The effects of copper organic solderability preservative (Cu-OSP) and electroless nickel immersion gold (ENIG) surface finish reflowed on Sn-3.0Ag-0.5Cu (SAC305) solder have been investigated in detail. Besides conventional cross-sectional microstructure observation, advanced characterization techniques such as synchrotron radiography imaging and synchrotron micro-x-ray fluorescence (ยต-XRF) were utilized to elucidate the microstructural evolution in the solder joints during soldering. Additionally, high-speed shear testing was performed to understand the influence of the surface finish on the solder joint strength. The results indicated that the presence of nickel (Ni) from the ENIG surface finish decreased the growth rate but increased the amount of small Cu6Sn5 primary intermetallics, resulting in a slight reduction of the average interfacial intermetallic compound (IMC) thickness in the SAC305/ENIG solder joints. Due to the refined control of the solder joint microstructure, the average high-speed shear strength was higher for as-reflowed SAC305/ENIG versus SAC305/Cu-OSP solder joints. These results indicate a significant influence of the surface finish on SAC305 solder joint microstructure and strength and could provide a basis to improve solder joint strength.
