Comparison of IMC growth in flip-chip assemblies with 100- and 200-μm-pitch SAC305 solder joints

In this work, the intermetallic compound (IMC) evolution in Cu pad/SnAgCu solder interface and SnAgCu solder/Ni pad interface was investigated using thermal shock cycling experiments with 100-μm-pitch and 200-μm-pitch flip chip assemblies. The study mainly focused on the size effect of solder joints on the microstructure of IMC. The experiments showed that lower stand-off height of solder joint and higher thermo-mechanical stress played a greater role on the IMC microstructure evolution for the smaller size solder joint under thermal shock test. By comparing the IMC growth of 100-μm-pitch with that of 200-μm-pitch solder joints, it was found that on the chip side with Ni pad, the amount of (Ni, Cu)₃Sn₄ reduced gradually with more cycles for both cases. However, after 1200 cycles, (Ni, Cu)₃Sn₄ completely disappeared for the 100-μm-pitch case, while a thin layer of (Ni, Cu)₃Sn₄ still remained for the 200-μm-pitch flip-chip assembly. In addition, the growth rate of (Cu, Ni)₆Sn₅ was faster for 100-μm pitch on both die and substrate sides, especially before 400 cycles. Finite-element analysis also showed that 100-μm-pitch solder joints experienced larger regions of higher stresses under thermal shock. The faster IMC growth rate for smaller joints could be explained through shorter diffusion path and higher thermo-mechanical stresses.