Effect of Cu stud structure and eutectic solder electroplating on intermetallic growth and reliability of flip-chip solder bump

In electroplating-based flip-chip technology, the Cu stud and solder deposition process is one of the most important factors affecting the reliability of solder joints. The growth of Cu-Sn intermetallic compounds (IMC) also plays a critical role. In this paper, the effect of Cu stud surface roughness and microstructures on the reliability of solder joint was studied. The micro-structural morphology of the Cu-Sn IMC layer was affected by Cu stud surface structure. The Cu stud with sloped edge can impacted the adhesion of solder bump and UBM (Under Bump Metallurgy) layer. Insufficient solder wetting at edge of the Cu stud can further degrade the reliability of solder joints. We observed the thickness of ε-phase Cu3Sn layer increased continuously instead of η-phase Cu6Sn5 due to the deficiency of tin at the bottom of solder bump after extended thermal aging. The adhesion of Cu stud and UBM layer was weakened due to the growth of Cu3Sn at the edge of Cu stud. Both of the Cu-Sn IMC layers grew at the top of Cu stud as the aging time increased. The mean thickness of two IMC layers increased linearly with the square root of aging time. We found that cracks formed easily at the interface of Cu-Sn IMC layer and solder bump, especially at the Pb-rich layer and IMC layer interface. Cracks led to low bump shear strength after extended thermal aging. We did not observe cracks formed at the Cu6Sn5 and Cu3Sn interface. The SEM and EDAX analysis suggested that the fracture surface structure was influenced by the Cu stud microstructure and solder bump deposition process.