Deformation and crack growth characteristics of SnAgCu vs 63Sn/Pb solder joints on a WLP in thermal cycle testing

Abstract

SAC (SnAgCu) lead free solder is currently the alloy of choice by the electronics industry for lead free applications. In this study multiple WLPs (Wafer Level Packages) called the Ultra CSP/sup TM/ from Flip Chip Technologies were put into a TC (Thermal Cycling) test. The goal was to see if the current AI/NiV/Cu UBM (Under Bump Metallurgy) system that has been used for eutectic SnPb solder Ultra CSP would be suitable for the SAC lead free solder version. Both SAC and eutectic SnPb solders were tested together. In this TC test, two parts were taken out of the TC chamber after every 200 cycles for monitoring the characteristics of deformation and crack growth in the solder joints. The result showed eutectic SnPb solder joints might have a global and uniform deformation in the high temperature regime. On the other hand, in the low temperature regime, the deformation is localized only at chip side solder joint while maintaining the global deformed shape from the previous high temperature regime. This localized deformation at low temperature regime created a large shear dislocation at chip side solder joint, with the crack initiating at the outside corner of the solder joint and growing toward the inside of chip. On the other hand, the SAC solder joints did not show that kind of large sliding at chip side solder joint. Instead two cracks initiated at both the outside and inside of chip side solder joint and grew at almost the same rate. There was very good agreement between Weibull life and the time that the cracked length (%) goes to 100% in eutectic SnPb solder. Extending this correlation to SAC lead free solder appears to be possible. Indications are that there will be an improvement, but there was insufficient data to make a conclusive statement as to reliability improvement. Tests are underway to confirm this.