fine solder particles).

[0137] It is here noted that processing of the material by each device and delivery of the material between the adjacent devices according to the flowchart of FIG. 1 were automated to the greatest extent practicable while numerical data are checked against those from the sensor attached to each device, and as many devices as possible were computer-controlled pursuant to the program for the whole production process. It is also noted that the devices are connected with one another by way of piping, and so the raw materials can continuously be processed into the end product.

[0138] Observation under a scanning electron microscope (SEM) of the obtained fine solder particles showed that they are in true spherical forms having no satellite particle whatsoever, and laser diffraction analysis indicated that the average particle diameter is 9.5 &mgr;m and the particle size distribution is 0.65 as expressed by &egr;=(D90−D10)/D50 where D90, D10 and D50 stand for the diameters of particles that account for 90%, 10% and 50%, respectively, of all particles as counted in ascending diameter order. The yield of the obtained fine solder particles was 90%. The smaller the value of &egr;, the narrower and sharper the particle size distribution becomes.

EXAMPLE 2

[0139] As shown in the flowchart of FIG. 2, solder powders were prepared as in Example 1 with the exception that two additional steps were added to the flowchart of FIG. 1.

[0140] In one additional step, the solid-liquid separator was run such that the liquid residue separated from the solid solder particles was used as the spent dispersion medium and the dispersion medium regenerator was run such that the solid matter was removed by a centrifugal decanter and the resulting clear liquid was used as the recovered dispersion medium in an amount of 90% relative to the original amount. The recovered dispersion medium was used as a part of the dispersion medium in the aforesaid dispersion medium heating tank (a 10% deficiency was made up by a fresh dispersion medium).

[0141] In another additional step, the washer was run such that the liquid residue composed mainly of ethyl acetate as the spent detergent was distilled out and recovered in a single distillation device working as the detergent regenerator. The washer was also operated such that the rate of recovery of ethyl acetate (the proportion of the recovered amount relative to the original amount) was 90%. That is, the washer was operated such that the recovered ethyl acetate could be used as a part of the detergent (the deficient was made up by a fresh detergent).

[0142] It is understood that the aforesaid two steps added to the flowchart of FIG. 2, too, can be automated in such a way as to be computer-controlled. In this example, the respective devices are again connected with one another by way of piping, so that the raw materials can continuously be processed into the end product. The obtained solder powders were the same as in Example 1.

[0143] According to the present invention, the step of fine granulation of the molten metal and the solidification-by-cooling step, solid-liquid separation step, washing step and drying step added thereto are controlled in the form of a series of mutually correlative steps, so that fine powder particles of the metal can be mass-produced in an industrially efficient manner, thereby achieving remarkable cost reductions. Especially if the dispersion medium and detergents are regenerated and recovered for recycling, then the production cost can be much more reduced.

[0144] Moreover, metal fine powders that can be applied even to fine soldering portions on wiring substrates can be mass-produced in an industrially efficient manner, and a solder paste composition using such metal fine powders can be used for metal mask printing of fine patterns, so that surface mounting of electronic parts, etc. can be carried out at high densities, resulting in the achievement of multifunctional, miniaturized wiring substrates for electronic equipments.

Claims

1. A process of producing metal powders, comprising steps of:

(a) melting a raw low-melting metal to obtain a metal melt,

(b) mixing a particle dispersion medium and a particle coalescence-preventing agent to obtain a dispersion medium that may or may not be heated,

(c) supplying the melt of said low-melting metal from said step (a) and supplying said dispersion medium from said step (b) with application of dispersion energy that disperses the melt of said low-melting metal in the form of fine particles, thereby obtaining a molten metal particle dispersion wherein molten metal particles are dispersed in said dispersion medium,

(d) cooling said molten metal particle dispersion to solidify said molten metal particles into solid metal particles,

(e) separating said solid metal particles from a liquid residue,

(f) washing said separated solid metal particles with a detergent to remove depositions to said solid metal particles, and

(g) drying said washed metal particles, wherein:

in said step (c), said particle coalescence-preventing agent adsorbs to and/or reacts with said molten metal particles to prevent coalescence of at least said molten metal particles so that said solid metal particles can be finely granulated in said steps (c) to (g), and said steps (a) to (g) are controlled in the form of a series of mutually correlative steps.

