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88 The PCB Magazine • August 2016 A very thin layer of nano copper ink is created on polyimide by coating techniques. A picture of a coating made in a roll-to-roll (R2R) manner on a slot die coating machine is shown at left in Figure 2. After drying the ink (done by air impingement here), a film of cop- per nanoparticles in dispersant/binder of about 0.5−1.0 microns thick is produced. This film can be turned into a continuous film of conductive copper metal by photonic sintering with a flash lamp system. There have been developed special R2R machines with moving conveyors, multi- ple flash bulbs, and algorithms to control and interleave flashes to give uniform sintering of the coating[2]. On the other hand, the lab scale coatings are made by using an automatic Mayer bar coater (Figure 2, right), dried in a vacuum oven, and sintered by a single flash lamp unit it the lab. The resulting copper films are less uni - form than the ones generated by the R2R pro- cess, and banding where the flashes overlap is usually observed. Alternatively, a uniform con- ductive copper film can be generated by sinter- ing by immersing in a formic acid/argon reduc- ing atmosphere at 190°C for an hour (Figure 3). This conductive layer can be plated in an electroplating bath to give copper anywhere from 3−50 microns of copper. As stated earlier, FACTORS AFFECTING THE ADHESION OF THIN FILM COPPER ON POLYIMIDE Figure 1: A process for preparing very thin copper foil on polyimide. Figure 2: Two ways to coat uniform layers of nano copper ink: a) roll-to-roll coating; b) auto- matic Mayer bar coater. Figure 3: A uniformly sintered Mayer bar coating of nano copper ink on an epoxy BT rigid substrate. Figure 4: A profilometry graph of an etched trace (a), and cross section photographs of plated (b) and etched (c) traces.