Interfacial reactions between Pb-free solders and metallized substrate surfaces

The interfacial reactions and the resultant intermetallics between lead free solders, i.e., Sn-3.8Ag-0.7Cu, Sn-3.5Ag and Sn-0.7Cu, and substrates with different metallisation, namely, Cu, electroless Ni (EN), immersion Ag on Cu (CuImAg) and electroless Ni immersion gold (ENIG), were investigated after multiple reflows and subsequent ageing. For Cu and CuImAg substrates, the intermetallic compounds (IMCs) Cu₃Sn and Cu₆Sn₅ formed at the interface were identified, compared to Ni₃Sn₄ IMC in the case of EN or ENIG substrates reacting with Sn-3.5Ag. However, for EN or ENIG substrates, when using Sn-3.8Ag-0.7Cu and Sn-0.7Cu solders, the IMCs formed at the interface was (Cu,Ni) ₆Sn₅. It has been revealed that the growth of Cu₆Sn₅ was faster than that of (Cu,Ni) ₆Sn₅ and Ni₃Sn₄, which indicates the introduction of Ni into the Cu₆Sn₅ IMC may provide an optimised solution to enable the stability of the interfaces formed during soldering. As for the morphology of the IMCs, when they formed at the interface, the (Cu,Ni) ₆Sn₅ IMCs were needle- or facet-like, the Cu₆Sn₅ and Ni₃Sn₄ IMCs were facet-like and Ag₃Sn IMCs were plate- or pebble- or needle-like. In the solder, the Cu₆Sn₅ IMC had prism-like shape with some of them appearing hollow with the Ag₃Sn IMCs embedded inside. Kirkendall voids were found in the Cu₃Sn and Ni₃P layers, and the formation of Kirkendall voids in the Cu₃Sn layer is likely related to the immersion Ag surface finish.