Boundary Stress
Adhesive bonds and composite structures experience localized mechanical forces acting parallel to the plane of contact between the joined materials. This interfacial shear arises due to differences in the elastic moduli or thermal expansion coefficients of the bonded substrates. It determines the ultimate load-bearing capacity and durability of the multi-material interface.
Deformation Cause
When a multi-layer sensor assembly is subjected to thermal cycling, the materials expand at different rates. This differential expansion generates high interfacial shear at the boundaries of the silicon die and the substrate. The resulting stress concentration can lead to delamination or mechanical shifts in the electrical characteristics of the sensing element.
In severe cases, these thermal stresses shear the bonded electrical connections, leading to intermittent signal losses or complete device failure.
Measurement Method
Die shear testing measures the force required to push a bonded chip off its substrate to calculate the stress at the point of failure. This evaluation quantifies the interfacial shear resistance of the adhesive or solder joint under controlled mechanical loads. The test is commonly performed during packaging qualification to verify bonding process consistency.
Mitigation Strategy
Selecting materials with matched coefficients of thermal expansion minimizes the magnitude of the generated stresses. Modifying the surface roughness of the substrates prior to bonding increases the mechanical interlocking and enhances resistance to interfacial shear failure. These design practices ensure the long-term mechanical reliability of the sensor assembly under harsh operational conditions.
