Effect of gold layer thickness on the characteristics of flip-chip interconnects

In this study, the effects of gold layer thickness on the microstructure and mechanical properties of solder interconnects of flip chip specimens are investigated. SAC lead free solder (95.5Sn-3.9Ag-0.6Cu) was used on three gold thickness which are named as: a) Flash gold (<1μm), b) Thin gold (1-2 μm) and c) Standard gold (> 2μm). The results of microstructural characterization studies revealed the presence of Ag-Sn and Cu-Sn-Ni-Au intermetallics. The size and amount of Ag-Sn intermetallics in the bulk solder was found to increase with the increase in the gold layer thickness while the thickness of Cu-Sn-Ni-Au intermetallic layer decreased with an increase in the gold layer thickness. The results of the mechanical shear test revealed that specimens with thin gold layer thickness exhibited the lowest shear strength as compared to specimens with Flash gold and Standard gold layers and the latter two had shear strengths comparable to each other. Particular emphasis was placed on examining the relationship between the microstructure and shear strength of the interconnects and to make recommendations for practical applications.