Study of Raw Materials Effect on the Dielectric Properties of Epoxy Molding Compound for High Voltage Power Devices

There is a strong growth of high voltage power semiconductor devices due to increasing applications in various end use industries such as automotive, industrial, renewable energy, and more. Some of the new applications that are driving the growth of high voltage power semiconductor devices are electric and hybrid electric vehicles and industrial robotics. Power devices in these advanced applications operate at high power (high voltage and/or high current) and are subjected to high ambient temperature. Epoxy molding compound (EMC) has been widely used as the main material for encapsulation and protection of these power semiconductor devices due to its superior properties such as high mechanical strength, high electrical and chemical resistance, excellent adhesion and cost effectiveness. EMC material plays an important role in the quality and reliability performance of semiconductor devices. An efficient high voltage power semiconductor device package is required to have low energy loss, good heat dissipation, and high reliability. HTRB (High Temperature Reliability bias) is one of the key reliability test for power devices that evaluates deterioration of switching performance, i.e. break down voltage, after high temperature storage with high voltage bias. Electric stability of EMC often affects to HTRB reliability. Dielectric properties, i.e. dielectric constant (Dk) and dielectric loss factor (Df), is evaluated as a substitute characteristic of electronic stability of EMC during HTRB process. EMC with good dielectric properties i.e. low Dk and low Df is required for encapsulation of high voltage power devices to minimize the electrical energy loss. EMC is a composite of many ingredients mainly epoxy resin, phenol hardener, silica filler, catalyst, wax, coupling agent and other additives. In this study, we have evaluated the effect of several types of each key ingredient on the dielectric properties (Dk/Df) of EMC. Dielectric properties were tested at high temperature (up to 200°C) and low frequency (100Hz). Capacitance measurement method was used to measure the Dk/Df. Base on evaluation results, we were able to identify some raw materials that resulted in significantly low Dk/Df. Furthermore, we selected some EMC with high Dk/Df and low Dk/Df and subjected them to HTRB test, key reliability test for power devices. For HTRB testing, EMC was molded on TO package and the device used was MOS-FET with rated voltage of 500V. Stress parameters used for HTRB testing were 150°C and 450V and the duration was 1000hrs. It was found out that the EMC with low Dk/Df results in good HTRB performance with no shift in the breakdown voltage, while the EMC with high Dk/Df deteriorate the HTRB performance i.e. lowers the breakdown voltage. This study will show an index of EMC design for high voltage and high temperature application.