Method of forming composite solder by cold compaction and composite solder

Abstract

A reinforced solder is formed by mixing particles of a solder material capable of forming a metal matrix and a reinforcing particulate to form a particulate mixture; compressing the mixture at room temperature to form a solid compact; and sintering the compact to form a particulate composite in which the reinforcing particulate is embedded in a metal matrix formed from the solder material. The solder material can be a lead-free solder material. There is also provided a reinforced solder which includes a lead-free metal matrix and a reinforcing particulate embedded in the metal matrix. The metal matrix includes a combination of tin, copper, silver, and indium having volume ratios of about 91.4:0.5:4.1:4. The reinforcing particulate has an average diameter less than 100 nm. The reinforced solder has a melting point between 180° C. to 230° C.

Description

FIELD OF THE INVENTION

The present invention relates generally to solders, and particularly to composite solders and methods of forming composite solders.

BACKGROUND OF THE INVENTION

Composite solders include both a solder material and a reinforcement material. The solder material is typically an eutectic or low-melting-temperature alloy and forms a metallic matrix. The reinforcement material is typically dispersed in the solder matrix in the form of particulates. Composite solders have improved properties over monolithic solders, which typically have some deficiencies due, for example, to their coarse microstructural features.

Conventional processes for synthesizing composite solders include melting a solder material, adding a reinforcement material to the solder material, and stirring the resulting mixture, to facilitate the uniform dispersion of the reinforcement material in the mixture. The mixture is then solidified to a predetermined shape. In some cases, a wetting agent is added to the mixture. A drawback of such processes is that the resulting solder has inferior physical and mechanical properties than can be realized using powder based methods. Moreover, it is difficult to incorporate and uniformly disperse particulates of reinforcement material that are smaller than about one micron. It is also difficult to produce composite solder materials with high reproducibility and reliability. The production cost can be high. Further, contamination or undesirable chemical reactions can occur during melting or when the wetting agent is added.

Accordingly, there is a need for an improved method of synthesizing composite solders and for composite solders having improved properties.

SUMMARY OF THE INVENTION

In accordance with an aspect of the present invention, there is provided a method of forming reinforced solder. The method includes mixing particles of a solder material capable of forming a metal matrix and a reinforcing particulate to form a particulate mixture. The mixture is compressed at room temperature to form a solid compact, which is sintered to form a particulate composite in which the reinforcing particulate is embedded in a metal matrix formed from the solder material. The solder material can be a lead-free solder material.

In another aspect of the invention, there is provided a reinforced solder formed in accordance with this method.

In accordance with a further aspect of the invention, there is provided a reinforced solder including a lead-free metal matrix and a reinforcing particulate embedded in the metal matrix. The metal matrix includes a combination of tin, copper, silver, and indium having volume ratios of about 91.4:0.5:4.1:4. The reinforcing particulate has an average diameter less than 100 nm. The reinforced solder has a melting point between 180° C. to 230° C.

Other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures, which illustrate exemplary embodiments of the invention,

FIG. 1 is a plurality of field emission scanning electron microscopy (FESEM) images of exemplary composite solders.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

An exemplary embodiment of the invention is a method of forming a reinforced composite solder, wherein a mixture of particles of a solder material and a reinforcing particulate is cold compacted to form a compact. The compact is a particulate composite having the reinforcing particulate embedded in a metal matrix formed by the solder material.

As used herein, a particulate composite refers to a composite material composed of particles embedded in a matrix. A metal matrix refers to a mass of metal particles in which a reinforcing particulate can be embedded. The metal matrix can hold the compact together in a stable shape.

The solder material can be primarily made of an alloy suitable for soldering and capable of forming a metal matrix when compressed and subsequently sintered. The solder material melts at relatively low temperatures, such as below 220° C. Any such alloy known to persons skilled in the art may be used. For example, lead-free alloys, such as tin-based alloys, may be suitable. A tin-based alloy may include one or more other metals such as antimony, bismuth, copper, indium, silver, zinc, and the like. As is known, solder alloy having an eutectic composition is advantageous because a low melting temperature is usually desirable. An example of suitable alloys is a Viromet™ 349 alloy, which has volume ratios Sn/Cu/Ag/In=91.4/0.5/4.1/4. As is known, when the volume ratio of the alloy is larger than 0.5, it will likely form a metal matrix on compaction.

