Sources of Creep Data Scattering of Solders in Micro-electronic Packaging

A large scattering of creep data of Pb-free solders existing in literature was investigated by conducting controlled experiments using Sn-3.5Ag alloy. Tensile specimens with different solidification rate from the melt were prepared to study the effect of solder microstructure, while simple shear (SS) and lap shear (LS) tests were added to find a mechanistic effect of multiaxial loading. Effects of the solder ball geometry was studied using 3D-FEM, and varying amounts of Cu (0.75 and 1.5 wt%) were added to Sn-3.5Ag solder and reflowed over Cu or Ni pad to investigate effects of pad metal dissolution in solder. Results identified the solder microstructure and the loading method as primary factors which could vary the creep deformation rate by 3-4 orders of magnitude. Relatively milder effects were found regarding to variations of solder ball geometry and of solder composition by pad metal dissolution.