GB2171720A

GB2171720A - Grain refining a solder alloy

Info

Publication number: GB2171720A
Application number: GB08604331A
Authority: GB
Inventors: W Reif; Magdy Abdel-Reihim; Norbert Hess
Original assignee: London and Scandinavian Metallurgical Co Ltd
Current assignee: London and Scandinavian Metallurgical Co Ltd
Priority date: 1985-02-21
Filing date: 1986-02-21
Publication date: 1986-09-03
Also published as: GB8604331D0

Abstract

A tin-rich solder alloy comprising 35 to 90 mass % tin, optionally 0.5 to 3.3 mass % antimony, balance lead and incidental impurities may be grain refined by incorporating selenium and/or tellurium into the alloy when in a molten condition prior to solidifying it, the selenium and/or tellurium being incorporated in such a way that it is at least partly chemically combined with lead to form nucleant particles. The selenium and/or tellurium may be added by means of a master alloy, e.g. one comprising antimony or tin and selenium or one comprising antimony or tin and tellurium. The invention is of especial use in connection with tin-rich solder alloys used for soldering electronic components.

Description

SPECIFICATION Grain refining a solder alloy This invention relates to grain refining a tinrich solder alloy, i.e. a solder alloy which comprises lead and 35-90 mass % tin. The invention is especially, although not exclusively, concerned with grain refining such a solder alloy which is to be used for soldering electronic components.

In order to improve the mechanical, electrical and metallurgical properties of soldered joints made using tin-rich solder alloys, especially those to be used for soldering electronic components, it would be desirable to find a way of causing the grain structure of the solder used in such joints to be more refined. So far as we are aware, no successful method of grain refining these alloys has ever been found.

According to the present invention, there is provided a method of grain refining a tin-rich solder alloy which comprises lead and 35-90 mass % tin, the method comprising incorporating selenium and/or tellurium into the alloy when in a molten condition prior to solidifying it, the said selenium and/or tellurium being incorporated in such a way that it is at least partly chemically combined with lead to form nucleant particles.

We prefer that the tin-rich solder alloy to be grain refined by the method of the invention should comprise at least 40 mass % tin, or, for oxidation resistance, at least 50 mass % tin, and also that it should comprise up to 70 mass % tin. When soldering electronic components, it is particularly desirable to avoid high soldering temperatures, both to reduce the risk of exposing the components to excessive amounts of heat, and also to reduce the energy requirements of the process. In order to facilitate the use of relatively low soldering temperatures, it is desirable that the tin-rich solder alloy should be a near-eutectic one, and thus the solder alloy preferably comprises about 60 mass % tin.

It is believed that in the method of the invention the nucleant particles take the form of an intermetallic compound(s) of lead and (as the case may be) tellurium and/or selenium.

Of the latter two elements, tellurium is the preferred one. Preferred tin-rich solder alloys to which the method of the invention may be applied contain antimony, preferably in the amount from 0.5 to 3.3 mass % antimony.

The inventors have discovered that without grain refinement, such tin-rich solder alloys comprise a Pb-Sn matrix containing relatively fine polyhedra of an SbSn intermetallic compound, together with much larger, relatively soft, dendrites of Sn-Sb solid solution. After grain refinement by the method of the invention, the solidified tin-rich solder comprises a Pb-Sn matrix again containing the polyhedra of SbSn intermetallic compound, but in this case also containing further SnSb polyhedra which have been nucleated by the above-described nucleant particles, in place of the above-described large, soft dendrites.

A preferred tin-rich solder alloy for grain refinement by the method of the invention has the composition 35 to 90 mass % tin, 0.5 to 3.3 mass % antimony, balance lead and incidental impurities. A particularly preferred such alloy has the composition: about 60 mass % tin, about 3.2 mass % antimony, balance lead and incidental impurities.

We have found the invention to be very effective when the solder alloy is in accordance with the German industrial standard DIN 1707.

The work of the inventors has shown that the usual impurities found in commercial solder alloys do not have any noticeable adverse effect on the grain refinement which can be achieved by the method of the invention.

