I’m having difficulties with bonding epoxy resin coated polyimide to other polyimide substrate.
The defect is local separations between those two surfaces (after heat press c-staging).
I’d like to mention that those separations appears as small air bubbles (under optical microscope).
Epoxies absorb moisture from the atmosphere and when heated liberate the moisture as steam, resulting in porosity. To prevent this, thoroughly dry adherends before bonding and also minimise exposure of the adhesive to humid environments, no more than 65% RH at 18C (65F) and 45%RH at 23C (about 75F).
Above is a snippet.