Failure prediction of the solder joints in the ball-grid-array package under thermal loading

Abstract

This paper studies the numerical failure mechanism of the solder joints in the ball grid array (BGA) package under thermal reliability process. The package consists of the silicon die, the Flame Retardant 4 (FR-4) substrate and the FR-4 printed circuit board (PCB). A total of 64 95.5Sn-4.0Ag-0.5Cu (SAC405) solder joints with a diameter of 0.46 mm are arranged together in area array fashion with a pitch distance of 0.8 mm. Only a quarter-model of the package is simulated since all the geometry, loading and boundary conditions (BC) is symmetry at the centre of the package. The package is exposed with thermal loading, initially at the liquidus temperature of 220°C to room temperature (25°C). Then, it follows with 3 additional thermal cycles between 125°C and -40°C with a ramp rate of 11°C/min and 15 minutes dwell time, respectively. Unified inelastic strain model (Anand model) was used to compute the inelastic behaviour of the solder joints. Results show that the stress level at the critical solder joints and the corresponding inelastic strain are 39.91 MPa of 0.2083%, respectively after the end of the solder reflow cooling process. As predicted, the inelastic strains accumulate continuously in the solder joint throughout the temperature cycles. Additionally, in the critical solder joint, both high stress and inelastic strain gradients are localized near to the solder-IMC interfaces. Prolong the thermal cycles can extensively accumulate the inelastic strains which lead to fatigue crack and subsequently crack propagation in the solder joints. After the end of the FE simulation, the highest stress and inelastic strain predicted are 57.96 MPa and 0.5781%, respectively.