The dependence of the activation energies of intermetallic formation on the composition of composite Sn/Pb solders

The kinetics of the growth of Cu/Sn intermetallics at composite solder/Cu substrate interfaces is examined. The composite solders consist of a eutectic Sn/Pb matrix plus additions of Cu, Cu/sub 6/Sn/sub 5/, Cu/sub 3/Sn or Ni particles. Samples were annealed at 110 to 160 degrees C for up to 64 days. The activation energies measured for Cu/sub 6/Sn/sub 5/ and Cu/sub 3/Sn formation with the eutectic solder alone are in good agreement with previously published values. The Cu-containing particles increase the activation energy for Cu/sub 6/Sn/sub 5/ formation and reduce the activation energy for Cu/sub 3/Sn. It is proposed that the particles (1) act as Sn sinks and (2) reduce the cross-sectional area of Sn diffusion. Ni is a diffusion barrier which prevents Sn from diffusing into Cu. It drastically increases the activation energies for the formation of both Cu/Sn intermetallics. Only Cu/sub 6/Sn/sub 5/ with a substantial volume fraction of voids is observed for this composite solder. The voids form on the Cu side of the intermetallic due to differential mass transport.<>