Fundamental study of 95 high-lead solder bump on substrates pre-soldered with eutectic PbSn

This study investigated the intermixing of 95Pb5Sn solder bumps and 37Pb63Sn pre-solder in flip-chip solder joints. The reaction conditions included multiple reflows (up to 10) at 240°C, whereby previously solder-coated parts are joined by heating without using additional solder. We found that the molten pre-solder had an irregular shape similar to a calyx (i.e., a cuplike structure) wrapped around a high-lead solder bump. The height to which the molten pre-solder ascended along the solid high-lead solder bump increased with the number of reflows. The molten pre-solder was able to reach the UBM/95Pb5Sn interface after three to five reflows. The molten pre-solder at the UBM/95Pb5Sn interface generated two important phenomena: (1) the molten solder dewetted (i.e., flowed away from the soldered surface) along the UBM/95Pb5Sn interface, particularly when the number of reflows was high, and (2) the molten pre-solder transported Cu atoms to the UBM/95Pb5Sn interface, which in turn caused the Ni-Sn compounds at the chip-side interface to change into (Cu_0.6Ni_0.4)₆Sn₅.