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86 The PCB Magazine • August 2016 Factors Affecting the Adhesion of Thin Film Copper on Polyimide by David Ciufo, Hsin-Yi Tsai and Michael J. Carmody INTRINSIQ MATERIALS Abstract The use of copper foils laminated to poly- imide (PI) as flexible printed circuit board pre- cursor is a standard practice in the PCB industry. We have previously described[1] an approach to very thin copper laminates of coating uni- form layers of nano copper inks and converting them into conductive foils via photonic sinter- ing with a multi-bulb conveyor system, which is consistent with roll-to-roll manufacturing. The copper thickness of these foils can be aug- mented by electroplating. Very thin copper layers enable etching fine lines in the flexible circuit. These films must adhere tenaciously to the polyimide substrate. In this paper, we in- vestigate the factors which improve and inhibit adhesion. It was found that the ink composi- tion, photonic sintering conditions, substrate pretreatment, and the inclusion of layers (metal and organic) intermediate between the copper and the polyimide are important. Ink factors include the intensity of photonic sintering. Better sintering leads to better cohe- sive strength of the nano copper layer. The ink solvent and the dispersant used to suspend the nanoparticles are significant both for adhesion and the colloidal stability of the dispersion. Pretreatment of the substrate by plasma rough- ening did not improve adhesion. We describe the effects of chromium and nickel interlayers which are typically used in standard foil lami- nates. Finally, we describe the types of peel strength testing used to assess adhesion. Introduction The goal of our ultra-thin film project is to create a thin (1−2 micron) film of uniform cop- per on flexible polyimide for application to flex circuits. Ultra-thin films allow very narrow cop- per lines on patterns created by photolithogra- phy, plating, and etching. Figure 1 depicts the invented process. ARTICLE

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