Fracture Behavior of IMCs at Cu/Pb-Free Solder Interface

Abstract

Since the adhesive properties of the Pb-free solder joints are significantly affected by the Cu–Sn intermetallic compounds (IMC) at the joint interface, fracture behavior of the IMCs induced by different factors were investigated. The indentation test on the Cu–Sn grains reveal that they fracture in a cleavage mode when the shear loading increases to a certain value, the fracture location depends on the shape and size of the Cu–Sn grain, and the shear strength is about 670 MPa. Serious stress and strain concentration occurs around the joint interface under tensile loadings, makes the shear stress applied on the protrudent Cu–Sn about 300–400 MPa, therefore fracture of the long-term reflowed joint interface usually occurs in the interfacial IMC layer. Due to the dislocation pile-up at the Cu/IMC interface, the IMC layers at the Cu/lead-free solder interface fracture shortly after yielding of the Cu substrate, resulting in cracking vertical to the joint interface. Yield of the Cu substrates can be regarded as the sufficient condition for fracture of the IMC layer, and the yield strength of the Cu substrate has dominative influence on the fracture behaviors.