SOLIDIFICATION AND MICROSTRUCTURE OF EUTECTIC PB-SN MICROSOLDER BONDS

The increased use of microsoldered circuits has brought with it concer n over the reliability of the Pb-Sn solder joints in such assemblies. It is well known that bulk samples of the eutectic Pb-Sn alloy solidif y as a lamellar eutectic. This has often been assumed to be the micros tructure of considerably smaller volumes of the same alloy, although s tudies of atomised drops in the Pb-Sn and other systems have shown tha t non-equilibrium structures commonly develop in alloy systems during the solidification of small volumes. In the present work the solidific ation of small volume truncated sphere flip-chip microsolder bonds was studied using differential scanning calorimetry of the arrays of bond s, and the microstructures were assessed. Cooling traces obtained usin g differential scanning calorimetry revealed undercoolings up to 31 K before solidification, although the integrated solidification exotherm is insufficient to encompass the solidification of the entire array o f bonds examined. Metallographic examination revealed non-equilibrium microstructures, and none of the bonds examined contained a lamellar e utectic structure. This is attributed to the bonds solidifying via a m etastable reaction, involving the nucleation of a lead rich phase whic h enriched the remaining liquid in tin, resulting in a final microstru cture of massive lead dendrites in a pure tin matrix.