The selenium and/or tellurium (as the case may be) can be incorporated by inoculating the molten solder alloy with the respective element or a mixture of the elements; the required nucleant particles form without any special steps being taken. Conveniently, the element or elements may be enclosed within a non-deleterious wrapping material, such as lead foil, for example.

However, we prefer to incorporate the selenium and/or tellurium by inoculating the molten solder alloy with a master alloy comprising the respective element or elements. This is a particularly convenient method of adding the element(s), and, once again, the required nucleant particles form without any special steps being taken.

Where the solder alloy is to be inoculated with selenium, the master alloy can be an antimony-selenium master alloy, preferably one comprising from 2 to 50 mass % selenium, balance antimony and incidental impurities: most preferably, the selenium content in this master alloy is about 5 mass %. Alternatively, a tin-selenium master alloy may be used.

Where the solder alloy is to be inoculated with tellurium, the master alloy can be an antimony-tellurium master alloy, preferably one comprising from 40 to 70 mass % tellurium, balance antimony and incidental impurities: most preferably, the tellurium content in this master alloy is about 50 mass %. Alternatively, a tin-tellurium master alloy may be used, preferably of a near-eutectic composition, containing about 80 mass % tellurium.

An alternative way to incorporate the selenium and/or tellurium would be to inoculate the molten solder alloy with a material comprising the respective element or elements at least partly chemically combined with lead to form nucleant particles. For example, the molten solder alloy could be inoculated with a master alloy comprising the said nucleant par tides.

We prefer that the amount of selenium and/or tellurium (as the case may be) incorporated into the solder alloy be such that the total amount of any selenium plus the total amount of any tellurium so incorporated is from 0.005 to 0.5 mass %, most preferably about 0.1 mass %.

The present invention also comprehends a solder alloy, whenever grain refined by the method of the invention.

The invention further comprehends a method of soldering using a tin-rich solder alloy, characterised in that the solder alloy has been grain refined by a method in accordance with the invention. In accordance with the presently preferred application, the soldering method is applied to the soldering of electronic components. Soldering may be carried out by normal soldering techniques, starting with a solder alloy which has already been grain refined by the method of the invention.

Indeed, we have found that repeated melting and solidifying of the solder alloy (up to 10 times, for example) does not have a substantially adverse effect on the degree of grain refinement in the final soldered joint. Similarly, we have found that prolonged holding (e.g.

more than one hour) of the molten solder alloy (e.g. in a soldering bath), for periods up to 20 hours, has made substantially no adverse effect on the grain refining efficiency.

It is possible to make a grain refined soldered joint in accordance with the invention starting with a solder alloy into which no selenium or tellurium has been incorporated, the selenium and/or tellurium content being added while the solder alloy is molten, during the making of the joint, so that grain refinement takes place for the first time when the molten solder alloy solidifies to form the joint.

We have found, surprisingly, that once the selenium and/or tellurium has been incorporated into the solder alloy, the melt has a noticeably improved fluidity. While not wishing to be bound by this theory, we suggest that this could be due to an absence of dendrite formation which otherwise would hinder the flow of the metal, and that this gives rise to an improved filling ability.

We have also found, again surprisingly, that a solder alloy which has been grain refined in accordance with the invention has the following further advantages for soldering purposes, as compared with a solder alloy of the same composition but not grain refined: (a) it is easier to melt; and (b) when molten, it spreads faster on the surfaces of substrates which are to be soldered, and thus has a lower surface tension and a higher wettability.

Easier solder melting means that soldering can be performed with a lower energy requirement and with less risk of damaging the substrates being soldered or components in thermal contact with them.

Better fluidity and lower surface tension substantially improve the soldering process, because they help the solder to enter, by the capillary effect, into the gaps between the substrates being soldered. Improved wettability improves the ability to achieve the required adhesion of the solder to the substrate materials.

Solder alloys which have been grain refined by the method of the invention can have the following additional advantages: 1. Improved mechanical properties, especially shear strength, which is important for sol dered joints.

2. Better resistance to fluctuation in tempera tures; lack of such resistance with known, un-grain refined solder alloys sometimes gives rise to joint cracking, due to micros tresses.

