Thermal Fatigue and Failure Analysis of SnAgCu Solder Alloys With Minor Pb Additions

The thermal fatigue performances of Sn_98.5Ag_1.0Cu_0.5 (SAC105), Sn_97.5Ag_2.0Cu_0.5 (SAC205), Sn_96.5Ag_3.0Cu_0.5 (SAC305) and Sn_95.5Ag_4.0Cu_0.5 (SAC405) solder alloys with Pb terminations were investigated by accelerated temperature cycling with and without thermal preconditioning. The performance of the SAC alloys was compared to eutectic SnPb and aged SAC alloys. The test vehicle consists of commercial 2512 ceramic chip resistors soldered to printed wiring boards using the different solder alloy compositions. The solder joints were monitored continuously during a thermal cycle of 0^°C -100^°C with a ramp rate of 9^°C/min and a 30 min dwell between temperature extremes. Failures were defined in accordance with the IPC-9701 A industry test guidelines and failure data are reported as characteristic life η (number of cycles to 63.2% failure) from a two-parameter Weibull distribution. The microstructural evolution was characterized using metallographic techniques and back-scattered scanning electron microscopy. The findings show that the lifetime of the alloys can be ranked as follows: SAC 305 ~ SAC 405 >; SAC 205 >; SAC 105 >; SAC305 aged >; SAC 105 aged >; SnPb and to determine mechanisms of failure, electron microscopy analysis and fractography were performed on post-cycled components.