Solders

Abstract

A solder comprising: between 87.2% and 89.5% tin; between 4.0% and 4.8% bismuth; between 3.5% and 4.5% indium; and between 3.0% and 3.5% silver.

Description

[0001] THIS INVENTION relates to solders, and in particular to solders which are substantially lead-free.

[0002] Many conventional solders contain lead as a major constituent thereof. Such solders often have desirable physical properties, and the use of lead-containing solders is widespread among several industries, including those concerned with the production of printed circuit boards.

[0003] However, there are increasing demands, due, for example, to environmental considerations, for solders to be lead-free, and it seems likely that, within the next few years, it will be a legal requirement in several countries for solders used in the manufacture of many items to contain little or no lead.

[0004] Previous attempts to formulate lead-free solders have met with limited success. Conventional lead-free solders generally have undesirable physical properties, including poor wetting properties, low fluidity, poor compatibility with existing component coatings and excessive drossing.

[0005] As a result, some manufacturers are finding that existing soldering processes that have functioned effectively for many years must be adapted to accommodate the use of lead-free solders. In addition, existing materials that are employed in the production of printed circuit boards may have to be replaced to be compatible with the use of lead-free solders. This adaptation of processes and materials is widely regarded as a poor use of resources, particularly as the standard of articles manufactured using conventional lead-free solders is, as described above, often below that achievable using leaded solders.

[0006] It is an object of the present invention to seek to provide a lead-free solder that alleviates some or all of the above drawbacks of lead-free solders.

[0007] Accordingly, one aspect of the present invention provides a solder comprising: between 87.2% and 89.5% tin; between 4.0% and 4.8% bismuth; between 3.5% and 4.5% indium; and between 3.0% and 3.5% silver.

[0008] Advantageously, the solder is a substantially lead-free solder.

[0009] Preferably, the solder is a lead-free solder.

[0010] Conveniently, the solder comprises 88.3% tin, 4.5% bismuth, 4.0% indium and 3.2% silver.

[0011] Another aspect of the present invention provides a method of preparing a solder, comprising the step of mixing tin, bismuth, indium and silver such that: the proportion of tin in the solder is between 87.2% and 89.5%; the proportion of bismuth in the solder is between 4.0% and 4.8%; the proportion of indium in the solder is between 3.5% and 4.5%; and the proportion of silver in the solder is between 3.0% and 3.5%.

[0012] Advantageously, the method comprises the addition of substantially no lead to the solder.

[0013] Preferably, the method comprises the addition of no lead to the solder.

[0014] Conveniently, the method comprises the step of mixing tin, bismuth, indium and silver such that: the proportion of tin in the solder is 88.3%; the proportion of bismuth in the solder is 4.5%; the proportion of indium in the solder is 4.0%; and the proportion of silver in the solder is 3.2%.

[0015] A further aspect of the present invention provides a method of soldering, comprising the step of using a solder comprising: between 87.2% and 89.5% tin; between 4.0% and 4.8% bismuth; between 3.5% and 4.5% indium; and between 3.0% and 3.5% silver.

[0016] Advantageously, the method comprises the step of using a solder comprising: 88.3% tin; 4.5% bismuth; 4.0% indium; and 3.2% silver

[0017] Preferably, the method comprises the step of wave-soldering.

[0018] In order that the present invention may be more readily understood, examples thereof will now be described, by way of example, with reference to the accompanying drawings, in which:

[0019] FIG. 1 shows a table of wetting times, in seconds, for a plurality of different solders, at a variety of temperatures;

[0020] FIG. 2 shows a graph representing the data expressed in the table of FIG. 1;

[0021] FIG. 3 shows a table of maximum wetting force, for a plurality of different solders, at a variety of temperatures;

[0022] FIG. 4 shows a graph representing the data expressed in the table of FIG. 3;

[0023] FIG. 5 shows a table of wetting times, for a solder embodying the present invention and a prior art solder, when the solders are applied to various coatings;

[0024] FIG. 6 presents the experimental conditions of an experiment involving use of a solder embodying the present invention in a wave-soldering machine; and

[0025] FIGS. 7 and 8 show the results of the experiment of FIG. 6.

