Thermo-mechanical reliability during technology development of power chip-on-board assemblies with encapsulation

In this paper we examine the thermo-mechanical reliability of polymer-encapsulated chip-on-board (COB) assemblies for power applications by simulation and experiment. Thereby the focus is set on the low cycle fatigue failure behaviour of the die-attach material under thermal cycling conditions. As die-attach material different solder materials and Ag-filled thermal adhesives have been used. The encapsulation was performed with a soft silicone-based and hard silica-reinforced epoxy-based material, respectively. An other process variable takes into account die-tilt. The study was carried out as a feasibility analysis in the course of a COB technology development. To this end lifetime models have been employed to correlate crack growth in the, i.e. attach to a computational accumulative failure criterion which allows to consistently describe ad predict quantitatively the lifetime of the assemblies. Thereby a considerable influence of the encapsulation was found. In particular it could be shown that a hard encapsulation largely increases reliability for solder die-attach.