A study on the soldering reaction between Sn3Ag0.5Cu and electrolytic-Ni coated with a Au/Pd(P) bilayer surface finish

The Ni-P alloy [or Ni(P)] with a 6-13 wt.% P content is a very common surface finishing material used in microelectronic packages. When a high P-content Ni(P) finish (≃13 wt.%) was jointed to the Sn-based solders, there are several possible P-containing intermetallics, e.g., Ni₂SnP, Ni₂P, Ni₁₂P₅, and Ni₃P, can form at the interface. Among these intermetallics, the Ni₂SnP layer played a major role in intermetallic compound (IMC) spalling behavior that has become a serious reliability concern of solder joints. Very recently, we found that the Ni₂SnP can also form in a low P-content Ni(P) case (≃7 wt.%) if an additional Pd(P) layer was deposited above. It is hypothesized that the P element within the Ni₂SnP resulted from the separation of P atoms from the Pd(P) layer as the Pd was converted into PdSn₄ [or (Pd, Ni)Sn₄] during soldering. The remaining P then reacted with Ni and Sn, forming a Ni₂SnP layer at the interface. However, there is no direct proof to support this view. To simply the solder/Au/Pd(P)/Ni(P) interfacial reaction and to clarify the validity of the Ni₂SnP formation mechanism aforementioned, soldering reactions between a Sn3Ag0.5Cu (96.5 wt.% Sn -3 wt.% Ag -0.5 wt.% Cu) alloy and a Au/Pd(P)/electrolytic-Ni tri-layer structure were investigated in this study. The results of this study showed that a layer of P-containing compound grew at the interface immediately after the Pd was converted into (Pd, Ni)Sn₄. This verified the hypothesis made previously. A more detailed analysis on the reaction kinetics about the Sn3Ag0.5Cu alloy with the Au/Pd(P)/electrolytic-Ni, and the possible reliability concern arising from the Pd(P) layer will be presented in this study.