Impact of SMT Assembly Process on Tin Whisker Risk Mitigation

Electroplated pure tin or high-tin finishes on Pb-free components remain a reliability concern due to spontaneous tin whisker growth which may cause failures in electronic equipment. To ensure system reliability, component suppliers and system manufactures have placed significant emphasis on component level tin whisker testing using the methods recommended by JEDEC. At the board level, several studies have shown that the SMT reflow process is, in general, beneficial for whisker mitigation as compared to loose components. Nonetheless, the effects of various assembly parameters, such as the reflow temperature or paste volume, are not fully investigated. To better understand the effects of assembly process on whisker growth propensity, JESD-201 tin whisker thermal cycling testing was performed on both SnPb and SnAgCu assemblies manufactured with components from multiple suppliers, various reflow temperatures, and different paste volumes.

The experimental results indicate that the SMT assembly process generally reduces the occurrence of tin whiskers in comparison to the loose components. This reduction in occurrence is most noticeable in SAC assemblies. For SnPb assemblies only, the reflow temperature effect is notable but the paste volume effect is minor. Sporadically, long whiskers are found in both SnPb and SAC assemblies. This observation suggests that the board assembly process is not always effective in mitigating whisker risk, especially in consideration to the maximum tin whisker length. Therefore, it is always critical to select components with acceptable tin whisker test result per JESD-201, coupled with board level testing for the verification purposes.