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

摘要

SAC (SnAgCu) lead free solder is currently the alloy of choice bythe electronics industry for lead free applications. In this studymultiple WLPs (Wafer Level Packages) called the Ultra CSPTMfrom 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 besuitable for the SAC lead free solder version. Both SAC and eutecticSnPb solders were tested together. In this TC test, two parts were takenout of the TC chamber after every 200 cycles for monitoring thecharacteristics of deformation and crack growth in the solder joints.The result showed eutectic SnPb solder joints might have a global anduniform deformation in the high temperature regime. On the other hand,in the low temperature regime, the deformation is localized only at chipside solder joint while maintaining the global deformed shape from theprevious high temperature regime. This localized deformation at lowtemperature regime created a large shear dislocation at chip side solderjoint, with the crack initiating at the outside corner of the solderjoint and growing toward the inside of chip. On the other hand, the SACsolder joints did not show that kind of large sliding at chip sidesolder joint. Instead two cracks initiated at both the outside andinside of chip side solder joint and grew at almost the same rate. Therewas very good agreement between Weibull life and the time that thecracked length (%) goes to 100% in eutectic SnPb solder. Extending thiscorrelation to SAC lead free solder appears to be possible. Indicationsare that there will be an improvement, but there was insufficient datato make a conclusive statement as to reliability improvement. Tests areunderway to confirm this