Authored By:
Won Sik Hong, Ph.D., Jinju Yu, BA., and Chulmin Oh, Ph.D.
Korea Electronics Technology Institute
Gyeonggi-do, Korea
Summary
Due to End-of-Life Vehicle (EVL) banning, automotive electronics and its manufacturing system was exchanged, and also heavy construction equipment is demanded to use lead free electronics. Thus, in this study, degradation behavior of lead free solder joint was quantitatively compared with mounted position and component density of vehicle engine control unit (VECU) for excavator. Sn-3.0Ag-0.5Cu (SAC305) Pb-free solder and FR-4 PCB plated with electroless nickel immersion gold (ENIG) were used. Thermal shock test was performed at -40-125 ℃, 10 min dwell at each temperature for 1500 cycles. We obtained that shear strength degradation of high component density area was larger than that of small component density. Based on this result, we have known that it was important to consider the component density for uniform heat distribution at electric circuit design under harsh environment.
Conclusions
We investigated the shear strength degradation of the SAC305 solder joints with mounted components density in the same board, and finally obtained the followed results.
We confirmed the deviation of absorption and desorption capability with mounted components density in printed wiring board. Due to the cooling rate deviation with components density, the heat desorption rate of a high components density region was very slow than that of low density area, and a large volume component showed a slow heat desorption rate. Therefore, in the region of high components density, shear strength degradation rate and IMC thickness of solder joints of high components density were larger and thicker than that of small density area due to insufficient time for recovery of heat stress.
Consequently, the degradation rate of Group 1 (a high components density) area showed a faster than that of Group 2 (a small components density). During the thermal shock cycle, a high components density area within the same module could be extended a soak time at the high temperature range, which caused a thicker IMC growth and finally, these reason leaded that the bonding strength of solder joints showed to be smaller caused by intermetallic compound growth.
(Ni,Cu)3Sn4 and Ag3Sn IMCs were formed at the solder joint interfaces and the solder matrix, respectively. The initial crack generated at the edge of the solder joint and propagated along the IMC interface of solder joint. To improve the solder joints reliability is necessary to consider the component size and mounted components density in the electrical board design.
Initially Published in the SMTA Proceedings
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