Interfacial Reaction Between 95Pb-5Sn Solder Bump and 37Pb-63Sn Presolder in Flip-Chip Solder Joints

Interdiffusion and interfacial reaction of 95Pb-5Sn solder bumps and 37Pb-63Sn presolder in flip-chip solder joints during high-temperature storage were studied. Reaction temperatures included 100°C, 130°C, 150°C, and 175°C. It was found that Cu₆Sn₅ and Cu₃Sn formed on the board side and (Ni,Cu)₃Sn₄ formed on the chip side after 100 h of aging. After 2000 h of aging at 175°C, the Ni under-bump metallization (UBM) was exhausted. This caused the (Ni,Cu)₃Sn₄ layer at the chip-side interface to be gradually converted into (Cu_0.6Ni_0.4)₆Sn₅. It was also found that the consumption of the Ni UBM was faster than the case where eutectic Sn-Pb solder was used for the entire joint. Nevertheless, the consumption of the Cu on the substrate side was slower than the case where pure eutectic Sn-Pb solder was used for the entire joint.