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)(3)Sn-4 and/or (Cu,Ni)(6)Sn-5 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 Sri 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 degrees 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.