Effects of substrate metallizations on solder/underbump metallization interfacial reactions in flip-chip packages during thermal aging

Effects of Ni and Au from Ni/Au substrate metallizations on the interfacial reactions of solder joints in flip-chip packages during long-term thermal aging were systematically investigated. It was found that both Au and Ni influenced the solid-state interfacial reactions, underbump metallization (UBM), and intermetallic compound (IMC) evolution. Because large amounts of Ni could incorporate into IMC to form a multicomponent (Cu, Ni)₆Sn₅ phase during assembly reflow, while Au could only affect the reaction during thermal aging through the reconfiguration of AuSn₄ phase, Ni had stronger effects on solid-solution type Ni–V UBM consumption than Au. It was found that the UBM consumption process was faster in the eutectic SnPb solder system than that in the SnAgCu solder system during aging. A porous structure was formed in the UBM layer after Ni in UBM was consumed. Electrical resistance of flip-chip packages increased significantly after the porous structure reached certain extents. The results showed that the diffusion process of Ni from UBM and Sn from solder in the presence of (Cu, Ni)₆Sn₅ or (Ni, Cu)₃Sn₄ phase at solder joint interfaces could be much faster than that in the case of binary Cu₆Sn₅ or Ni₃Sn₄ IMC.