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Microstructure Modification for SnBi Low Temperature Solder Alloy
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Authored By:Albert T. Wu National Central University Taoyuan City, Taiwan Chao-Chin Chang, Tsao-Cheng Huang, Yu-Cheng Chen Formosa Plastics Corporation Kaoshiung City, Taiwan Chang-Meng Wang Shenmao Technology Inc. Taoyuan City, Taiwan SummaryLow-temperature solder alloys lower the processing temperature required for electronic packaging. SnBi alloy system stands out due to its eutectic temperature, which can reach 139°C. However, Bi segregation during aging can compromise the joint's strength. This paper introduces a novel structure for the SnBi-based solder joint at the board level. A layer of SAC alloy or hot air solder leveling (HASL) is applied over the OSP surface finish. An alternative strategy involves the addition of carbon nanofiber (CNF) to adjust the SAC alloy microstructure. The incorporated CNF acts as nucleation sites, transforming the microstructure. Both microstructural modification techniques bolster mechanical strength and shift the fracture mode from brittle to ductile. ConclusionsThe research presents a distinctive two-layer configuration, with SnBi positioned over either SAC or HASL Sn. This design aids in achieving low-temperature assembly, especially beneficial for electronic devices. By integrating CNF into the SAC solder, the microstructure undergoes significant refinement. Detailed morphological evaluations, coupled with mechanical strength assessments, validate a reduction in Bi segregation within these joints. Such a configuration consequently paves the way for an enhancement in the long-term reliability and durability of the soldered joints. Initially Published in the SMTA Proceedings |
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