EVOLUTION OF MICROSTRUCTURE OF Sn-Ag-Cu LEAD-FREE FLIP CHIP SOLDER JOINTS DURING AGING PROCESS

Flip chip bonding has become a primary technology that has found application in the chip interconnection process in the electronic manufacturing industry in recent years. The solder joints of the flip chip bonding are small and consist of complicated microstructures such as Sn solution, eutectic mixture, and intermetallic compounds (IMCs), whose mechanical performance is quite different from the original solder bulk. The evolution of microstructure of the flip chip solder joints under thermal aging was analyzed. The results show that with an increase in aging time, coarsening of solder bulk matrix and AuSn4 IMCs occurred within the solder. The IMCs that are formed at the bottom side of the flip chip bond were different from those on the top side during the aging process. (Cu,Ni,Au)6Sn5 were formed at the interfaces of both sides, and large complicated (Au,Ni,Cu)Sn4 IMCs appeared for some time near the bottom interface after aging, but they disappeared again and thus (Cu,Ni,Au)6Sn5 IMC thickness increased considerably. The influence of reflow times during the flip chip bonding (as-bonded condition) on the characteristics of interfacial IMCs was weakened when subjected to the aging process.