Joint properties of Au stud bumps joined with Sn-3.5Ag solder by flip chip bonding

Joint properties of Au stud bumps joined with Sn-3.5Ag solder by flip chip bonding were investigated. Au stud bumps were bonded on SOP (solder on pad) at bonding temperature of 260°C and 300°C for 10sec, respectively. Aging treatment was carried out at 150°C for 100 and 300 hrs. After flip chip bonding, intermetallic compounds (IMCs) of AuSn, AuSn₂, and AuSn₄ were formed at interface between Au stud bump and Sn-3.5Ag solder. At bonding temperature of 300°C, AuSn₂ IMC clusters, which were surrounded by AuSn₄ IMC, were observed in the Sn-3.5Ag solder bumps. Also, (Au, Cu)₆Sn₅ IMC was formed at Sn/Cu interface. After flip chip bonding, bonding strength was approximately 22.5gf/bump. A typical fracture surface of as-bonded samples was intergranular fracture between AuSn₂ and AuSn₄ IMCs. Such phenomenon was similar for all specimens irrespective of bonding temperature. As aging time increased, AuSn and AuSn₂ IMCs continuously grew while AuSn₄ IMC decreased. After 300 hrs aging, thickness of AuSn₂ IMC was higher than that of other Au-Sn IMCs. The bonding strength was decreased as aging time increased. After 100 hrs aging treatment, the bonding strength of 300°C flip chip bonding sample was lower than that of 260°C sample due to fast growth rate of AuSn and AuSn₂ IMCs. Initial joint microstructures after flip chip bonding were closely related to the bonding strengths of aged samples.