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Traction-separation relations can be used to represent the adhesive interactions of a bimaterial interface during fracture. In this paper, a direct method is proposed to determine mixed-mode tractionseparation relations based on a combination of global and local measurements including load-displacement, crack extension, crack tip opening displacement, and fracture resistance curves. Mixed-mode interfacial fracture experiments were conducted using the end loaded split (ELS) configuration for a silicon-epoxy interface, where the epoxy thickness was used to control the phase angle of the fracture mode-mix. Infrared crack opening interferometry was used to measure the normal crack opening displacements, while both normal and shear components of the crack-tip opening displacements were obtained by digital image correlation. For the resistance curves, an approximate value of the J-integral was calculated based on a beamon-elastic-foundation model that referenced the measured load-displacement data.sive zone model with mixed-mode traction-separation relations was then adopted in finite element analyses, with the interfacial properties determined directly from the experiments. With the mode-I fracture toughness from a previous study, the model was used to predict mixed-mode fracture of the silicon/epoxy interfaces for phase angles ranging from −42 • to 0 • . Results from experiments using ELS specimens with phase angles that differed from those employed in parameter extraction were used to validate the model. Additional measurements would be necessary to further extend the reach of the model to mode-II dominant conditions.

Interfacial delamination has been a major reliability issue for both BEoL and packaging systems. The failure is often due to poor adhesion of interfaces. Thus characterization of interfacial properties is critical for material selection and process control. Conventional methods for interfacial adhesion and fracture toughness measurements are generally based on linear elastic fracture mechanics. More detailed local measurements are required to fully characterize the interfaces based on a nonlinear cohesive interface model. With the experimentally determined interfacial properties, cohesive interface modeling can be set up to predict the initiation and evolution of interfacial failure in chip-package systems. In this study, two model systems are considered by approaches of both linear elastic fracture mechanics (LEFM) and cohesive interface modeling (CIM). First, for a brittle thin film on a compliant substrate, the initiation and propagation of delamination from the root of a channel crack is simulated. The effects of the cohesive strength and fracture toughness of the interface on channel cracking of thin films on compliant substrates are analyzed. Second, a four-point bend test is considered, in comparison with experimental measurements of the local crack opening displacements.

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