2. The process of claim 1, wherein there is provided a dispersion medium recycle step in which the liquid residue separated in said step (e) is directly used as a part or the whole of the dispersion medium in said step (b) or a dispersion medium regenerated from said liquid residue in a dispersion medium regeneration step (h) is recycled as a part or the whole of the dispersion in said step (b), wherein said dispersion medium recycle step is continuously controlled in correlation to said step (b).

3. The process of claim 1, wherein there is provided a detergent recycle step in which a spent detergent that may contain the depositions removed in said step (f) is directly used as a part or the whole of the detergent used in said step (f) or a detergent regenerated from said spent detergent in a detergent regeneration step (i) is recycled as a part or the whole of the detergent used in said step (f), wherein said detergent recycle step is continuously controlled in correlation to said step (f).

4. The process of claim 2, wherein there is provided a detergent recycle step in which a spent detergent that may contain the depositions removed in said step (f) is directly used as a part or the whole of the detergent used in said step (f) or a detergent regenerated from said spent detergent in a detergent regeneration step (i) is recycled as a part or the whole of the detergent used in said step (f), wherein said detergent recycle step is continuously controlled in correlation to said step (f).

5. The process of claim 1, wherein the detergent used in said step (f) has a vapor pressure of at least 15 kPa at 40° C. and a latent heat of vaporization of at most 100 kJ/kg.

6. The process of claim 2, wherein the detergent used in said step (f) has a vapor pressure of at least 15 kPa at 40° C. and a latent heat of vaporization of at most 100 kJ/kg.

7. The process of claim 3, wherein the detergent used in said step (f) has a vapor pressure of at least 15 kPa at 40° C. and a latent heat of vaporization of at most 100 kJ/kg.

8. The process of claim 4, wherein the detergent used in said step (f) has a vapor pressure of at least 15 kPa at 40° C. and a latent heat of vaporization of at most 100 kJ/kg.

9. The process of claim 1, wherein in said step (g), said washed solid metal particles are dried such that the liquid residue deposited to said solid metal particles accounts for 0.01 to 1% of said solid metal particles, thereby reducing oxidization and dusting of powders of said solid metal particles.

10. The process of claim 2, wherein in said step (g), said washed solid metal particles are dried such that the liquid residue deposited to said solid metal particles accounts for 0.01 to 1% of said solid metal particles, thereby reducing oxidization and dusting of powders of said solid metal particles.

11. The process of claim 3, wherein in said step (g), said washed solid metal particles are dried such that the liquid residue deposited to said solid metal particles accounts for 0.01 to 1% of said solid metal particles, thereby reducing oxidization and dusting of powders of said solid metal particles.

12. The process of claim 4, wherein in said step (g), said washed solid metal particles are dried such that the liquid residue deposited to said solid metal particles accounts for 0.01 to 1% of said solid metal particles, thereby reducing oxidization and dusting of powders of said solid metal particles.

13. The process of claim 1, wherein said step (d) of cooling the molten metal particle dispersion to solidify said molten metal particles into solid metal particles is carried out while said molten metal particles dispersion is passed through an inner pipe of a double pipe structure and a coolant is passed through an outer pipe of the double pipe structure.

14. The process of claim 2, wherein said step (d) of cooling the molten metal particle dispersion to solidify said molten metal particles into solid metal particles is carried out while said molten metal particles dispersion is passed through an inner pipe of a double pipe structure and a coolant is passed through an outer pipe of the double pipe structure.

15. The process claim 3, wherein said step (d) of cooling the molten metal particle dispersion to solidify said molten metal particles into solid metal particles is carried out while said molten metal particles dispersion is passed through an inner pipe of a double pipe structure and a coolant is passed through an outer pipe of the double pipe structure.

16. The process of claim 1, wherein said double pipe is located at an angle of 45 to 90 degrees with respect to horizontal.

17. The process of claim 2, wherein said double pipe is located at an angle of 45 to 90 degrees with respect to horizontal.

18. The process of claim 3, wherein said double pipe is located at an angle of 45 to 90 degrees with respect to horizontal.

19. The process of claim 1, wherein in said step (b), the dispersion medium that is a mixture of the particle dispersion medium with the particle coalescence-preventing agent is heated in such a way that said dispersion medium is preheated in a preheating tank, and then passed through a heated delivery pipe for a residence time that does not exceed 10 minutes at most.

20. The process of claim 2, wherein in said step (b), the dispersion medium that is a mixture of the particle dispersion medium with the particle coalescence-preventing agent is heated in such a way that said dispersion medium is preheated in a preheating tank, and then passed through a heated delivery pipe for a residence time that does not exceed 10 minutes at most.