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Microstructure Modification for SnBi Low Temperature Solder Alloy
This paper introduces a novel structure for the SnBi-based solder joint at the board level. A layer of SAC alloy is applied over the OSP surface finish.
Technical Paper
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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
Summary
Low-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.
Conclusions
The 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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Comments
I'm not a highly educated SMT expert so please ignore my ignorance if it's blatant...I ask, wouldn't this imply that using CNF in SAC305 strengthen all solder joints, and therefore be a good addition as a matter of course for any PCB that might undergo physical shocks frequently or from time to time?
Brian Martel, MadgeTech
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