Effects of Bi in Sn-Cu based lead-free solder alloys and interconnects

Additions of 1.5wt%Bi to Sn-0.7Cu-0.05Ni (SN100C) were investigated for their influence on mechanical properties and the IMC layer formed between the solder and Cu substrates. Solder balls of Sn-0.7Cu (Sn07Cu), SN100C, Sn-0.7Cu-0.05Ni-1.5Bi (SN100CV) and Sn-3Ag-0.5Cu (SAC) were reflowed onto Cu ball grid arrays (BGAs). They were examined in the as reflowed condition and after a heat treatment of annealing at 150°C up to 1,500 hours. The mechanical properties of SN100C, SN100CV and SAC solder balls were investigated by nano-indentation, and cross-sections of the interfacial IMC layer were observed by SEM to determine the morphology and average interfacial IMC layer thickness. It was found that the effect of Bi additions was to increase the lattice parameters and alter the mechanical properties. The near-eutectic microstructure and suppression of Cu
₃
Sn at the IMC layer that are associated with Ni additions are not altered by the presence of 1.5wt%Bi.