Correlation of residual stress and adhesion on copper by the effect of chemical structure of polyimides for copper-clad laminates

BACKROUND: Polyimide films coated on copper are a potential new substrate for fabricating printed circuit boards; however, adhesion between the copper and polyimide films is often poor. The relations between residual stress and adhesion strength according to the development of molecular orientation of polyimide films with different chemical backbone structure coated on copper were studied. RESULTS: The effect of chemical structures on properties including the residual stress and the adhesion strength were widely investigated for four different polyimides. Diamine 4,4'-oxydianiline (ODA) and dianhydrides 1,2,4,5-benzenetetracarboxylic dianhydride (PMDA), 4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), 4,4'-oxydiphthalic anhydride (ODPA) and 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA) were used to synthesize polyimide. In an attempt to quantify the interaction of thermal mismatch with the polyimide films depending on various structures, residual stress experiments between polyimide film and Cu-Si wafer were carried out over a range of 25-400 degrees C using in situ thin film stress analysis. A universal test machine was used to conduct 180 degrees peel test (ASTM D903-98) of polyimide film from cooper foil. The residual stress on Cu-Si (100) wafer decreased in the order 6FDA-ODA > BTDA-ODA > ODPA-ODA > PMDA-ODA, and the interfacial adhesion strength decreased in the order BTDA-ODA (5 N mm(-2)) > ODPA-ODA > PMDA-ODA > 6FDA-ODA. The results may suggest that the morphological structure, degree of crystallinity of chain orientation and packing significantly relate to the residual stress and adhesion strength in polyimide films. Wide-angle X-ray diffraction was used for characterizing the molecular order and orientation and X-ray photoelectron spectroscopy was used for the analysis of components on copper after polyimide films were detached to confirm the existence of copper oxide chemical bonding and to measure the binding energy of elements on the copper surface. CONCLUSION: In this research, it is demonstrated that BTDA-ODA polyimide has a low residual stress to copper, good adhesion property, good thermal property and low dielectric constant. Therefore, BTDA-ODA would be expected to be a promising candidate for a two-layer copper-clad laminate. (C) 2007 Society of Chemical Industry