| Microelectromechanical systems (MEMS) and microelectronics devices and their packages contain many multimaterial interface corners. They are often an inevitable byproduct of many bonding and encapsulation technologies that are used for the microassembly and packaging of accelerometers, pressure, tactile and flow sensors, and micropumps. These multimaterial interface corners are severe stress raisers and are often sites of failure initiation. At present, no approach exists to predict failure at such interface corners; the development of such an approach has been identified as one of the key challenges for highly-reliable microelectronics packaging.;We propose an approach to characterize fracture initiation from such two- and three-dimensional bimaterial interface corners, using a combination of analytical, numerical and experimental techniques. The approach is based on the universal singular stress field that exists at an interface corner in the context of linear elasticity, the magnitude of which is scaled by the corresponding stress intensities. The basic idea is to correlate fracture initiation at an interface corner with critical values of the stress intensities. The approach is in the spirit of interface fracture mechanics, but is applicable to a different, and arguably more technologically important class of problems; specifically, when no cracks exist (or can be observed economically), and when the crack runs into one of the adherends instead of along the interface. In order to validate the approach, we designed and fabricated silicon/glass anodically bonded specimens with different interface corner geometries that are commonly used in the microsensor industry resulting from different silicon etching techniques. We also designed and fabricated aluminum/epoxy specimens with different interface surface finishes. From a rigorous analysis of the stress state at the interface corner (where fracture initiates), the corresponding stress intensities were determined for the applied loading and far-field geometry. Mechanical fracture testing of the specimens showed that, although failure stresses varied significantly with specimen size, the corresponding critical stress intensity for a given interface corner geometry and surface preparation was constant, thus validating its use to correlate fracture initiation in a universal manner. This approach shows promise for many practical applications where the structural integrity of bonds is critical to design for long-term reliability. |
