Effect of glass transition slope of underfill on solder joint fatigue life

In flip chip microelectronic packages, solder bumps are used to connect the silicon die and package substrate for electrical functionality. However, due to the large mismatch of silicon and organic package in the coefficient of thermal expansion (CTE), the solder bumps undergo large viscoplastic deformation in temperature cycling test and in field operation. The viscoplastic damage accumulates cycle by cycle, which leads to the bump failure by fatigue cracking after hundreds or thousands of thermal cycles. Underfill plays a key role in solder joint fatigue lifetime. Besides the effects of underfill modulus, CTE, and the glass transition temperature T_g, the slope of temperature-dependent modulus curve in the range of glass transition is also critical. Since the solder deformation is very sensitive to underfill modulus, the viscoplastic work is much larger when the temperature is above T_g than below T_g. Since the glass transition occurs within a range of temperature, there are various definitions of Tg in practice. Besides, different materials could have the same T_g based on some definition, but the transition behaviors could be quite different. As long as the accurate temperature-dependent properties of underfill are not used, the predicted solder fatigue life based on a single value of T_g and a sharp transition is subject to skepticism. This paper studies the effect of glass transition slope on solder fatigue life under accelerated thermal cycling (ATC:0~100°C) and deep thermal cycling (DTC:-40~125°C) test conditions. The results show that lifetime variation can be 80% less for steeper slope and doubled for shallower slope.