Simulation and experimental analysis of substrate overmolding

This paper presents new experimental and numerical methods to characterize the transfer overmolding of substrates with epoxy polymer. We investigated Multi Chip Modules on ceramic panels as well as on printed circuit boards encapsulated by a MAP-type molding process. Experiments show that the polymer flow during the overmolding process depends significantly on the mold height: While standard MAP-type mold cavities are filled homogeneously and symmetrically by the polymer, low cavity heights (<500 mum) can cause the flow front to concentrate on a few flow paths. We developed a numerical method to describe this inhomogeneous polymer flow. The reason for this unusual behavior seems to be the shear thinning of epoxy molding compounds: Within preferred flow paths the polymer shear rate is increased, resulting in reduced viscosity which enforces a necking on distinct flow paths. Inhomogeneous flow behavior can cause the formation of air traps and excessive wire sweep. We also developed a new experimental method to measure the pressure distribution within cavities: our sensor is based on commercially available, passive pressure sensitive films and is operational at temperatures up to 200deg.