Numerical analysis of fine lead bonding-effect of pad thickness on interfacial deformation

Deformation process of lead and pad during inner or middle lead bonding for packaging integrated circuit (IC) is simulated by a finite element method (FEM) in order to understand the effect of pad thickness on lead deformation behaviors, especially on the interfacial extension between lead and pad. The simulated bonding process was simplified so that we could easily understand the effect of pad thickness. Because of the simplification, we assumed that lead and pad were both made of gold and the cross section of lead was square. The mode of the interfacial deformation largely depends on the size ratio of δ₀/h ₀, where δ₀ is the pad thickness and H₀ is the lead height. The distributions of equivalent stress on the lead/pad and pad/substrate interfaces were also analyzed to discuss the bondability. It was suggested that the size ratio controls the position of the maximum stress at the pad/substrate interface which can damage silicon substrates