Wirebond Deformation During Molding of IC Packages

Abstract

During the transfer molding of IC packages, wirebonds are deformed by the action of flow-induced viscous forces acting along them. Excessive deformation of wirebonds could give rise to short circuits and bond pull-outs. In this paper, the deformation of gold wirebonds during transfer molding of IC packages is studied using the finite element method. Hitherto, only elastic deformation of wirebonds has been considered. In this paper, a more realistic elasto-plastic large-deflection model is employed. The gold wire is assumed to be made of a bilinear strain hardening material. It is shown that plastic deformation in the wirebond can occur even if the melt flowrate is not very high. However, wirebond deflection may still be within acceptable limits even though certain portions of the wirebond have yielded plastically. The deformation of parabolic wirebonds under the action of melt flow, both normal and parallel to the plane of the wirebond, is also studied. The melt flow within the cavity is simulated assuming creeping flow. Parametric studies of the effects of wirebond dimensions, namely bond height, span and wire diameter, on wirebond deformation are also carried out.