Effect of Pd(P) thickness on the soldering reaction between Sn–3Ag–0.5Cu alloy and ultrathin-Ni(P)-type Au/Pd(P)/Ni(P)/Cu metallization pad

• Strong effect of Pd(P) thickness on a submicron-scale Ni(P) film in soldering.
• Reaction between Sn–3Ag–0.5Cu solder and Au/Pd(P)/Ni(P)/Cu multilayer.
• A thick Pd(P) film might induce a substantial depletion in the Ni(P)/Cu pad.
• Pd(P)-enhanced Ni2SnP growth.
• The Pd(P) thickness is a key factor for the reliability of solder joints.

The strong effect of the Pd(P) thickness (δPd(P)) on the interfacial reaction between Sn–3Ag–0.5Cu solder and ultrathin-Ni(P)-type Au/Pd(P)/Ni(P)/Cu pad was revealed. We observed that the P of the Pd(P) film would redeposit over the Ni(P) platings and formed layered Ni2SnP in the early stage of soldering reaction. An excessive growth of Ni2SnP inhibited the formation of layered (Cu,Ni)6Sn5, which is argued to be an effective diffusion barrier of Ni. Thus, an extremely fast depletion of the Ni(P) film accompanied by a substantial dissolution of the Cu pad might be induced for large δPd(P). These observations demonstrated that δPd(P) plays a dominant role in determining the interfacial microstructure of solder joints and that a precise control on the δPd(P) value is required for microelectronic packaging reliability.