An analytical model describing residual stresses in thin die attach layer

There is a strong drive to replace soft lead-based solders as die attach materials with environmentally friendly technologies such as copper and silver-based sintering. The main challenge hampering the widespread acceptance of sinter technology over solder has been its reliability with respect to thermomechanical loading caused by coefficient of thermal expansion (CTE) mismatch of die and lead frame. The residual stresses occur during package manufacturing i.e., sintering and moulding, and affect life cycle testing. Material properties of die attach materials are tuned to improve the package reliability through reducing residual stresses, limiting crack propagation and optimizing the material stiffness. A generic die attach material that can be universally applied is however elusive and estimations of their working envelop is imperative. 3D Finite Element (FE) simulations are the de facto standard to quantify the stresses and strains in package due to thermal loads. However, they require expertise in meshing, material model and applying the correct boundary conditions and they become computationally intensive with increasing complexity of packages. Also, understanding the effect of each individual parameter i.e, geometrical or material on package mechanical response under thermal loads is challenging due to unavailability of die attach material properties and its interaction with components in package. An alternative step is to build an analytical model for a package with a bare die bonded to a substrate or lead frame with a die attach material. This analytical model computes the warpage of package and, stresses and strains in die attach layer. Hence, this analytical model can be applied in the design phase of packages with bare die i.e., packages with simple geometry to avoid establishing FE simulations. In this work we present a description of analytical model providing an accurate approximation of residual stresses in the die attach layer. The analytical model takes into consideration the three coupled deformation modes of the package: (1) bending mode, (2) shear mode and (3) thermal dilatation. Comparison of the model is achieved through FE simulations and experiments showing good coincidence. With the model in place, sensitivity analysis was performed to study the influence of the die attach layer thickness and substrate thickness on the deformation modes, stresses and strains in die attach layer and warpage of package. To experimentally validate the model, packages are manufactured with different die attach layer thicknesses and substrate thicknesses. Warpage is the key quantitative parameter that can be measured through a non-destructive test in addition to the shear force that can be quantified through a destructive test.