Effects of isothermal aging and in-situ current stress on the reliability of lead-free solder joints

The microstructure and mechanical behavior of Sn-Ag-Cu (SAC) soldering alloys can change significantly over time when exposed to isothermal aging and in-situ current stress. Electronics assemblies built with SAC solder joints are exposed to elevated ambient temperatures and relatively high current densities for prolonged periods of time. Therefore it is important to understand the impact of these stresses on the long term reliability. In this study, board level thermal cycle reliability tests have been performed on daisy chain lead-free 0.5 mm pitch CTBGA, 1.0 mm pitch PBGA, and 0.5 mm pitch MLF packages. The tests were performed with and without isothermal pre-aging. During the thermal cycling tests, the daisy chained solder joints were subjected to different levels of constant current density. The thermal cycle profiles were tailored in order to evaluate the effect of current density on solder joint fatigue, while minimizing the secondary effects of joule heating. The results show that the effects of both thermal aging and current stress are consistent for a given package type, but the trends can be different for different package designs. In 0.5 mm pitch CTBGA, isothermal aging significantly reduced the solder joint life, while current stress caused the solder joint life to slightly increase at moderate current densities, and eventually degrade at higher current densities. In PBGAs, the effect of thermal aging was insignificant but current stress caused a slight degradation in life. In MLF packages isothermal aging caused a serious degradation in solder fatigue life, while the effect of current stress was not significant.