3. Better corrosion resistance.

In order that the invention may be more fully understood, some embodiments in accordance therewith will now be described, by way of example only, in the following Examples, with reference to the accompanying drawings, wherein: Figure 1 is a typical microstructure of a commercially pure Pb, 60 mass % Sn, 3.2 mass % Sb soft soldering alloy, which has not been grain refined, at a magnification of 200:1; Figure 2 is a typical microstructure of the same soldering alloy as in Fig. 1, but grain refined in accordance with the invention by a 0.2 mass % addition of an SnTe 80 mass % master alloy also at a magnification of 200:1.

Figure 3a is a photograph, at a magnification of 1.5:1, of two brass plates which have been heated under identical conditions at a relatively low temperature, the plates having on their surfaces cubic samples of: (a) on the left-hand side, the un-grain re fined alloy which is the subject of Fig. 1; (b) on the right-hand side, the grain refined alloy which is the subject of Fig. 2.

Figure 3b is a photograph, also at a magnification of 1.5:1, of two brass plates which have been heated exactly as in Fig. 3a, except at a higher temperature.

Example 1 About 50g of a commercially pure PbSn60Sb3.2 soldering alloy in conformity with German Industrial Standard DIN 1707-LSn 60 was melted in a steel crucible and superheated to 400 degrees C.

0.2 mass % of an SnTe 80 mass % master alloy, which had been produced in the laboratory, was added, and the melt stirred by means of a ceramic rod. After a holding time of 20 minutes, the melt was poured into an unheafted steel mould having an internal diameter of 20 mm and a height of 30 mm.

Fig. 1 shows the microstructure of the alloy without addition, and Fig. 2 shows the effect of adding 0.2 mass % of the SnTe 80 master alloy to the tin-rich alloy in accordance with this Example. As can be seen, substantial refinement of the grain structure resulted.

The structure of LSn60, as shown in Fig. 1, comprises primary polyhedra and primary dendrites in a eutectic matrix. Studies made by the inventors have indicated that the eutectic matrix comprises Sn and Pb; the dendrites are Sn-Sb solid solution; and the polyhedra are the intermetallic compound SbSn.

The structure of the grain refined alloy, as shown in Fig. 2, comprises only polyhedra in the eutectic matrix. Further studies by the inventors have indicated that the grain refined alloy comprises the Sn-Pb matrix; and polyhedra of the intermetallic compound SbSn. Thus, it appears that the nucleant particles have suppressed the formation of the relatively large and soft Sn-Sb solid solution dendrites, nucleating instead relatively fine, relatively hard polyhedra of the intermetallic SbSn compound.

This can account for the fact that the grain refined alloy has been found to be 10% harder than the un-grain refined alloy. It can also account for the better melting, wetting and flow properties of the grain refined alloy, as described in the next Example.

When the grain refined solder alloy was used for soldering electronic components, in a conventional manner, the grain structure of the solder in the resulting joint was similar to that shown in Fig. 2.

It was found that repeating this Example, but replacing the tellurium addition by a corresponding addition of selenium, resulted in a grain refined microstructure similar to that shown in Fig. 2.

Scanning electron microscope studies have shown that, in the selenium- and telluriumgrain refined alloys described in this Example, the nucleant comprises, respectively, lead and selenium, and lead and tellurium. At present we believe that the respective nucleants are Pb-Se and Pb-Te intermetallic compounds. A sample of the nucleant in the test described at the beginning of this Exaple had a Te content of about 38 mass %: the intermetallic compound PbTe can contain from 20-45 mass % Te.

As indicated above, the experiments described in the above Example can be repeated using other master alloys to make the Se and Te additions, for example, Sb, Se 5 mass % or Sb, Te 50 mass %.

Example 2 Two brass plates (a) and (b) were coated with Tinol 4200 soldering oil and electrically heated, while supporting a cube-shaped sample of soft solder alloy, under identical conditions, except that the samples differed, as follows: (a) the un-grain refined alloy used as a start ing material in Example 1; (b) the grain refined alloy of Example 1 treated with 0.2 mass % SnTe 80.

