A study of moisture diffusion in plastic packaging

The absorption and desorption processes of moisture in plastic material were studied with experimental measurement and finite element (FE) simulation. The diffusion coefficient and the saturate concentration were determined by experiment and simulation results with Fick’s law. The water molecules inside the plastic material were chemically bonded with polymers by hydrogen bonds in the microholes formed by the polymer molecule chains. On the saturate concentration, the moisture density in the effective volumes was 100 times larger than the vapor density in standard state. However, it is only 8% of liquid water. The water inside the plastic material was in a liquid situation. The delamination and the delamination recovery of flip-chip packaging inspected by C-mode scanning acoustic microscropy (C-SAM) during a high-temperature and high-humidity accelerating test could be explained with the state change of the water in plastic material space. The delamination recovery resulted from the increase in content of liquid water. The bonding of water molecules and polymers reduced the adhesive strength at the interface between epoxy material and die, and the delamination on the interface was initiated. A comparison of three cases with noncoated film, top-side SiCx coated, and both-sides SiCx coated indicated that the delamination would occur when the moisture concentration was between 50% and 95% of the saturate concentration. During the reflow process, the low interface-adhesive strength and the high vapor pressure of the wet sample might cause popcorning of the plastic packaging. Popcorning could be predicted by simulation of the moisture concentration at the interface.