Internal stress state measurements of the large molded electronic control units

In this paper internal stress state measurements in a large molded electronic control unit are discussed. Measurements were done during transfer molding process in a high volume production environment. In addition, the effect of the post mold cure (PMC) was investigated. Tests were done utilizing piezoresistive stress sensors and thin film strain gages. For some parts post mold cure was done in parallel to warpage measurements using TDM Insidix apparatus. In this case the stress and deformation measurements were conducted at the same time. It was observed that the initial warpage of the module before the PMC at room temperature was reduced by 40%. It is shown that the reduction of the deformation is caused by shifting of the glass transition temperature (T_g) to 180°C, which was for not fully cured sample at 125°C. As a result of increase of the T_g, the internal stresses at room temperature after the PMC were changed as well. Driven by the fact that the out-of-plane component of stress is affecting the measurement of the in-plane components of stress, numerical simulations were used in an iterative way to correlate the results of experiment. Presented algorithm improved results of the measurements.