Finite element analysis of the effect of process-induced voids on the fatigue lifetime of a lead-free solder joint under thermal cycling

Abstract

Process-induced voids remain one of the key concerns in thermo-mechanical reliability of solder alloys. Previous studies reported that the void effect on fatigue failure reliability of solder joints depends on the void configuration and some other specific characteristics of the electronic package. This paper investigates the void effect on the solder material layers used in power modules subjected to thermal passive cycles. The Anand's visco-plastic model of the solder alloy is identified based on experimental data obtained with a micro-tester. The constitutive model is then used in a finite element analysis to study the behaviour of Innolot Pb-free solder joint used in an electronic assembly. An algorithm called Monte Carlo Representative Volume Element Generator is used to generate, based on the statistical probability law for the diameters, the 2D disk distribution of the voids (thereafter extruded in the form of cylinders) within the solder layer. The dissipated plastic energy is considered as a damage variable indicator representing the void effect on the fatigue lifetime of the solder. Results suggest that the fatigue reliability of solder joints depends not only on the size, location and ratio of the voids but also on their statistical distribution. The critical sites for damage are located at the corners of the joint, as well as at the border of voids. Fatigue lifetime of the solder joint decreases as the volume fraction of voids increases. Moreover, voids near the critical sites facilitate initiation of damage significantly. On the contrary, the solder joint behaviour is almost not affected by voids located far from the critical sites.