Mechanically relevant chemical shrinkage of epoxy molding compounds

Abstract

One of the most prominent failure modes in microelectronics devices is the delamination of epoxy materials (adhesives, molding compounds). The thermal mismatch at the interface between materials leads to stresses that build up during processing steps at different temperatures and in the following thermal cycling through use of the device or reliability testing. These stresses are well understood and are commonly investigated by finite element modeling. Epoxy molding compounds undergo a chemical reaction during processing called curing. Here the two components epoxy and hardener react to form a 3D network giving the molding compound its final material properties. During this process, the volume of the compound decreases, a phenomenon called cure shrinkage. The shrinkage itself can be experimentally determined, e.g. using volumetric measurements. However, due to relaxation processes that take place at higher temperatures and the changing thermal-mechanical properties during the curing process, the stresses that build up due to chemical shrinkage are more complex to consider. In this work, the mechanically relevant cure shrinkage was investigated by a combination of experiments and finite clement simulations. Samples of molding compound on Cu-leadframe material were manufactured using standard procedures. Thermal expansion experiments were performed at several temperatures recording the warpage of the samples. To extract the mechanically relevant shrinkage FE-simulations were performed mimicking the process temperatures. The resulting data was evaluated and discussed with respect to: qualitative behaviour for five different molding compounds; qualitative agreement between simulation and experiment; error margins of simulation results with respect to material properties input data; and error margins of experimental data due to processing variations and experimental setup.