Undercooling and solidification behavior of Sn-Ag-Cu solder balls and Sn-Ag-Cu/UBM joints

The influence of the cooling rate and the type of under bump metallization (UBM, i.e., Cu and Ni) on the undercooling and solidification behavior of Sn-Ag-Cu (SAC) based solders (i.e., Sn-1.0Ag-0.5Cu, Sn-3.0Ag-0.5Cu and Sn-3.8Ag-0.7Cu) and their joints was characterized. The results show that the cooling rate has little influence on the undercooling of Sn-1.0Ag-0.5Cu and Sn-3.0Ag-0.5Cu solder balls, while Sn-3.8Ag-0.7Cu solder exhibits some variation in undercooling values at different cooling rates. The undercooling value of Sn-1.0Ag-0.5Cu solder itself is significantly higher than that of Sn-1.0Ag-0.5Cu/Cu or Sn-1.0Ag-0.5Cu/Ni joints; the value of undercooling of Sn-3.0Ag-0.5Cu solder itself is slightly lower than that of Sn-3.0Ag-0.5Cu/Cu or Sn-3.0Ag-0.5Cu/Ni joints; while for Sn-3.8Ag-0.7Cu solder, the undercooling of Sn-3.8Ag-0.7Cu/Cu joints is slightly higher than that of the solder itself, but the undercooling of Sn-3.8Ag-0.7Cu/Ni joints is significantly higher than that of the solder itself. In addition, for all three SAC solders, the undercooling of SAC/Ni joints is larger than that of SAC/Cu joints.