Interface Tension
Mechanical force concentrations measure the internal loading at material boundaries where coefficients of thermal expansion differ. These intensities are typically calculated using finite element analysis to identify areas prone to mechanical failure. Shear stress intensity describes the magnitude of the force trying to slide one material layer past another.
These loads are highest at the sharp corners of a silicon die or at the edges of a large solder joint. If the intensity surpasses the strength of the bond, the materials will separate.
Fracture Risk
Calculation of these values allows for the identification of areas prone to mechanical failure. High shear stress intensity often occurs during the cooling phase of the reflow process as the board and components shrink at different rates. The interface between the die attach material and the leadframe is a common site for failure.
Load Gradient
Distribution of these forces is not uniform across the surface of the component. The shear stress intensity increases toward the perimeter of the die and reaches a peak at the extreme corners. Softening the adhesive or changing the geometry of the pad can reduce these concentrations.
A balanced design minimizes the risk of package cracking during thermal cycling.
