Role of the Insulating Fillers in the Encapsulation Material on the Lateral Charge Spreading in HV-ICs

High electric fields and temperatures in high-voltage ICs (HV-ICs) can induce charge transport phenomena in the encapsulation material leading to reliability test failures. In this paper, the resistivity of epoxy-based resins with insulating microfiller weight fraction exceeding 70% has been experimentally and theoretically investigated for the first time. Electrical conductivity has been measured at high temperature (150°C) using both dielectric spectroscopy analysis on bulk samples and charge-spreading characterizations on a dedicated test chip with integrated charge sensors. The use of a charge sensor close to the internal HV metallization leads to results more pertinent with the active area of HV-ICs. Remarkably, both experiments show an unexpected increase and a significant variability of the electrical conductivity as the microfiller fraction is increased. The strong correlation between bulk and lateral experiments clearly indicates that those features should be attributed to the bulk material. Numerical simulations of diffusion phenomenon in mold structures with random arrangements of spherical microfillers demonstrate that the conductivity increase with filler content can be ascribed to the role of the epoxy/filler interfaces.