Moisture diffusion monitoring in globtop encapsulant with microdielectric sensors

Dielectric analysis (DEA) can be an invaluable tool for characterizing the curing properties of polymers used in the electronics industry. It is also useful for moisture diffusion studies of polymer encapsulants. Its main advantage is that it can monitor cure rate or moisture uptake in-situ. The sensor used in this study is a Si chip with metal interdigitated electrodes across a polyimide film. The sensor works with the dielectrometer to monitor changes in ionic conductivity due to the presence of free ions in the polymer. The polymer permittivity was measured while it was curing or absorbing moisture. This microdielectric sensor was applied to compare the properties of globtop materials used in COB products. Good sample preparation by cleaning the sensor surface with solvent was key to preparing defect free samples. The encapsulated sensor was cured for different curing times and placed inside a temperature humidity chamber at 85/spl deg/C/85 RH% for in-situ testing. The glob top thickness above the die was 100 /spl mu/m. The results showed an increase in permittivity with moisture uptake. The magnitude of the change in permittivity after equilibration was also related to the total amount of water uptake. This curve, as has been shown by Day et al (1992), is in direct correlation to Fick's law of diffusion. Sensors encapsulated with voids or delamination in the globtop showed much faster rates of moisture uptake. This paper presents moisture diffusion rates of two globtop materials. The effects of variations in curing time (cure state) and presence of microvoiding on water diffusion rate and changes in amount of moisture uptake are also shown.