A particulate herein refers to a substance or matter in the form of separate and fine particles. The reinforcing particulate has small particle sizes. The term “particle size” as used herein refers to the average diameter of the particles. As the particles may have non-spherical shapes and different sizes, the term “diameter” refers to the average or effective diameter. An effective diameter of a non-spherical particle is the diameter of a spherical particle that has the same volume as the non-spherical diameter. Small particle sizes typically refer to sizes less than 100 nm. However, depending on the application and the desired properties of the end product, particles sizes of the reinforcement particulate can be in the range of submicrons or microns.

The reinforcing particulate is made of one or more materials that can be embedded in the metal matrix formed from the solder particles such that the resulting particulate composite has a melting temperature suitable for soldering and has one or more improved properties over a monolithic solder formed from the solder alloy only. For example, the reinforcing material should be stable, and should not transform into a different phase, at the soldering temperatures of the composite solder. Example properties that can be improved by addition of the reinforcing particulate include density, porosity, mechanical strength (0.2 yield strength (YS) and ultimate tensile strength (UTS)), dimensional and thermal stability, ductility, and the like. As is known, the reinforcing particulate may also include any material that, when embedded in the solder matrix, can suppress grain-boundary sliding, improve intermetallic compound morphology, reduce grain growth, and redistribute internal stress more uniformly. The reinforcement material may have a melting temperature higher than the melting temperature of the solder alloy so that during soldering the reinforcement material can remain in solid form.

Many materials that can be used to reinforce composite solders are known to persons skilled in the art. Materials that have been used in other existing composite solders as reinforcement materials may be used. Example suitable reinforcement materials include ceramic materials such as alumina (aluminium oxide, Al₂O₃), titanium dioxide (TiO₂), and the like. Some metals, alloys and intermetallics may also be suitable. However, reactive materials, unstable materials or materials having low melting temperatures are generally not suitable.

The choice of reinforcement material may depend on the particular application for which the resulting composite solder is to be used. As can be understood, within a limit, a higher proportion of the reinforcing particulate in the mixture may produce a higher reinforcement effect; however, the proportion of the reinforcing particulate in the mixture should not be too high because when the proportion of the reinforcing particulate is too high, the properties of the resulting composite solder can be adversely affected. Further, if the content of reinforcing material is too high, it may be difficult or impossible to form metal matrix from the solder material. Experiments show that including up to 5% of reinforcing particulate may produce composite solders having improved properties. However, in different embodiments, the proportions of the reinforcing particulate may vary, depending on the materials used, the size of the reinforcing particulate, the type of application and the desired properties of the end product.

Particles of the solder material, which are capable of forming a metal matrix, and the reinforcing particulate can be mixed to form a particulate mixture. The mixture may be blended so as to uniformly disperse the particles of the solder material and the reinforcing particulate. Particulates of the solder material and the reinforcement material, and their mixture, can be obtained or formed using techniques known to persons skilled in the art, such as those described in the following articles: D. C. Lin et al., Journal of Metastable and Nanocrystalline Materials, (2005), vol. 23, pp. 145-148; Japanese patent no. 6031486 to K. Sasapi and K. K. K. D. Tanaka, published Feb. 8, 1994; and U.S. Pat. No. 5520752 to Jr. G. K. Lcey et al., published May 28, 1996, each of which is incorporated herein by reference.

The mixture is compressed at a temperature beneath the melting point of the solder material. Typically, the mixture is compressed at room temperature. Compressing at room temperature can be less expensive and easy to perform. The mixture can be compressed using a technique known as cold compaction, which typically includes applying a pressure to the mixture, for example, with a hydraulic press. Cold compaction generally is more particularly described in J. P. Schaffer et al., The Science and Design of Engineering Materials, WCB/McGraw-Hill, 1999, pp. 689-691, which is incorporated herein by reference. Typically, the pressure is applied steadily. However, it may be applied cyclically. Cold compaction under cyclic pressure can be advantageous and can lead to improved compact density, a reduction in the density gradient and a more uniform reinforcement distribution. The compact may be of any desirable shape and size. It can be in the form of an ingot. Optionally, the compact may be coated with colloidal graphite to minimize oxidation.

