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90 The PCB Magazine • August 2016 difference in particle morphology, i.e., less par- ticle necking and densification in the dark area. The result indicates that the sintering condi- tion of the nano copper ink is important to the adhesion. This also points out the criticality of using a flash apparatus specifically designed to minimize such banding. When the coated and dried ink is treated with the flash lamp system, the binder/disper- sant decomposes and flashes off allowing the copper nanoparticles to sinter via necking. Va- pors from this process can be observed on sinter- ing. However, electron micrographic evidence suggests that some binder is left at the copper/ substrate interface of a nano copper film plated up to ~5 µm thick. As the plated film was peeled off, the deformed polymer fibers (by the peel force) were left on both nano copper (Figures 7a, 7b) and the substrate (Figure 7c). Further- more, the polymer layer left on the substrate (visible by the naked eye) can be wiped off by good solvents but not by bad solvents of the binder. The results clearly indicate the presence of binder/dispersant at the end of the sintering and plating process (data not shown). The results suggest that the dispersant/bind- er is playing an important role as an adhesive layer at the interface, therefore affecting the ad- hesion. To investigate the binder effect, we ex- amined a number of binders of different chemi- cal structures and different solvent systems for their ability to disperse the nano copper and to FACTORS AFFECTING THE ADHESION OF THIN FILM COPPER ON POLYIMIDE Figure 6: SEM images of the copper nanoparticles on copper colored area (left) and darker band (right) of a coated ink sample sintered by repetitive flashes. Figure 7: SEM images of gold sputtered samples of a sintered and plated copper film, peeled from the substrate: (a) top view of the peeled film; (b) tilted view of the peeled film; (c) top view of the substrate underneath.