Intermetallic formation in the Sn-Ag solder joints between Au stud bumps and Cu pads and its effect on the chip shear strength

The Au stud bump, which is based on the wire bonding technology, offers a low cost flip chip solution for low I/O count ICs. The Au stud bumps were formed on the Al pads and coined to produce uniform bump height. Lead free 96.5wt%/oSn3.5wt%Ag Solder paste was deposited on the Cu pads of the printed circuit board (PCB) by stencil printing. The solder joints were fabricated by reflowing the solder paste after aligning Au stud bumps on the corresponding Cu pads. The solder joints were underfilled with epoxy. The strength of the joints before underfill was measured by die shear test. Au-Sn intermetallic compounds were formed as a result of the Au stud reaction with Sn-Ag solder. Au-Sn intermetallic compounds spread over in the whole joints and the solder remained randomly as island shape due to the fast. dissolution of Au into the solder. AlAu₄ was formed as a result of the Au stud reaction with Al pad. The continuous layer of scallop-like Au-Cu-Sn intermetallic was formed at the solder/Cu pad interface. The microstructure of the solder joints did not changed significantly even after multiple reflows and the thickness of the intermetallic layers increased with the number of reflows