The compact may then be sintered in an inert environment, such as in an inert gas. As can be understood, sintering can cause particles to bind and thus make the compact denser. The compact can be sintered at suitable temperatures which may vary depend on the particular circumstances but should not be too high to avoid melting the compact melts or otherwise significantly deteriorating the physical properties of the resulting composite solder. For example, the sintering temperature can be in the range of about 70% to 90% of the absolute melting temperature of the compact. Sintering in an inert environment can prevent or reduce oxidation of the compact.

The compacts may be extruded into a desired shape such as a cylindrical solder rod. The extrusion temperature, which can vary between room temperature and about 160° C. depending on the desired property of the end product, and the extrusion ratio can be varied depending on the desirable rod sizes.

As can be appreciated, the forming process does not require melting of the solder particulate. Nor is it necessary to add wetting agents. Therefore, some of the problems associated with the conventional processes for forming composite solders can be avoided. For example, contamination or undesirable chemical reactions can be reduced or avoided. Production cost can also be reduced.

The resulting composite solders have a metal matrix and reinforcing particulate embedded therein and exhibit improved properties over a monolithic solder consisting of the solder alloy only. To illustrate, the results of tests performed on sample composite solders formed in an exemplary process are shown in FIG. 1 and Tables 1 to 3.

The sample composite solders were formed as follows.

Mixtures of particulate of Viromet 349 alloy and particulate of alumina having particle sizes of about 50 nm were blended at a typical speed of 50 rpm for about 10 hours. The proportion of the alumina particulate varied from one (1) to five (5) percent by volume.

The blended mixtures were cold compacted into ingots, typically of 35 mm diameter and 40 mm length, by applying a typical pressure of 50 tons using a 150-ton hydraulic press.

The compacted ingots were coated with colloidal graphite and were subsequently sintered at a temperature of about 150° C. for about two (2) hours in an argon gas.

The sintered ingots were extruded into cylindrical rods, some at room temperatures and others at about 150° C.

The resulting sample solders were tested and examined for their microstructural and physical properties including mechanical properties. Some results of these tests are shown in Tables 1 to 3 and FIG. 1, in comparison with results obtained from a pure VIROMET solder sample. The samples I to IV were extruded at room temperature and the samples I′ to IV′ were extruded at about 150° C.

FIG. 1 shows FESEM images of samples II to IV. As can be seen and appreciated by persons skilled in the art from images in the left column, the alumina particulates (shown as small white dots) were relatively uniformly distributed in the composite solder samples. As can be appreciated from images in the right column, near-equiaxed intermetallic phases with good integrity are present in the metallic matrix. Images in both columns show that the metallic matrix in each sample has pores in the submicron and nano range rather than micron size pores. Further, as shown in the Tables, in comparison to Viromet 349, the density (hence the weight) of the composite solder samples were reduced (up to ˜7.5%; for 5 volume % alumina); their CTEs were reduced (up to ˜16%; for 5 volume % alumina), hence enhancing the dimensional stability; their hardness was increased (up to ˜18.5%; for 5 volume % alumina); and their strength was significantly increased. The 0.2% yield strength was increased by up to ˜31% for 5 volume % alumina, and the ultimate tensile strength was increased by up to ˜28% for 3-5 volume % alumina).

Other features, benefits and advantages of the present invention not expressly mentioned above can be understood from this description and the drawings by those skilled in the art.

Of course, the above described embodiments are intended to be illustrative only and in no way limiting. The described embodiments are susceptible to many modifications of form, arrangement of parts, details and order of operation. The invention, rather, is intended to encompass all such modification within its scope, as defined by the claims.

TABLE 1
Structural Properties
	Mixture	Al2O3	Density	Porosity	Pore Size
Sample	Materials	Wt %	(g/cc)	(%)	(μm)
I	Pure	0.0	7.202 ± 0.042	1.14	0.26
	VIROMET
II	VIROMET/	0.53	7.182 ± 0.075	3.3	0.56
	1% Al2O3
III	VIROMET/	1.63	6.948 ± 0.074	2.1	0.33
	3% Al2O3
IV	VIROMET/	2.74	6.665 ± 0.035	3.03	0.85
	5% Al2O3
I′	VIROMET	0.0	6.773 ± 0.369	4.50	0.73
II′	VIROMET/	0.53	6.621 ± 0.193	1.66	0.58
	1% Al2O3
III′	VIROMET/	1.63	6.598 ± 0.159	2.17	0.33
	3% Al2O3
IV′	VIROMET/	2.74	6.329 ± 0.129	6.31	0.52
	5% Al2O3

