Intermetallic compound growth suppression at high temperature in SAC solders with Zn addition on Cu and Ni-P substrates

The effect of adding 0.5-1.5 wt.% Zn to Sn-3.8Ag-0.7Cu (SAC) solder alloy during reflow and solid state ageing has been investigated. In particular, the role of the Zn addition in suppressing interfacial Intermetallic Compound (IMC) growth on Cu and Ni-P substrates has been determined. Solder-substrate couples were aged at 150 °C and 185 °C for 1000 h. In the case of 0.5-1.0 wt.% Zn on Cu substrate, Cu₃Sn IMC was significantly suppressed and the morphology of Cu₆Sn₅ grains was changed, leading to suppressed Cu₆Sn₅ growth. In the SAC-1.5Zn/Cu substrate system a Cu₅Zn₈ IMC layer nucleated at the interface followed by massive spalling of the layer into the solder, forming a barrier layer limiting Cu₆Sn₅ growth. On Ni-P substrates the (Cu,Ni)₆Sn₅ IMC growth rate was suppressed, the lowest growth rate being found in the SAC-1.5Zn/Ni-P system. In all cases the added Zn segregated to the interfacial IMCs so that Cu₆Sn₅ became (Cu,Zn)₆Sn₅ and (Cu,Ni)₆Sn₅ became (Ni,Cu,Zn)₆Sn₅. The effect of Zn concentration on undercooling, wetting angles and IMC composition changes during ageing are also tabulated, and a method of incorporating Zn into the solder during reflow without compromising solder paste reflow described.