[0026] As described above, conventional lead-free solders suffer from several drawbacks, including poor wetting properties, low fluidity, poor compatibility with existing component coatings and excessive drossing when compared with commonly-used leaded solders.

[0027] However, it has been found that a solder composed of a lead-free alloy comprising between 87.2% and 89.5% tin, between 4.0 and 4.8% bismuth, between 3.5% and 4.5% indium and between 3.0% and 3.5% silver possesses significantly improved properties when compared with conventional lead-free solders. Such a solder embodies the present invention. Indeed, the properties of solders embodying the present invention are comparable to conventional leaded solders with regard to wettability, fluidity, compatibility with existing component coatings and drossing.

[0028] In order to demonstrate the advantageous physical properties of solders embodying the present invention, a number of tests were carried out, as will be described below.

[0029] The first test concerned the wettability of a solder embodying the present invention, as compared to a number of existing lead-free solders and a conventional leaded solder (comprising 63% tin and 37% lead). The solder of the present invention employed in the first test comprised 88.3% tin, 4.5% bismuth, 4.0% indium and 3.2% silver.

[0030] A first aspect of the first test comprised the measurement of the wetting time, based on the ANSI/J Std-003 standard, for the solders under consideration at a variety of temperatures ranging from 235° C. to 265° C. In this test, a specimen of copper was immersed in a quantity of each molten solder. A sensitive force measuring device was connected to the copper specimen, and arranged so that vertical forces on the specimen could be measured and recorded.

[0031] The variation in the vertical force on the specimen during immersion thereof in the molten solders was due to two main factors. The first of these, the buoyancy force, arose from the upward force exerted on the specimen due to the displacement of solder, which was equal to the weight of solder displaced by the specimen. Since the volume of the part of the specimen that was immersed in the solder, and the density of the solder, are known, this upward force can be calculated and accounted for.

[0032] The second factor is a force acting on the specimen due to the change in contact angle between the surface of the solder and the surface of the specimen. The wetting time in each particular case was defined as the time taken for the wetting force acting on the specimen to be equal to zero.

[0033] The results of the first aspect of the first test are shown in FIG. 1. In summary, the solder embodying the present invention exhibited a wetting time, at each of the temperatures, that was comparable to that displayed by the conventional leaded solder. In addition, the solder embodying the present invention exhibited a wetting time, at all but one of the temperatures considered, that was lower than that displayed by the any of the existing lead-free solders. The wetting time is a measure of the rapidity with which a solder adheres to a substrate, and clearly a low wetting time is a desirable property for a solder. Hence, it can be seen that the solder embodying the present invention performed better overall in the first aspect of the first test than any of the existing lead-free solders.

[0034] The results of the first aspect of the first test are displayed in graph form in FIG. 2. It will be seen from this graph that the results representing the performances of the conventional leaded solder and the solder embodying the present invention follow each other closely when compared to those representing the performances of the existing lead-free solders.

[0035] A second aspect of the first test comprised the measurement of the maximum wetting force at 2.0 seconds after immersion of the specimen in the respective solders. The wetting force is, as described above, the adhesive force between the solder and the specimen. Clearly, the wetting force provides a useful indication of the strength with which a solder binds to a substrate, and a high wetting force is a desirable property for a solder.

[0036] The results of the second aspect of the first test are shown in FIG. 3. To summarise these results, the solder embodying the present invention exhibited a maximum wetting force 2.0 seconds after immersion of the specimen therein, at each of the considered temperatures, that was comparable to that displayed by the conventional leaded solder. While some of the existing lead-free solders displayed a wetting force that was close to that of the conventional leaded solder at some of the temperatures, the solder of the present invention displayed a wetting force that was close to that of the conventional leaded solder at all of the considered temperatures. It will be appreciated that such properties allow the solder of the present invention to display similar behaviour to conventional leaded solders under a variety of temperature conditions, or where soldering takes place under varying temperature conditions.

