A method for evaluating delamination between epoxy moulding compounds and different plated leadframes

Interfacial fracture toughness acting as true adhesion strength has been identified as a key factor in evaluating delamination problems of IC plastic packages. However, since most conventional adhesion tests are not configured to obtain this bi-material fracture toughness constant (only apparent adhesion strengths varying with specimen geometry and loading position), their results cannot be used for structural design and material selection purposes. This study employed an end notch flexure (ENF) test to determine, by means of finite element analysis (FEA), the true adhesion strength expressed in terms of resultant stress intensity factor of mixed mode I and mode II fracture. The ENF test was successfully applied to specimens composed of epoxy moulding compounds (EMCs) and 3 different leadframes such as bare Cu, Ni plated Cu and black oxide Cu for adhesion evaluation without any moisture preconditioning. The results indicated that most epoxy compounds adhering to black oxide Cu can achieve relatively high fracture toughness, but in opposite behaviour to Ni plated Cu. It has been verified that there is a strong correlation between these findings and the occurrence of package delamination in real cases. Indeed, for a specific IC package, the selection of plating materials for adhesion improvement may directly rely on the corresponding fracture toughness found, as the mechanical effect of thin plating material can be negligible.