Shear Failure Analysis of Sub-50 μm Flip Chip Lead-free Solder Bumps

Sn-0.7Cu, Sn-3.5Ag, Sn-4Ag-0.5Cu lead-free solder bumps were produced by stencil printing of solder pastes on wafers having electroless Ni-P immersion Au (ENIG) under bump metallization (UBM). The diameter of the solder bumps was 45˜47 μm. Shear tests were carried out to evaluate the bonding quality of the solder bumps after multiple reflows and thermal aging at 150°C. The fracture surface, cross section microstructure and interfacial intermetallic compounds (IMCs) growth kinetics were investigated to identify the correlation between failure mode and the metallurgical characteristics of the LBM/solder interface IMC layers. The interfacial IMC phase was mainly (CuNi)₆Sn₅ for Sn-4Ag-0.5Cu and Sn-0.7Cu solders and Ni₃Sn₄for Sn-3.5Ag solder. The coarsening of Ni₃Sri₄ IMC phase for Sn-3.5Ag solder was much greater than that of (CuNi)₆Sn₅ for Sn-0.7Cu and Sn-4Ag-0.5Cu solders with increasing reflow times. Even if only a small amount of Cu is presented at the interface during reaction, the type, size, morphology and evolution of the interfacial IMCs are greatly different.