Intermetallic Growth and Failure Study for Sn-Ag-Cu/ENIG PBGA Solder Joints Subject to Thermal Cycling

The growth of interfacial IMC compounds and the interfacial voids/crack formation between 95.5Sn-3.8Ag0.7Cu lead-free solders and Electroless Nickel/Immersion Gold (ENIG) surface finish on 35x35mm 316 I/O PBGA soldered assembly subject to thermal cycling aging are reported. The thicknesses of interfacial Ni-Cu-Sn IMC compounds were observed on all the solder joints. It is noted that IMC layer thickness at different locations are different after 3000 thermal cycling aging. The highest IMC layer thickness was observed at the outermost solder joint. Compared with IMC growth subjected to isothermal aging exposure at 125 o C, IMC layer growth rate under thermal cycling is accelerated and it is subject to the locations, suggesting the effect of thermal stress generated by CTE mismatch during temperature cycling. The correlation between void/crack formation and IMC layer growth was then discussed.