Alpha Papers Investigate High-Reliability Solder Alloys

Estimated reading time: 6 minutes

Andy Shaughnessy: So, Morgana, tell us about the second paper that you presented.

Morgana Ribas: Its title is, "Effect of Thermo-mechanical Stresses on the Reliability of Lead-Free Low-Silver Alloys." In this paper, we concentrated a little bit more on the fact that SAC 305 has better thermal cycling reliability than the Sn-37Pb, but it does not match the Sn-Pb mechanical reliability, especially under shock and vibration conditions.

Low-silver alloys have been studied since the introduction of Lead-free solders, when the industry started thinking that maybe SAC 305 was a too expensive option for electronics assembly. Actually, Alpha started working on a low-silver Sn-Ag-Cu solder since the late 1980s. We were one of the first companies to actually work on low-silver SAC alloys. ALPHA SACX family of solders particularly targeted on improving the mechanical reliability shortcomings of SAC305. Later we improved these solders further by adding performance additives, which extended the low-silver family of alloys with the ALPHA SACX Plus solders. In this paper, we showed a comparison between ALPHA SACX Plus 0307 SMT and SAC 305, sometimes with Sn-37Pb, and check how this low silver alloy compares with these other two solders.

There are economical reasons for using low-silver solder alloys, which include their reduced cost and higher price stability that comes from reducing the silver content. But are there any other advantages? Are these low-silver solders drop-in replacements for the SAC 305? These are the type of questions we would like to answer with this paper. We started our analysis by comparing the bulk alloy properties of these three alloys. We also performed thermal cycling test, drop shock test, and solder joint evaluation.

The bulk alloy properties are comparable and both SAC alloys have higher thermal conductivity, ultimate tensile strength, and yield strength than the tin-lead. From the bulk alloy properties perspective, the low silver alloy SACX Plus 0307 SMT is very good, including its wetting properties, which are very important for forming a good solder joint. We performed wetting balance test to compare the SACX Plus 0307 SMT with the SAC 305, and we observed that both alloys have zero wetting time below one second, passing the IPC standard requirement.

We used copper OSP on the test vehicles and saw that, in the thermal cycling test, the tin-lead was the first one to fail at 665 cycles, followed by the SACX Plus 0307 SMT at 1866 cycles, and SAC 305 at 1928 cycles. It was very interesting to observe that SACX Plus 0307 SMT first failure was so close to the SAC 305. At the end of the 6,000 cycles, all components assembled using the tin-lead and the low-silver had failed, but several of the SAC 305 components survived, which does highlight its superior thermal cycling performance compared to these two other alloys. Weibull plots of the censored data showed that SACX Plus 0307 SMT characteristic life is 44% higher than the tin-lead and 58% lower than the SAC 305, which also highlights that SACX Plus 0307 SMT performance is right in the middle between SAC305 and Sn-37Pb.

Shaughnessy: I always thought that tin-lead was the most robust.

Ribas: Of course, this type of data is always subjected to the type of test vehicle and components used. However, similar trend was verified in other independent studies.

Moving from thermal cycling to drop shock performance evaluation, we verified that the tin-lead had the best drop shock performance, as expected. The SACX Plus 0307 SMT characteristic life is 59% higher than SAC 405, which we used as a lower bound, but only 14% lower than the tin lead.

We also compared copper dissolution of the low-silver and high-silver solders and observed that SACX Plus 0307 SMT has much lower copper dissolution than SAC 305. We also collected SACX Plus 0307 SMT intermetallic behavior when the testing boards were processed with various TAL (time above liquidus) and peak temperatures. The intermetallic thickness was found to be below three microns, which is an acceptable level of intermetallics. Finally, we performed very comprehensive analysis of the soldering on leaded components, and observed that all solder joints were acceptable under the IPC-A-610 standard.

Shaughnessy: Well, that’s good.

Ribas: The final conclusion here is that we demonstrated that the SACX Plus 0307 SMT is a very versatile alloy, which can be used as a drop-in replacement of SAC 305 solder alloy, depending on specific thermal cycling reliability requirements.

Shaughnessy: Very good. Thanks for sharing this with us.

Morgana: Thank you.

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