The reliability study of sub 100 microns SnAg flip chip solder bump on FR4 substrate under thermal cycling

Due to the advantages of small-footprint, short-lead, high performance, high-packaging-density and thin profile, flip-chip-on-board (FCOB) technology is becoming an attractive choice in todaypsilas high density electronic packaging industry. With the trend toward lead-free and miniaturization in consumer electronics, the fatigue reliability of the small size lead-free FC solder joint on low cost PCB substrate are becoming one of the important issues. In this study, the reliability of sub 100 microns Sn-3.0Ag flip chip solder bump on FR4 substrate was investigated under thermal cycling between -40degC to 125degC. The influences of the shape of solder joint on the failed plane and the fatigue life were studied. The failed plane of the solder joint was discussed by the stress state analysis of the solder joint. Using the metallography, SEM, and live testing of the resistance, failure character and failure mechanism of the solder joint before and after underfilling were analyzed. The increasement of the fatigue life with the use of underfill was interpreted by plastic mechanics.