Electronics Assembly Knowledge, Vision & Wisdom
Low Melting Temperature Interconnect Thermal Cycling Performance
Low Melting Temperature Interconnect Thermal Cycling Performance
Low melting temperature for solder interconnection comes with benefits but one disadvantage is inferior thermal cycling performance due to the higher creep rate.
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Corrosion, Contamination, Data Acquisition, ESD and EOS, Inspection, Measurement, Profiling, Reliability, R&D, RFID, Solder Defects, Test, Tombstoning, X-ray and more.
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Authored By:
Andy Hsiao and Tae-Kyu Lee,
Portland State University
OR, USA

Imbok Lee and Young-Woo Lee
MK Electron Co., Ltd., Yongin, Korea, Sejong University, Seoul, Korea
University of Seoul
Seoul, Korea

Edward Ibe and Karl Loh
Zymet
NJ, USA

Summary
The adaptation of low melting temperature for solder interconnection comes with significant benefits to less warpage and component defect risk due to the lower assembly temperature. But one disadvantage is inferior thermal cycling performance due to the higher creep rate. Adding to the recent active efforts to improve the thermal cycling performance with maintaining the melting temperature, the degradation mechanism in low melting
temperature solder alloys is critical to understand. Since the mitigation of the degradation mechanism will provide the key mechanism improving the thermal cycling performance.

In this study, 12x12 mm chip array BGA (CTBGA) components on 62mil thick boards were thermal cycled from -40ºC to 125ºC with Sn based low melting temperature solder with control elements including In and Bi. The microstructure evolution during thermal cycling were observed and the correlation between crack propagation and localized recrystallization were compared in a series of cross section analyses via polarized imaging and Electron–backscattered diffraction (EBSD) imaging. It was found that the elemental impact, of In and of Bi, enhance thermal cycling due to creep rate changes compared to conventional Sn based alloys.

To further enhance the performance, an edgebond adhesive was applied and investigated, to determine if it can increase the thermal cycle performance. An improvement of over 140% with dot-cornerbond and280% with full-edgebond was observed.

Conclusions
12x12 mm chip array BGA (CTBGA) components on 62mil think boards were thermal cycled from -40ºC to 125ºC with Sn based low melting temperature solder with control elements, including In and Bi. The microstructure evolution during thermal cycling were observed and the correlation between crack propagation and localized recrystallization were compared in a series of cross section analyses using polarized imaging and Electron-backscattered diffraction (EBSD) imaging. The Sn based In and Bi containeingd solders show a reasonable thermal cycling performance with a capability to lower the assembly reflow temperature for lower level of warpage during assembly.

It was found that the inclusion of In or Bi enhances thermal cycling due to creep rate changes compared to conventional Sn based alloys. To further enhance the performance, a reworkable edgebond adhesive was applied and investigated, to determine if it can increase the thermal cycle performance. Improvements of over 140% with dot-cornerbond configuration and 280% with full-edgebond configuration were observed. The edgebond adhesive can be used to enhance the board level reliability of boards assembled with low temperature solder. The microstructure of Sn-Ag-In-Bi shows a fine-grained structure with lower level of grain rotation and recrystallization behavior, which can be the reason for relatively stable microstructure during thermal cycling.

Initially Published in the SMTA Proceedings

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