Interfacial Reactions of Ag and Ag-4Pd Stud Bumps with Sn-3Ag-0.5Cu Solder for Flip Chip Packaging

Abstract

The wettability and growth of intermetallic compounds (IMCs) of stud bump materials Ag, Ag-4Pd, Cu, and Au with Sn-3Ag-0.5Cu (SAC305) solder have been investigated. Stud bumps produced using wire bonding techniques are widely employed in flip chip assembly, especially in 3D integrated circuit packaging. The material of stud bumps is a key factor in the reliability of the bonds. The wettability of IMCs as studied in this study decreased in the order Au > Ag ≈ Ag-4Pd > Cu, indicating that Ag and Ag-alloy bumps were only slightly inferior to those of Au but superior to those of Cu. Moreover, the morphologies and growth behaviors of the IMCs were evaluated by aging at 100°C to 200°C for 100 h to 1000 h. Without the gold embrittlement effect found in Au or the Kirkendall void issue of Cu, IMCs contained mainly Ag3Sn at the interface after long-term aging tests, and thickened IMCs with Ag3Sn and PdSn4 were observed in the case of Ag-4Pd after prolonged aging for 1000 h at 200°C. The growth kinetics of the IMCs in Ag and Ag-4Pd stud bumps with SAC305 solder followed the Arrhenius equation, with activation energies of 56.45 kJ/mol and 57.37 kJ/mol, respectively. The formation mechanisms of the IMCs in Ag and Ag-4Pd stud bumps were found to be diffusion controlled. The results indicated that Ag and Ag-4Pd are better than Au and Cu for application as stud bump materials.