Adhesion issues in epoxy-based chip attach adhesives

This paper focuses on the effects of surface chemistry and processing conditions on the adhesive strength of epoxy-based chip attach adhesives on typical leadframe substrates. The surface chemistry is characterized using a three liquid probe method, which quantifies the polar and apolar contributions to the thermodynamic work of adhesion. Practical adhesion is quantified using interfacial fracture mechanics. The effect of processing conditions on adhesion is summarized using 'bondability diagrams,' which are modeled after moldability diagrams used in reaction injection molding processes. Preliminary bondability diagrams are given for several silver-filled, epoxy-based chip attach adhesives. These diagrams are constructed from a fundamental understanding of cure kinetics, degradation mechanisms, void phenomena, and contraction during cure. Verification of such diagrams is in progress and the agreement thus far is very satisfactory.