Effect of Intermetallics Spalling on the Mechanical Behavior of Electroless Ni(P)/Pb-free Solder Interconnection

A systematic investigation of mechanical testing was conducted to correlate the brittle fracture observed in Ni(P) metallization with Ni-Sn intermetallic spalling in Pb-free solder joints. To produce lap shear test samples, two FR4 PCBs finished with Ni(P)/Au were reflowed using two Pb-free solders, Sn-3.5Ag and Sn-3.0Ag-0.5Cu (in wt.%). Brittle fracture was found in the solder joints made of Sn-3.5Ag, while only ductile fracture was observed in the Cu-containing solder (Sn-3.0Ag-0.5Cu). For Sn-3.0Ag-0.5Cu joints, (Ni,Cu)₃Sn₄ and/or (Cu,Ni)₆Sn₅ IMCs were formed at the interface between the solder and Ni(P) film. For Sn-3.5Ag, Ni₃Sn₄ was formed and spalled off the Ni(P) film, causing brittle fracture in solder pads where Ni3Sn4 had spalled. From the analysis of fracture surfaces, it was found that the brittle fracture occurs through the Ni₃SnP layer The growth of the Ni₃SnP layer appears to be responsible for Ni3Sn4 spalling and thereby for the brittle fracture of Ni(P)/solder interconnection.

To prevent IMC spalling from the Ni(P) metallization, a thin intermediate layer of Sn or Cu was deposited by electrolytic or electroless plating method on the Ni(P) film. During the reflow reaction of Sn-3.5Ag solder paste, the intermediate layers effectively suppressed Ni₃Sn₄ spalling during the reflow reaction at 250°C for 30 min, while Ni₃Sn₄ easily spalled off the Ni(P) film in a few minutes in the control samples without an intermediate layer.