Quantitative characterization of underfill/substrate interfacial toughness enhancement by silane additives

The apparent fracture toughness of the interfaces between epoxy-based underfill materials and various substrates is experimentally measured. A small amount of silane coupling agents is added to the base underfill to form various underfill derivatives. Double layer specimens with preexisting interfacial cracks are made for four-point bending tests. The substrates tested include aluminum, polyimide, BCB, and printed wiring boards (FR-4) with solder mask. The measurements are qualitatively correlated to each silane additive. Although the purpose of adding silane additives was to enhance the adhesion, it is found that the enhancement of interfacial toughness depends 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.