Characterization of underfill/substrate interfacial toughness enhancement by silane additives

This work experimentally measures the apparent fracture toughness of the interfaces between epoxy-based underfill materials and various substrates including aluminum, polyimide, BCB, and printed wiring boards (FR-4) with solder mask. A small amount of silane coupling agent is added to the base underfill in order to form various underfill derivatives, and double layer specimens with preexisting interfacial cracks are prepared for four-point bending tests. The measurements are qualitatively correlated to each silane additive. The purpose of adding silane additives was to enhance the adhesion; the enhancement of interfacial toughness was found to strongly depend on the type of substrate. The results of this study have important implications in flip-chip reliability where interfacial cracking is one of the major failure modes.