Influence of Pad Surface Finish on the Microstructure Evolution and Intermetallic Compound Growth in Homogeneous Sn-Bi and Sn-Bi-Ag Solder Interconnects

Low reflow temperature solder interconnect technology based on Sn-Bi alloys is currently being considered as an alternative for Sn-Ag-Cu solder alloys to form solder interconnects at significantly lower melting temperatures than required for Sn-Ag-Cu alloys. Microstructural evolution after reflow and aging, especially of intermetallic compound (IMC) growth at solder/pad surface finish interfaces, is important to understanding fatigue life and crack paths in the solder joints. This study describes intermetallic growth in homogeneous solder joints of Sn-Bi eutectic alloy and Sn-Bi-Ag alloys formed with electroless nickel-immersion gold (ENIG) and Cu-organic surface protection (Cu-OSP) surface finishes. Experimental observations revealed that, during solid state annealing following reflow, the 50nm Au from the ENIG surface finish catalyzed rapid (Ni,Au)Sn4 intermetallic growth at the Ni-solder interface in both Sn-Bi and Sn-Bi-Ag homogeneous joints, which led to significant solder joint embrittlement during fatigue testing. Intermetallic growth of (Ni,Au)Sn4 was decreased by Ag alloying of eutectic Sn-Bi solder and was completely eliminated by changing the metallization from ENIG to Cu-OSP on the board side of the assembly. The reduction in (Ni,Au)Sn4 growth rate with Ag additions is attributed to changes in grain boundary wetting of the IMC by Bi with Ag alloying.