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 silicon chip with metal interdigitated electrodes across a polyimide film. The sensor works with the dielectrometer to monitor changes of ionic conductivity due to the presence of free ions in the polymer. The permittivity (relative to the permittivity of vacuum) of the polymer was measured while it was curing or while it was absorbing moisture. This microdielectric sensor was applied here to compare the properties of globtop materials used in Chip-on-board products. Good sample preparation by cleaning senor surface with solvent was key to preparing defect free samples. The encapsulated sensor was cured for different curing time and placed inside temperature humidity chamber at 85 degreesC/85RH% for in-situ testing. The thickness of the glob top above the die was 100 mum. 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 [1,2], is in direct correlation to Fick's Law of Diffusion. Sensors encapsulated with voids or delamination in the globtop showed much faster rate of moisture uptake. This paper presents moisture diffusion rates of two globtop materials from different suppliers. 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 in this paper