Investigation on moisture diffusion in COB packaging [chip on board]

Abstract

The moisture diffusion in globtop material, globtop coated with SiNx film, globtop coated with silicone and globtop coated with SiNx plus silicone were measured by humidity sensors wire-bonded on FR4 boards in three different temperature/humidity environments. The experimental results were simulated by the finite element method and Fick diffusion law. The moisture diffusion coefficients were calculated to quantitatively compare various coatings' moisture-resistance. Our experimental and simulation results show that a double-layered coating with SiNx plus silicone has excellent moisture-resistance because it can not only smooth the steps on the PCB but also keep the good moisture-resistance of inorganic films.