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Crack Propagation Mechanism Study on Bismuth Contained Base Lead Free Solder Under Thermo-Mechanical Stress
Crack Propagation Mechanism Study on Bismuth Contained Base Lead Free Solder Under Thermo-Mechanical Stress
The thermal cycling reliability characteristics of solder containing a small amount of Bismuth which is below 4wt% in accordance with the new requirements were investigated.
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Authored By:
Imbok Lee and Young-Woo Lee
MK Electron Co., Ltd., Sejong University
Seoul, Korea

Aakash Valliappan and Tae-Kyu Lee, Ph.D.
Portland State University
OR, USA

Summary
The needs of the higher thermo-mechanical reliability performance in the Tin-based lead-free solder interconnections are drastically increased in wafer-level chip scale packages implemented in automotive field application. This study reveals that a few wt% Bismuth makes 64% improvement in thermo-mechanical reliability results compared with SAC405 solder. In this study, 12mm X 12mm X 1mm thickness 228LD CTBGA package assembled on 62mil thickness board with NSMD Cu-OSP pad using a SAC305 solder paste. This study is aimed to verify the key mechanism, which enabled the reliability improvements. The crack propagation path shifted with Bismuth contents variation in the lead-free solder interconnections under thermal cycling with -40 to 1250C temperature range and 10OC ramp rate.

Microstructure has been studied via polarized optical imaging and High-speed EBSD (electron backscattering diffraction) analysis. The associated grain structure evolution was observed, revealing identifiable signature grain structure development during thermal cycling. Along with the microstructure evolution during thermal cycling, detail resistance monitoring for each localized daisy chain enabled the comparison between various solder materials showing the crack initiation and propagation phenomenon. With this analysis, the decoupling of crack initiation and propagation was performed.


Conclusions
The thermal cycling test results identified that Bismuth added solder mitigates recrystallization and grain refinement, which plays a role as a strengthening mechanism and result in thermal cycling reliability performance improvement. It was observed that shift in the crack propagation path from the bulk region to the package side interface, without a subgrain structure development resulted in decreasing the crack propagation rate. Also, it has been observed that the resistance variation, from the initiation to final failure in Bismuth added solder is longer than normal SAC solder.

Initially Published in the SMTA Proceedings

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