Delamination modeling for power packages by the cohesive zone approach

Ensuring reliability of interfaces of dissimilar materials is one of the most critical design aspects of electronic assemblies. A study on the interface delamination phenomenon of different power packages was undertaken. Thermally induced stresses caused by the thermal mismatch between molding compound (MC), Si-die, and Cu- heatsink act as combined shear and normal loadings on the respective interfaces. Interface fracture- or damage mechanics was implemented to avoid the difficulties linked with the singular stress field at bi-material interface edges and to allow for inclusion of different interface adhesion parameter as well as damage progress. The cohesive zone approach was adopted for these purposes. A parametric study with different adhesion properties was performed to determine the critical interface energies causing delamination of different power packages. It turned out that delamination failure is not an issue for good adhesion properties, but depends on the package type. For lower critical fracture energies, delamination sensitive areas were found at the sawing edge of the die and at the MC-heatsink interface. Delamination onset was calculated based on a scalar damage metrics based on the critical interface energies. Delamination progress was followed and the delaminated areas were compared with experimental analyses. It turned out that at the critical interfaces MC-heatsink different types of delamination growth can occur which are stable and unstable delamination growth.