In a first experiment, heating was performed at a relatively low temperature; the results are shown in Fig. 3a. The experiment was then repeatd, at a somewhat higher temperature: the results are shown in Fig. 3b. In both of these Figs., the un-grain refined sample can be seen on the left-hand side, with the grain refined sample on the right-hand side.

The following observations were made: (i) The refined alloy melted much faster than the unrefined one. This is thought to be because of the better distribution of the eutectic phase by grain refinement (the eutectic phase has a lower melting po int).

(ii) The refined alloy spread faster on the sur face of the copper plate, which indicates a lower surface tension and a higher wet tability.

Claims

1. A method of grain refining a tin-rich solder alloy which comprises lead and 35-90 mass % tin, the method comprising incorporating selenium and/or tellurium into the alloy when in a molten condition prior to solidifying it, the said selenium and/or tellurium being incorporated in such a way that it is at least partly chemically combined with lead to form nucleant particles.

2. A method according to claim 1, wherein the solder alloy comprises at least 40 mass % tin.

3. A method according to claim 1 or claim 2, wherein the solder alloy comprises up to 70 mass % tin.

4. A method according to any one of claims 1 to 3, wherein the solder alloy comprises about 60 mass % tin.

5. A method according to any one of claims 1 to 4, wherein the solder alloy comprises from 0.5 to 3.3 mass % antimony.

6. A method according to claim 5, wherein the composition of the solder alloy is 35 to 90 mass % tin, 0.5 to 3.3 mass % antimony, balance lead and incidental impurities.

7. A method according to claim 6, wherein the composition of the solder alloy is about 60 mass % tin, about 3.2 mass % antimony, balance lead and incidental impurities.

8. A method according to any one of claims 1 to 7, wherein the solder alloy is in accordance with the German Industrial Standard DIN 1707.

9. A method according to any one of claims 1 to 8, wherein the selenium and/or tellurium is incorporated by inoculating the molten solder alloy with the respective element or a mixture of the elements.

10. A method according to claim 9, wherein the said element or elements is or are enclosed within a non-deleterious wrapping material.

11. A method according to claim 10, wherein the wrapping material is lead foil.

12. A method according to any one of claims 1 to 8, wherein the selenium and/or tellurium is incorporated by inoculating the molten solder alloy with a master alloy comprising the respective element or elements.

13. A method according to claim 12, wherein the master alloy comprises antimony and tellurium.

14. A method according to claim 12, wherein the master alloy comprises antimony and selenium.

15. A method according to claim 12, wherein the master alloy comprises tin and tellurium.

16. A method according to claim 12, wherein the master alloy comprises tin and selenium.

17. A method according to any one of claims 1 to 8, wherein the selenium and/or tellurium is incorporated by inoculating the molten solder alloy with a material comprising the respective element or elements at least partly chemically combined with lead to form nucleant particles.

18. A method according to claim 17, wherein the molten solder alloy is inoculated with a master alloy comprising the said nucleant particles.

19. A method according to any one of claims 1 to 18, wherein the total amount of any selenium incorporated into the solder alloy plus the total amount of any tellurium incorporated into the solder alloy is from 0.005 to 0.5 mass %.

20. A method according to claim 19, wherein the said total amount is about 0.1 mass %.

21. A method of grain refining a solder alloy, substantially as hereinbefore described in the foregoing Example 1.

22. A tin-rich solder alloy, whenever grain refined by a method in accordance with any one of claims 1 to 21.

23. A method of soldering using a tin-rich solder alloy, characterised in that the solder alloy has been grain refined by a method in accordance with any one of claims 1 to 21.

24. A method according to claim 23, whenever used for soldering an electronic component.

25. A method according to claim 23 or claim 24, wherein soldering is performed after the solder alloy has been soldified and then remelted after the incorporation into it of the selenium and/or tellurium.

26. A method according to any one of claims 23 to 25, wherein soldering is performed after the solder alloy has at least 10 times been solidified and then remelted, after the incorporation into it of the selenium and/or tellurium.

27. A method according to any one of claims 23 to 26, wherein soldering is performed after at least one hour and up to 20 hours after the incorporation into the solder alloy of the selenium and/or tellurium.