This paper details an investigation into the microstructure, thermal behaviors and joint strength of Sn-0.7Cu-1.5Bi solder alloy on electroless nickel immersion gold (ENIG) surface finish. Besides conventional techniques, the real-time synchrotron imaging was used to analyze the microstructure evolution in Sn-0.7Cu-1.5Bi/ENIG. This research investigated the growth behavior of the primary (Cu,Ni)6Sn5intermetallic compounds (IMCs) in the solder joint with the Bi alloying. The elemental distribution analysis showed the Ni diffused from the ENIG surface finish and dissolved into the bulk solder during solidification and that the size of the primary (Cu,Ni)6Sn5IMCs decreased due to the addition of 1.5 wt% Bi. The average kinetic growth rate of the primary (Cu,Ni)6Sn5IMCs in Sn-0.7Cu-1.5Bi/ENIG was lower than that of the Sn-0.7Cu/ENIG. The thermal analysis revealed that the pasty range slightly increased and the undercooling degree decreased due to the addition of 1.5 wt% Bi for free-standing solder and soldering on ENIG surface finish. The shear strength of the Sn-0.7Cu-1.5Bi/ENIG was determined using a high-speed bond tester, and it increased by ~12% at bulk solder fracture of ~15% within the solder joint interfacial fracture due to the addition of 1.5 wt% Bi into the Sn-0.7Cu. These occurrences can be attributed to the solid solution strengthening effect at the bulk solder and the formation of finer interfacial (Cu,Ni)6Sn5IMCs within the solder joints. The results indicated that the microstructural changes, especially the size reduction of IMCs, in Sn-0.7Cu-1.5Bi/ENIG impacted the joint strength.
