Thermal cycling effect on intermetallic formation with various surface finish of micro bump interconnect for 3D package

Micro bump interconnect with through-silicon via (TSV) is one of the critical issues for realizing three dimensional (3D) packages. This enabling technology provides more I/O in shrunken die area, and hence high density interconnection. Electroless Ni immersion Au (ENIG), electroless Ni electroless Pd immersion Au (ENEPIG), and plating Tin are commonly used surface finish for Cu pad in lead-free package. However, the majority of studies are focusing on Controlled Collapse Chip Connection (C4) solder. Intermetallic compounds (IMC) formation is a result of interaction between the solder tip, barrier layer, Cu pillar form the top die and micro pad finish from the interposer. As a result, microstructure made with different pad finish will major impact to solder reliability. In this study, three types of pad finish including ENIG, ENEPIG, and plating Tin were chosen to evaluate intermetallic formation during thermal cycling test. In addition, presence of Ni was also discussed to understand IMC formation in this microstructure. Multi reflow (at time zero, 1×, 3×) and thermal cycling test were performed for this evaluation. Metallurgy and growth kinetics of IMC were compared at different thermal cycling. The results show that SnAg bump with ENEPIG pad finish have more Cu consumption than ENIG after pre-con. ENEPIG pad finish exhibits crack formed between (Ni, Cu)₆Sn₅ and P-rich layer. The detailed influence of Ni with various pad finish on the growth kinetics of IMC formation was investigated and discussed.