Interface design and thermal stresses in layered microelectronic assemblies

In microelectronic components thermal stresses arise from the thermal and elastic mismatch of the bonded materials. The incorporation of continuum mechanics descriptions of interfaces into the finite element method and the application of this approach is demonstrated here. The present paper uses a fracture process zone model to describe the non-linear behavior of interfaces. A cohesive strength and a cohesive energy characterize the interface with both material parameters being embedded in a traction-separation law. With the fracture process zone model cracking in bi-material strips under thermal loading is studied. Initiation and propagation of cracks is predicted in dependence of the applied temperature change. Cracking occurs at a critical temperature which depends not only on the property mismatch of the two solids but more strongly on the properties of the interface. Subsequent material separation is dominantly in shear. The crack growth rate is found to be very large initially but decreases to zero as the crack propagates. Based on these simulations the model shows a path towards increasing robustness in electronic packaging by appropriate interface design.