Frequency dependent S-N curves for predicting drop impact robustness of Pb-free solder interconnects

In this paper, a series of PCB strain-to-fatigue life characteristics were generated at a constant bending frequency of 100Hz for different material combinations: eutectic SnPb solder, SAC101 solder, and In/Ni-doped SAC101 solder, assembled on PCB with OSP and ENIG finish respectively. As expected, SAC101 solder exhibits similar performance as eutectic SnPb solder, and solder joints on OSP finish show better performance than solder joints on ENIG. The effect of bending frequency in the range of 30Hz to 150Hz was investigated and the fatigue life was found to decrease with increasing bending frequency. Failure analysis has shown that failures tend to become more brittle with increasing cyclic bending frequency. The PCB strain-to-fatigue life characteristics were converted to peeling stress in the solder joint -to-fatigue life (S-N) using 3D finite element modeling supported by strain rate dependent stress-strain properties of the solder alloy. The S-N characteristics so obtained were found to be independent of the bending frequency.