Intermetallic compound growth mechanism and failure modes of flip chip solder bump with different UBM structure during electromigration

The growth mechanisms of intermetallic compound (IMC) and the failure modes in flip chip solder joint induced by electric current stressing have been investigated for various UBM and substrate metallizations. The UBM investigated was TiCuCuNi/TiCuNi while the substrate metallizations include electroless Ni(P)/electroless Pd/immersion Au (ENEPIG) and organic solderability preservatives (OSP) Cu substrate. The investigation was conducted with 7kA/cm at 160 °C. The main IMCs formed in the TiCuNi-ENEPIG bump were the Ni₃Sn₄, PdSn₄ and AuSn₄ phases. The nucleation of (Pd, Ni)Sn4 on Ni₃Sn4 layer and the dissolution of Au and Pd into the melting solder form the rod shape AuSn₄ and PdSn4 IMCs. The ENEPIG layer acts as barrier layer that inhibits the Cu pad consumption and forms Ni₃P layer under electric current stressing. Cu atoms from the Cu pad diffused through the Ni₃P layer to form the Cu₆Sn₅ IMC at the interface instead of the Ni₃Sn₄ IMC. The main IMCs formed in the bumps with TiCuNi-Cu and the TiCuCuNi-Cu as UBM were the Cu₆Sn₅ IMC as expected. The failure after the electromigration test occurs either through the expedite consumption of the bottom Cu pad, where no Ni barrier layer, or through the formation of voids and cracks at the IMC/solder interface on the Ni UBM side. The mechanism of void and crack formation was discussed in detail in this study.