Effect of Solder Volume on Interfacial Reactions between Eutectic Sn-Pb and Sn-Ag-Cu Solders and Ni(P)-Au Surface Finish

In this study, interfacial reactions between eutectic Sn-Pb and Sn-Ag-Cu with different solder volumes and electroless nickel/immersion gold (ENIG) surface finish are investigated. Soldering on copper finish has also been investigated for reference. Three different solder ball sizes (300, 500 and 700 mum) were used on the substrate surface finish pads with a diameter of 380 mum. The effect of solid state thermal ageing is also examined, focusing on the type of intermetallic compound (IMC) formed, the IMC thickness and growth rates. The results revealed that the solder volume has a significant effect on the thickness of intermetallics formed at the solder joints. The intermetallic formed in the solder with smaller volume was observed to be thicker than that in the solder with larger volume. It was also found that the eutectic Sn-Pb solder forms a thicker intermetallic and a much greater intermetallic growth rate compared to the Sn-Ag-Cu solder. Examination of solder joints microstructures revealed that several types and morphologies of intermetallics formed after reflow soldering and solid state ageing. However, the solder bump volume has no effect on the intermetallics types or morphologies. For the eutectic Sn-Pb solder, mainly Ni₃Sn₄ and Ni₃Sn₂ intermetallics formed at the solder joints whereas in the Sn-Ag-Cu solder joints (Cu, Ni)₆Sn₅ and (Ni, Cu)₃Sn₄ were observed. The investigation also revealed that more Ag₃Sn intermetallics were observed just ahead of the solder joint interface and in the bulk of the Sn-Ag-Cu solder with the larger volume (solder ball with diameter of 700 mum).