Optimization of Ni and Bi levels in Sn-0.7Cu-xNi-yBi solders for improved interconnection reliability

Sn-Cu-Ni solders were introduced to electronic assembly in 1999 and the Sn-0.7Cu-0.05Ni (wt%) composition was widely accepted as a Pb-free and Ag-free alternative to Sn-Ag-Cu alloys for wave and reflow soldering. The minor Ni addition of 0.05wt% was shown to have several beneficial effects on the Cu6Sn5 intermetallic that form during soldering. In recent years, Bi additions have been identified as a beneficial alloying component that significantly improves the mechanical properties of solder alloys. The present investigation explores BGA joint microstructures that form in Sn-0.7Cu-xNi-yBi solders and seeks to identify the Ni and Bi levels that deliver optimum performance in soldering processes and reliability in service.