CrCu based UBM (under bump metallization) study with electroplated Pb/63Sn solder bumps - interfacial reaction and bump shear strength

The electroplating solder bumping process offers fine pitch, high reliability, and cost effective advantages for flip-chip technology. In this technology, under bump metallization (UBM) is required for chemical solder deposition and mechanically reliable solder contact to Al pads. An evaporated Cr/phased CrCu/Cu structure UBM has been used with 95Pb/5Sn and also with 37Pb/63Sn solder for flip-chip interconnection. In this study, the intermediate CrCu layer is modified using various sputtering techniques, and the underlying Cr adhesion layer is compared with TiW. Six UBM systems were selected, and their interfacial reaction and bump shear strength were investigated using 100 μm and 50 μm size electroplated Pb/63Sn solder bumps. The results demonstrate that the final Cu layer should have a minimum thickness, more than 0.8 μm, for interface stability on CrCu based UBMs. Intermetallic compound growth and CrCu layer interface changes are more severe after 20 min reflow at 210°C compared with 1000 h aging at 125°C. Especially for small size bumps, a more stable interface between UBM and solder bump is required.