Reliability tests of alternative lead-free pastes
This article reviews the thermal reliability of six alternatives to SAC305, and the tests were conducted by none other than Flex itself.
Sn3.0Ag0.5Cu (SAC305) is currently the most popular almost eutectic lead-free alloy used in production processes. However, the price of silver has increased dramatically in the last few years, generating demand for alternative alloys with a lower silver alloy content.
As a result, there has been a significant increase in the number of alternative lead-free solders with low or even no silver content. Previous research has shown that many low silver solder paste alternatives had good printing performance and good wetting properties compared to SAC305. However, there was a lack of information on the reliability of soldering joints for alternative low-silver alloys - a team of researchers from Flextronics International conducted and presented in an extensive paper a joint reliability study. Six different lead-free pastes were tested with the thermal cycle test (3,000 cycles, 0 to 100 ° C), and of course, SAC305 was used as the basis for the comparison. The set of tested alloys includes low-silver solder pastes and SnBiAg low-temperature solder pastes - details on specific alloys are provided in the box below.
The entire test was described in detail by Flex scientists in the article available here.
First, the researchers found that there was no significant difference in the intermetallic layer thickness of the joints formed with SAC305 and other lead-free solder paste alternatives. Overall, the intermetallic layer thickness of these materials increased slightly after the thermal cycle tests were performed, but the change was insignificant. The intermetallic layer thickness of the joints formed with SnBiAg after reflow soldering was generally thinner than that of high-temperature lead-free alloys. The intermetallic layer of SnBiAg solder joints grew during the thermal cycle tests to a similar thickness as in the case of other lead-free alloys. However, both the thickness and the composition of the intermetallic layer were not determined by the test development team to affect joint reliability during thermal cycle tests.
On the other hand, the reliability of alternative solder pastes as tested by the thermal test varied depending on the type of housing and the size of the component. In this study, it was this variable that influenced the thermal reliability of the solder joint more than the alloy composition itself. The 2512 resistors failed first compared to other components tested. After 3,000 thermal cycles (from 0 ° C to 100 ° C), most of the 2,512 resistors were found to be completely inoperable and numerous solder cracks were found. However, in the case of small chip-type components - such as components 0603, 0402, 0201 - no failures were observed after testing.
Serious cracks and failures were also observed for the BGA196, BGA228, BGA97, and QFN88 components after the thermal cycle tests. Slight cracks were observed in the BGA1156, BGA64, QFN32, and QFP208 and QFP100 components but did not cause a failure.
In conclusion, the authors state that solder joints made with SAC305 performed better than solder pastes with low Ag content. Surprisingly, the SnBiAg low temperature soldering joint performed very well in the thermal cycle test, but only if only this alloy formed the joint. [i.e. was present on the balls and on the pad]. By combining the SAC305 and SnBiAg in one connector, the Flex team observed more defects and failures. As a summary of their work, the team concludes that further reliability studies are needed for alternative lead-free pastes.
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