Ferrites in Transfer Molded Power SiPs – Challenges in Packaging

Abstract

Abstract Transfer-Molding-Process is enjoying growing interest when aiming for novel high-power density System-in-Packages (Power SiPs), where not only transistors and diodes, but also drivers, passives, coils and transformers are supposed to be integrated in one package. Encapsulating modules in a Transfer-Molding-Process induces higher mechanical load onto module components compared to conventional silicone potting. Previous investigations have shown, that integration of delicate components as ferrite cores into molded packages is not as trivial as integration of conventional Surface Mount Devices (SMDs) or power semiconductors; the brittle ferrites tend to fracture during the encapsulation process, resulting in higher ferrite core loss. The present study aims to identify main root causes for ferrite core cracking during manufacturing of molded Power SiPs. The test vehicle is a symmetrical Printed Circuit Board (PCB) based package with three pairs of E-shaped ferrite cores. The Epoxy Molding Compound (EMC) deployed here is characterized to enable filling simulations. Since technical datasheets of ferrites typically lack specifications of mechanical properties, ferrite materials are analyzed in more detail. Filling simulations and thermo-mechanical simulations are performed in order to gain insight into process-induced stress, which may induce cracks in the ferrites. In addition, different ferrite designs are evaluated regarding core losses and mechanical stability, and thus their tendency to fracture.