Interfacial reactions between Pb-free solders and metallized substrate surfaces

Abstract

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) Cu/sub 3/Sn and Cu/sub 6/Sn/sub 5/ formed at the interface were identified, compared to Ni/sub 3/Sn/sub 4/ 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) /sub 6/Sn/sub 5/. It has been revealed that the growth of Cu/sub 6/Sn/sub 5/ was faster than that of (Cu,Ni) /sub 6/Sn/sub 5/ and Ni/sub 3/Sn/sub 4/, which indicates the introduction of Ni into the Cu/sub 6/Sn/sub 5/ 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) /sub 6/Sn/sub 5/ IMCs were needle- or facet-like, the Cu/sub 6/Sn/sub 5/ and Ni/sub 3/Sn/sub 4/ IMCs were facet-like and Ag/sub 3/Sn IMCs were plate- or pebble- or needle-like. In the solder, the Cu/sub 6/Sn/sub 5/ IMC had prism-like shape with some of them appearing hollow with the Ag/sub 3/Sn IMCs embedded inside. Kirkendall voids were found in the Cu/sub 3/Sn and Ni/sub 3/P layers, and the formation of Kirkendall voids in the Cu/sub 3/Sn layer is likely related to the immersion Ag surface finish.