Enhanced moisture protection of electronic devices by ultra-thin polyimide films

Thin films of polyimide that exhibit enhanced resistance to moisture, fabricated using the Langmuir-Blodgett (LB) technique, are discussed. The adhesion strength of both LB and spin-coated films of several different polyimides, deposited on fused silica, has been measured by subjecting the films to steam or water, followed by a tape test, and monitoring changes in their ultraviolet spectra. The results demonstrate that LB films adhere better to fused silica than the spin-coated films. Measurements of the rate of water vapor transmission through Kapton sheet coated by a monolayer of any of the polyimides show that a monolayer forms a moisture barrier. On the basis of these results the electrical performance of polyimide films has been tested at 85°C and 85% humidity by measuring leakage current between conducting paths under 180-V bias, using samples coated with various combinations of LB and spin-coated films. Composite films consisting of a LB monolayer either underneath or on top of a thick, spin-coated film exhibit superior electrical performance to either type of film by itself