Electrochemical capacitance based method applied to epoxy molded devices

Sensors as devices can be considered the most advanced microelectronics systems built by connecting different types of microchips as micro-mechanical structure (MEMS) and Application Specific Integrated Circuit (ASIC). These Systems in Package (SiP) use mostly an epoxy resin material whose major reliability concern is the moisture absorption after humidity exposure so that the electrical interconnections between the dies become surrounded by a non-waterproof material. This article describes how the real resin morphology influences the electrical signals provided by a MEMS to an ASIC in an inertial sensor SiP. In particular, the study is focused on the pads area of the MEMS, where a large surface of polysilicon is in direct contact with the resin. As the epoxy structure close to the above regions may be affected by a nonhomogeneous distribution of fillers, the compound humidity sensitivity may locally increase and the electrical properties may be conditioned in the same way. The goal of this study is to define a suitable method to quantify and characterize the local change of the resin properties after moisture absorption. After deeply analyzing the results of the standard electronics measurements methods an electrochemical approach has been selected as the best appropriate one to match the physical reality.