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92 The PCB Magazine • August 2016 Surface cleaning and modification by physi- cal and chemical means is often used to promote adhesion. A commercially available polyimide substrate, which was invented to promote the adhesion to copper, has been tested. However, we could not find significant improvement in adhesion comparing to regular PI substrate (Ta- ble 3). It is argued that the stronger adhesion to copper can be achieved through chemical functionalization and physical roughening of polyimide surfaces, by plasma or alkaline treat- ment[5-7]. However, the nano copper film on the plasma treated PI (Figure 8a) did not show stronger adhesion after plating and etching, compared to the film on untreated PI (data not shown). Further investigation on the effect of potassium hydroxide (KOH) treatment is in progress. We have found so far that sintering con- dition and ink formulation (binder/solvent selection) have more impact on the adhesion of plated and etched films, while the surface modification of PI substrate does not seem to make much difference. We believe that in the system we use, the binder layer remain- ing at the interface after sintering is acting as an adhesive layer, and the adhesion failure happens between the nano copper layer and the binder layer. The surface modification of PI may promote the adhesion of the binder layer contacting with PI, but does not prevent the adhesion failure between nano copper layer and the binder layer. Sintering condition plays a complex role here, since it not only affects the connection between copper nano- particles, but also the decomposition and therefore the remaining binder in the final film. The fact that the sintered nano copper lay- er adheres well on PI but not after plating and etching makes us consider if the plating/etch- ing chemistry is causing the adhesion failure. The sintered copper layer is not entirely ho- mogeneous. As shown by the SEM image of a sintered trace (Figure 9a), there are voids in the sintered copper film. The sintered copper film which densifies as copper nanoparticles are connected by necking. At the same time, the voids between the nanoparticles join to form larger voids. This explains why the conductiv - ity of this sintered copper by this technique is about 10% that of pure copper wire. When plating, those voids are filled in by much more uniform pure copper as shown in Figure 9b. Since this image is from the substrate facing side of a trace peeled from the PI, the pres- ence of plated copper clearly indicates that the plating solution can get into the voids and all the way down to the PI substrate. If there is a negative effect of plating solution on the in- terface, the solution certainly has access to it. Strategies to minimize the impact of the plat- ing/etching chemistry, such as mixing binary nanoparticles to create denser packing, will be further investigated in the near future. FACTORS AFFECTING THE ADHESION OF THIN FILM COPPER ON POLYIMIDE Figure 8: Micrographs of coated nano copper films on (a) plasma treated PI; (b) PI with 5−7.5 nm of chromium coating; (c) PI with 7.5 nm of titanium coating and (d) PI with 7.5 nm of chromium/100 nm of copper coating. Table 3: Maximum peel strength of 1 mm wide peel test coupons on the plated and etched cop- per nano ink coatings on different PI substrates (N=2-3).