Influence of pad size on shear strength and microstructure of fine pitch solder bump

Stencil printing process was optimized to obtain fine pitch flip chip solder bump. In order to evaluate the printing process, bump morphology were observed and bump heights were measured. Furthermore, shear force was tested by Pull-Shear Tester to investigate the effect of pad size on shear strength for the fine pitch solder bumps. Fracture surfaces were inspected and intermetallic compounds (IMC) at the surface and in the bump were separately detected and identified. It was found that bump height with small pad size was larger than that with large pad size as well as bump uniformity. Bump shear strength decreased with reducing pad size, which appeared size effect. This phenomenon was attributed to load condition difference during shear testing and IMC variation. Scallop IMC was observed at the bump interface with large pad size and needle like was found at the interface with small pad size, which possibly led to crack stress and decrease shear strength.