Microstructure evolution observation for SAC solder joint: Comparison between thermal cycling and thermal storage

This paper describes a reliable and reproducible method to reveal the microstructure of lead-free solder joints. This method is then used to follow the aging of lead free assembled chip resistors and BGA, with either ENIG or immersion Sn finishes. Two types of aging were applied: accelerated thermal cycles (ATC), and iso-thermal storage at three different temperatures. The associated failure mechanisms are compared and discussed.During ATC, microstructural changes under thermo-mechanical fatigue are evidenced, whereas thermal storage only induces intermetallic evolution, depending on the finish type.