Alpha Papers Investigate High-Reliability Solder Alloys

Estimated reading time: 8 minutes

During SMTA International, Morgana Ribas, manager of Metals Technology Group at Alpha Assembly Solutions, presented a pair of papers that focus on the company’s reliability testing of solder alloys. I-Connect007 Managing Editor Andy Shaughnessy sat down with Morgana to discuss these papers, and some of the surprising results that the testing yielded.

Andy Shaughnessy: I understand you presented two papers at SMTAI. Why don't you tell us about those papers?

Morgana Ribas: Yes. The common topic discussed in these two papers is "reliability of solder alloys," which is something increasingly interesting for the electronics industry. Alpha has a wide range portfolio of alloys and we feel that there is a solder that is just right for each application requirement. We would like to provide the best cost benefit for our customers, preventing them from using over-designed alloys and spending more money than is necessary. We feel it's important that our customers have the right dataset to make informed decisions on which alloy fits best their application.

These two papers deal with this fact, but in different ways. The first one, "High Thermo-mechanical Fatigue and Drop Shock Resistant Alloys for Ball-grid Array Package Applications," is a collaboration between Alpha Assembly Solutions and Alpha Advanced Materials.

Shaughnessy: OK. Tell us about the first paper.

Ribas: In this paper, we are trying to answer a question that is quite common when selecting an alloy for soldering: Considering high-silver, low-silver, and high-silver plus additives, which alloy is going to be a better fit for certain application? We tried to make this a very comprehensive study by comparing SAC 105, SAC 125N, and SAC 305 with three of the ALPHA® alloys: Low-silver SACX® Plus 0307, and high-reliability MAXREL™ (also known as Innolot) and MAXREL™ Plus alloys. MAXREL Plus is our latest high-reliability alloy, a high-silver alloy with additives for enhanced performance.

We assembled CTBGA packages using these alloys, and studied their properties using SAC 305 solder paste. The combination of MAXREL Plus as spheres and solder paste was also investigated. We tested these assemblies for single ball shear, drop shock and thermal cycling performance.

Shaughnessy: Okay. So, did you all find any surprising results?

Ribas: Yes, some results are surprising, starting from the single ball shear test results. We observed that MAXREL and the MAXREL Plus alloys have a much higher shear strength than the low-silver and the SAC 305 alloys. The average single ball shear strength is about 72% higher than the SAC 305 and about 100% higher than the low-silver alloys. The statistical plots also show that there is a significant difference in the single ball shear strength of these alloys. Similar to SAC305, both MAXREL and MAXREL Plus have high silver content, so this increased shear strength is related to the additives present in these two alloys. Both alloys have bismuth in their composition, which contributes to the solid solution strengthening. In addition to that, MAXREL has antimony addition, which also contributes to its higher strength. Despite that, MAXREL Plus, which is an antimony-free alloy, has the same single ball shear strength. Thus, it is very interesting to confirm that minor alloying additions can significantly affect the shear strength of a solder alloy.

However, depending on the application, single ball shear results alone may not be enough for deciding which alloy to use. Thus, we also performed a drop shock test (also known as mechanical shock test), which is a very important test for evaluating the mechanical reliability of a solder alloy, especially in portable electronics applications. From these results, MAXREL Plus showed the best drop shock performance among the alloys that were tested. We also saw that despite having excellent thermal cycling performance, MAXREL alloy has lower drop shock characteristics. Both SAC125N and MAXREL alloys showed lower drop shock, whereas MAXREL Plus had the highest drop shock performance, as high as the low-silver alloy, SACX Plus 0307, which is known for its higher mechanical reliability. In general one may think that lower silver is going to result in higher drop shock performance, and that lower silver will have poor drop shock performance. However, at the end of our testing, we found that low silver SACX Plus 0307, and high-silver with additives MAXREL Plus have very similar drop shock performance. Once more, this result shows the importance of using the right alloying additions when developing high reliability solders.

Thermal cycling performance is also key for most applications when deciding on a solder. For this study we performed a thermal cycling test from minus 40°C to 125°C for a total of 4,000 cycles. It is interesting to note that due to the higher-reliability nature of these alloys, the test duration was a little longer than what is usually necessary. Whereas SAC105 started failing around 1,000 cycles and had 100% failures by 3,000 cycles, SACX Plus 0307, despite its lower silver content, showed better thermal cycling performance. This can be attributed to the minor alloy additions present in this alloy. Next in performance was SAC125, followed by SAC305. Thus, SAC 0305 alloy is right in the middle between this group of low silver alloys and the higher reliability alloys. MAXREL Plus, with MAXREL solder paste and SAC305 solder paste, showed the best performance, between 9 and 20% failures by 4,000 cycles. In our actual paper we show very interesting additional cross-sections analyses that support this data.

Just to conclude, in order to obtain the maximum performance and cost benefit, it is important to pair the best solder alloy with each application. Therefore, our recommendation at the end of this study is to use SACX Plus 0307 as low-silver alloy for applications that require excellent mechanical shock and reasonable thermal cycling performance. For applications that need very good thermal cycling, but for which mechanical shock performance has lower priority, then SAC 305 is still the best recommendation. For applications that require thermal cycling performance better than SAC305, and for which drop shock is not a concern, then MAXREL is the best candidate. For applications that need both, very high thermal cycling performance and very high drop shock performance, then MAXREL Plus is the best recommendation.

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