Failure analysis of sub-50 /spl mu/m lead-free solder bumps on electroless Ni-P UBM for flip chip interconnects

Three lead-free solder bumps (Sn-0.7Cu, Sn-3.5Ag, Sn4Ag-0.5Cu) were produced by stencil printing of type 6 and type 7 solder pastes on wafers having electroless Ni-P immersion Au (ENIG) under bump metallization (UBM). The diameter and height of the solder bumps were measured to be 47 and 43 mum, respectively. Ball shear tests were carried out to evaluate the UBM solderability and the bonding quality of the solder bumps after 1 reflow, multiple reflows and thermal aging at 150degC. 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 UBM/solder interface IMC layers. The interfacial IMC phase was mainly (CuNi)
₆
Sn
₅
for Sn-4Ag-0.5Cu and Sn-0.7Cu solders and Ni
₃
Sn
₄
for Sn-3.5Ag solder. The coarsening of Ni
₃
Sn
₄
IMC phase for Sn-3.5Ag solder was much greater than that of (CuNi)
₆
Sn
₅
for Sn-0.7Cu and Sn-4Ag-0.5Cu solders with increasing reflows (solid/liquid reaction time). Even if only a small amount of Cu (0.5~0.7 wt %) is presented at the interface during reaction, the type, morphology and evolution of the interfacial IMC particles are greatly different. The growth kinetics of the interfacial IMC layers and the consumption of Ni-P UBM were also identified. Plate-like Ag
₃
Sn phase with a rectangle shape was found in both Sn-4Ag-0.5Cu and Sn-3.5Ag solder bumps. The redeposition of AuSn
₄
at the IMC/solder interfaces were observed for all of the solders after aging at 150degC for 1000 h. The failure mechanisms of the solder bumps were identified and analyzed. The morphology, size and chemical nature of the interfacial IMCs play a very important role on the reliability of the ultra fine pitch flip chip interconnects. P-enriched layer (Ni
₃
P) underneath the interfacial Ni
₃
Sn
₄
for Sn-3.5Ag solder was a weak interface, along which the failures were prone to occur