[0037] The results of the second aspect of the second test are displayed in graph form in FIG. 4, and it will be clear from this graph that the results representing solder embodying the present invention follow those representing the conventional leaded solder far more closely than those representing any of the existing lead-free solders.

[0038] From the results of the first test, it can be seen that the solder embodying the present invention exhibits very similar properties, with regard to wettability, than the existing lead-free solders studied. Clearly, this similarity in physical properties renders the solder embodying the present invention far more suitable for use as a replacement for the conventional leaded solder than any of the existing lead-free solders.

[0039] A second test concerned the compatibility of the solder of the present invention with existing component coatings. Components on, for example, printed circuit boards, may have coatings of various materials, and it is important that a solder to be used in conjunction with such components adheres readily to the coatings thereof.

[0040] In the second test, six common types of components were plated with metallic coatings of tin, gold, silver, a tin/lead alloy and a copper/phosphorous alloy. A comparative test was then conducted of the wetting time displayed by a solder embodying the present invention having a composition of 88.3% tin, 4.5% bismuth, 4.0% indium and 3.2% silver, and a conventional leaded solder comprising 60% tin and 40% lead, when applied to each of the coated components at a variety of temperatures, using the globule test method, which is rather similar to that described above in relation to the measurement of wetting force, but conducted on a smaller scale. The flux employed in the second test was a non-clean 4% solid content flux.

[0041] The results of the second test are presented in FIG. 5. It will be seen from these results that the solder embodying the present invention displays a similar wetting time to that exhibited by the conventional leaded solder for most of the coatings, at most of the temperatures considered (which ranged from 235° C. to 265° C.). In the great majority of cases, the solder embodying the present invention and the conventional leaded solder displayed wetting times less than half a second different from one another.

[0042] It will be clear, from the results of the second test, that the solder embodying the present invention is suitable for use as a direct replacement for conventional leaded solders, with regard to compatibility with existing component coatings.

[0043] A third test was concerned with the suitability for use of the solder of the present invention in a wave-soldering machine. In an example of wave-soldering, a circuit board is held just above the surface of a quantity of molten solder. A wave is then caused to propagate across the surface of the molten solder, of sufficient amplitude that the crest of the wave comes into contact with the surface of the circuit board. The wave is as wide as the circuit board (or the portions thereof that require soldering), and as the wave propagates across the surface of the molten solder all parts of the downward-facing surface of the circuit board are contacted with molten solder.

[0044] This method of application of solder to a circuit board entails a greatly reduced risk of solder coming into contact with the upward-facing surface of the circuit board, compared to a method where the circuit board is dipped directly into molten solder.

[0045] Existing lead-free solders, when used in a pot of molten solder in a wave-soldering machine, have been known to lead to high levels of contamination in the solder pot after a number of uses of the wave-soldering machine. In a wave-soldering machine, it is normal for two pot areas to be provided. When circuit boards are soldered using such a wave-soldering machine, they are initially manoeuvred over a first pot area, in which a preliminary “chip” wave is passed over the downward-facing surface of the circuit boards for the purposes of cleaning this surface. Subsequently, the circuit boards proceed to a position over a second pot area, where a further “laminar” wave is passed over their downward-facing surfaces to perform the required soldering. It will be clear that, since the chip wave is concerned with cleaning the surfaces of the circuit boards prior to soldering, there is a risk of a build-up of undesirable contaminants in the first pot area, and this problem has been found to be exacerbated by the use of existing lead-free solders.

[0046] In addition, the levels of dross present in the pot after several uses have been found to be unacceptably high when existing lead-free solders are employed.

[0047] In the third test, a solder embodying the present invention having a composition of 88.3% tin, 4.5% bismuth, 4.0% indium and 3.2% silver was put to use in a conventional wave-soldering machine. No alteration of the machine was made to accommodate the use of the solder. The wave-soldering machine was then used to solder circuit boards, in the same way as for a conventional solder comprising a tin/lead alloy.

