Microstructure, thermal behavior and joint strength of Sn-0.7Cu-1.5Bi/electroless nickel immersion gold (ENIG)

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)(6)Sn-5 intermetallic 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)(6)Sn-5 IMCs decreased due to the addition of 1.5 wt% Bi. The average kinetic growth rate of the primary (Cu,Ni)(6)Sn-5 IMCs 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 similar to 12% at bulk solder fracture of similar to 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)(6)Sn-5 IMCs 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. (C) 2021 Published by Elsevier B.V.

Automated summary

This study investigated the microstructure, thermal behaviors, and joint strength of Sn-0.7Cu-1.5Bi solder alloy on electroless nickel immersion gold (ENIG) surface finish. The addition of 1.5 wt% Bi resulted in a decrease in size of primary (Cu,Ni)(6)Sn-5 intermetallic compounds (IMCs) and increased shear strength of the solder joint. The results indicated that microstructural changes, especially the reduction in size of IMCs, in Sn-0.7Cu-1.5Bi/ENIG had a significant impact on joint strength.