The Influence of the Glass Transition Temperature of Epoxy Mold Compounds on the Reliability of a Semiconductor Device

Abstract:
Mold compounds used for chip encapsulation are an important part of discrete semiconductor packages. With increasing requirements such as high operational temperature, high voltage and increasing lifetime, the selection of the right mold compound is an important but challenging task. Thermomechanical properties of the mold compound, such as the storage modulus, the coefficient of thermal expansion (CTE) and the glass transition temperature (Tg), are key parameters in the selection. Various publications conclude that high Tg values (>>150 °C) are favorable or even required to ensure good device reliability. However, high Tg mold compounds can have several disadvantages such as lower temperature stability, higher water uptake and higher costs. For that reason, the influence of Tg on the overall package reliability is assessed in this publication by investigating 21 different materials with Tg values between 84 °C and 192 °C. Intrinsic material parameters such as weight loss at elevated temperature and humidity uptake have been measured. In addition, thermomechanical simulations were used to investigate the influence of Tg on the temperature dependent stress in the package. A package build up was carried out for some of the evaluated mold compounds to assess the performance at standard reliability conditions such as high temperature storage (HTS), temperature cycling (TC) and high temperature reverse bias tests (HTRB). The results of this study show that while the Tg of the mold compound has a big influence on the properties, the use of high Tg materials do not necessarily increase the package performance and can even be a disadvantage. Considering that high Tg materials are typically more expensive than their low Tg alternatives, the use of well selected low Tg materials can come along with better cost competitiveness, without negative impact on the reliability of the device.