Application of simulation of plastics in the development of power modules

Plastics are used as the material for the housing of power modules. The most obvious function of this part is the electrical insulation as well as protecting the chips from the environmental pollution (e.g. dust). The housing is also designed to carry and distribute mechanical loads, for the positioning of the pins and bus bars and for containing the silicon gel until it is hardened. The housing also provides a connection point for the circuit board. Plastics are chosen due to the fact, that their properties such as moldability, stiffness, strength, and chemical resistance can be optimized by the correct choice of the basis material, the production process and by the inclusion of additives. The material behavior of the plastics depends not only on the temperature or current load state, but also on production method, load profile, age and environmental conditions such as humidity that the plastic is subjected to. The difference in mechanical behavior of the housing frequently results in an additional stress on the metallic components of the module. Improving the design of the plastic parts can reduce this load on these other components. Simulation of the plastic housing can help to predict module behavior in the relevant tests, under application conditions, and can simplify and accelerate the module development to achieve a better mechanical behavior. In the presented paper, potential weak points of the simulation strategies for the plastic material have been addressed. No matter which material model is used for simulation of the plastic (elastic, viscoelastic or viscoplastic), it is usually described as a homogeneous and an isotropic material. The material properties of the plastics, published in different sources like scientific papers or material data sheets, have been evaluated from testing specimens. In practice the more complex forms show inhomogeneous anisotropic behavior, even with pure and unreinforced plastics. This situation is amplified by the use of the short fibre reinforced plastics. Here, the complete mechanical behavior of the housing is influenced by the fiber orientation resulting from the injection molding process as well as from the housing geometry and positions of the metallic inserts. It also depends on position of weld lines, which are considered as the weak points of any plastic component. If the anisotropy and inhomogenity of the plastic part is taken into account, a better prediction of the critical areas of the chosen part can be achieved.