TABLE 2
Coefficient of Thermal Expansion (CTE),
Hardness and Secondary Phases
			Microhardness
	Sample	CTE	(Hv)	Phases Present
	I	33.261 ± 1.64	15.8 ± 0.526	Sn, Ag3Sn, Ag
	II	30.639 ± 2.79	16.8 ± 1.080	Sn, Ag3Sn, Ag
	III	30.443 ± 0.31	16.9 ± 0.801	Sn, Ag3Sn, Ag
	IV	27.832 ± 2.54	18.7 ± 0.482	Sn, Ag3Sn, Ag
	I′	29.790 ± 1.68	16.6 ± 0.785	Sn, Ag3Sn, Ag
	II′	27.629 ± 3.22	15.2 ± 1.525	Sn, Ag3Sn, Ag
	III′	27.250 ± 0.11	15.4 ± 0.150	Sn, Ag3Sn, Ag
	IV′	26.962 ± 0.42	15.5 ± 0.051	Sn, Ag3Sn, Ag

TABLE 3
Tensile Properties
	Sample
	(%)	0.2% YS (MPa)	UTS (MPa)	Ductility
	I	56 ± 6	60 ± 8	37 ± 7
	II	72 ± 6	75 ± 6	21 ± 3
	III	73 ± 3	77 ± 3	11 ± 3
	IV	74 ± 3	76 ± 2	10 ± 0

Claims

1. A method of forming reinforced solder, comprising:

mixing particles of a solder material capable of forming a metal matrix and a reinforcing particulate to form a particulate mixture;

compressing said mixture at room temperature to form a solid compact; and

sintering said compact to form a particulate composite in which said reinforcing particulate is embedded in a metal matrix formed from said solder material.

2. The method of claim 1, wherein said solder material is lead free solder material.

3. The method of claim 2, wherein said solder material is Viromet.

4. The method of claim 1, wherein said reinforcing particulate comprises a ceramic.

5. The method of claim 4, wherein said ceramic comprises Al2O3.

6. The method of claim 5, wherein said reinforcing particulate has an average diameter less than 100 nm.

7. The method of claim 1, further comprising extruding said compact.

8. The method of claim 1, wherein said mixing comprises blending said mixture.

9. The method of claim 1, wherein said compact has a melting temperature at a first temperature and said sintering comprises heating said compact in an inert gas at second temperature between about 70% to about 90% of said first temperature.

10. The method of claim 9 wherein said second temperature is between about 84° C. and 163° C.

11. The method of claim 9, wherein said second temperature is about 150° C.

12. The method of claim 2, wherein said lead free solder material is a lead-free metal alloy.

13. The method of claim 12, wherein said lead-free metal alloy comprises an combination of tin, copper, silver, and indium.

14. The method of claim 5, wherein the proportion of said reinforcing particulate is between 0 to about 5% by volume of said mixture.

15. The method of claim 1, further comprising coating said compact with colloidal graphite.

16. The method of claim 1, wherein said reinforcing particulate is made of a material having a density lower than the density of said solder material.

17. A reinforced solder formed in accordance with the method of claim 1.

18. A reinforced solder comprising a lead-free metal matrix and a reinforcing particulate embedded in said metal matrix, said metal matrix comprises a combination of tin, copper, silver, and indium having volume ratios of about 91.4:0.5:4.1:4, said reinforcing particulate having an average diameter less than 100 nm, and said reinforced solder having a melting point between 180° C. to 230° C.

19. The reinforced solder of claim 18, wherein said reinforcing particulate is made of alumina and has an average diameter of about 50 nm.

20. The reinforced solder of claim 19, wherein said reinforced solder has a density lower than that of a monolithic solder formed from a material having said combination.

21. The reinforced solder of claim 19, wherein said reinforced solder has porosity up to 6.3%.

22. The reinforced solder of claim 19, wherein said reinforced solder has a coefficient of thermal expansion lower than that of a monolithic solder formed from a material having said combination.

23. The reinforced solder of claim 19, wherein said reinforced solder has hardness higher than that of a monolithic solder formed from a material having said combination.

24. The reinforced solder of claim 19, wherein said reinforced solder has 0.2% yield strength higher than 56 MPa.

25. The reinforced solder of claim 19, wherein said reinforced solder has an ultimate tensile strength higher than 60 MPa.

26. The reinforced solder of claim 19, wherein said reinforced solder has ductility lower than 37%.