Effect of intermetallic and Kirkendall voids growth on board level drop reliability for SnAgCu lead-free BGA solder joint

Lead free solder joints used in surface mount packages like ball grid array (BGA) have a great impact on the reliability of the end product. In accelerated thermal cycling (ATC) tests, cyclic excursions of temperatures cause concurrent degradation of the solder joint resulting in intermetallic compound (IMC) growth and thermal fatigue damage. The effect of thermal cycling aging on board level drop reliability for lead-free SnAgCu FPBGA packages is reported in this study. Drop impact tests were conducted after (1) thermal cycling aging of 500, 1000 and 1500 cycles, with TC profile of -40degC to +125 degC and (2) isothermal aging at 150degC with different periods of 120hrs, 240hrs and 390hrs. Significant decrease of drop life was observed for both SAC/ENIG and SAC/Cu-OSP packages after thermal aging. SAC/ENIG packages exhibits better drop impact reliability after thermal aging than that of SAC/OSP. The IMC growth, Kirkendall voids formation and solder joint failure mode are characterized by Scanning Electron Microscopy (SEM) after argon ion (Ar+) sputtering by a radio frequency (R.F.) magnetron sputtering equipment. It was found that growth of Kirkendall voids and IMC significantly weakened the solder joint interface during TC aging. Drop impact crack location changed from the inside of IMC to the IMC/Cu interface, which is related to Kirkendall void formation. For SAC/NiAu specimen, vertical and horizontal void in Ni(P) layer was found after TC aging, drop impact crack propagates between Ni(P)/Cu pad interface along the horizontal voids