Improved polyimide/metal adhesion by chemical modification approaches.

To increase their adhesion to silver, polyimide (PI) surfaces were modified by sulfonation reactions by means of different experimental procedures. The results of these modification reactions were analyzed in terms of the variation of the surface chemical composition examined by X-ray photoelectron spectroscopy. Long-time reactions were also followed by monitoring the increase in weight of the PI films. Changes in the total surface free energy were evaluated by contact angle measurements with the sessil drop method. The influence of the modification reactions on silver/PI adhesion was estimated by means of a 180° peeling test according to ASTM standards. Finally, the effect of chemical modification on film roughness was investigated by means of atomic force microscopy. The results obtained showed that a net improvement, that is, over the maximum rate according to the reference ASTM test, was attained via both the gaseous SO3 and concentrated H2SO4 procedures, with the latter procedure more effective in the presence of silver sulfate as a catalyst. However, the former was judged more convenient because of the low amount of reagents used and the simplicity of cleaning operations. Interestingly, optimum adhesion levels were attained after only very low contact times with all sulfonating agents, that is, after about 30 s. These results were compared with the effects on PI/silver adhesion of traditional etching procedures with KOH aqueous solutions at different temperatures. In this case, good adhesion was observed only after longer reaction times, that is, on the order of 1-2 min in the case of 1M KOH and 1 h in the case of 0.1M KOH.