SOLIDIFICATION AND MICROSTRUCTURE OF EUTECTIC PB-SN MICROSOLDER BONDS

The increased use of microsoldered circuits has brought with it concern 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 solidify as a lamellar eutectic. This has often been assumed to be the microstructure of considerably smaller volumes of the same alloy, although studies of atomised drops in the Pb-Sn and other systems have shown that non-equilibrium structures commonly develop in alloy systems during the solidification of small volumes. In the present work the solidification of small volume truncated sphere flip-chip microsolder bonds was studied using differential scanning calorimetry of the arrays of bonds, and the microstructures were assessed. Cooling traces obtained using 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 of bonds examined. Metallographic examination revealed non-equilibrium microstructures, and none of the bonds examined contained a lamellar eutectic structure. This is attributed to the bonds solidifying via a metastable reaction, involving the nucleation of a lead rich phase which enriched the remaining liquid in tin, resulting in a final microstructure of massive lead dendrites in a pure tin matrix.