[0048] The wave-soldering machine was used at four different pot temperatures, namely 235° C., 245° C., 255° C. and 265° C. These temperatures are all within the range of temperatures that would normally be used when using the wave-soldering machine with a conventional leaded solder. In addition, the conveyor speed (i.e. the speed at which the circuit boards were moved over the surface of the pot) was varied such that three different conveyor speeds were used—1 meter/minute, 1.4 meters/minute and 1.8 meters/minute. Each of these conveyor speeds was within the normal range of conveyor speeds that would normally be used when using the wave-soldering machine with a conventional leaded solder. The third test was carried out in a normal air environment.

[0049] To employ a totally lead-free process, a lead-free flux was developed for use in the process, the lead-free flux being a RMA 13% solid content flux. At the end of each day of testing, the level of contamination in the pot and the alloy composition of the pot were measured and recorded. In addition, the dross in the pot was removed and weighed to determine the amount of dross produced by the wave-soldering process.

[0050] The experimental conditions of the of the third test are presented in FIG. 6. The amounts of dross left in the pot at the end of each day are presented in FIG. 7. The levels of contamination of the solder in the pot (for both chip waves and laminar waves, where appropriate) are presented in FIG. 8.

[0051] A person of ordinary skill in the art, from consideration of these results, will appreciated that the levels of contamination in the pot, as well as the amount of dross produced, are significantly lower than those that would be encountered when using an existing lead-free solder in an unmodified conventional wave-soldering machine.

[0052] It will be clear that the present invention provides a lead-free solder that is more suitable for use as a direct replacement for conventional leaded solders than previously-proposed lead-free solders, due to the comparable characteristics of wettability, fluidity, compatibility with existing component coatings and drossing exhibited by the solder of the present invention.

[0053] An advantage of this suitability is that the need for manufacturers to replace existing machinery, processes or component coatings to accommodate use of a lead-free solder can be lessened or eliminated by employing the solder of the present invention. It will be clear that, as a result, the process of converting to use of a lead-free solder may be rendered far simpler and more economically viable for many manufacturers.

[0054] In the present specification “comprises” means “includes or consists of” and “comprising” means “including or consisting of”.

[0055] The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

1. A solder comprising:

between 87.2% and 89.5% tin;

between 4.0% and 4.8% bismuth;

between 3.5% and 4.5% indium; and

between 3.0% and 3.5% silver.

2. A solder according to claim 1, wherein the solder is a substantially lead-free solder.

3. A solder according to claim 2, wherein the solder is a lead-free solder.

4. A solder according to claim 3, wherein the solder comprises 88.3% tin, 4.5% bismuth, 4.0% indium and 3.2% silver.

5. A method of preparing a solder, comprising the step of mixing tin, bismuth, indium and silver such that:

the proportion of tin in the solder is between 87.2% and 89.5%;

the proportion of bismuth in the solder is between 4.0% and 4.8%;

the proportion of indium in the solder is between 3.5% and 4.5%; and

the proportion of silver in the solder is between 3.0% and 3.5%.

6. A method according to claim 5, wherein the method comprises the addition of substantially no lead to the solder.

7. A method according to claim 6, wherein the method comprises the addition of no lead to the solder.

8. A method according to claim 7, wherein the method comprises the step of mixing tin, bismuth, indium and silver such that:

the proportion of tin in the solder is 88.3%;

the proportion of bismuth in the solder is 4.5%;

the proportion of indium in the solder is 4.0%; and

the proportion of silver in the solder is 3.2%.

9. A method of soldering, comprising the step of using a solder comprising:

between 87.2% and 89.5% tin;

between 4.0% and 4.8% bismuth;

between 3.5% and 4.5% indium; and

between 3.0% and 3.5% silver.

10. A method according to claim 9, comprising the step of using a solder comprising: 88.3% tin; 4.5% bismuth; 4.0% indium; and 3.2% silver

11. A method according to claim 9, wherein the method comprises the step of wave-soldering.