Interface failure criterion of button shear test as a means of interface adhesion measurement in plastic packages

The button shear test is widely used to measure the interfacial bond strength between the molding compound and leadframe. In this study, the normal and shear stress fields along the interface for a button shear test specimen with axi-symmetric cylindrical shape are analyzed using the finite element method (FEM). There are stress singularities at the loaded edge for both the interface normal and shear stress components. The shear stress component varies little over the whole bond length, except for the free edge surface, whereas the normal stress component decreases consistently over the bond length from positive to negative values. Therefore, while the average normal stress increases as the loading position moves upward, the average shear stress remains almost constant, independent of loading position. The failure loads obtained for different loading positions in experiments are incorporated into the numerical results to derive the interface failure criterion based on the average stress concept. The interface shear strength was much higher than the interface normal strength, especially for bare copper, which showed 2.5 times difference in these interface strength values. This indicates that the leadframe surface roughness played a significant role in contributing to the interface bond strength. This is consistent with the predominant role of the normal stress component for interface debonding in the button shear test