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74 PCB007 MAGAZINE I FEBRUARY 2018 Understanding Surface Preparation Issues To fully grasp some of the surface prepara- tion issues encountered in the PCB fabrication process, a short primer on the copper foil man- ufacturing process follows. This will enhance the reader's understanding of the composition and topography of standard electrodeposited (ED) foils and reverse-treated foils (drum-side treated foils). Copper foil is manufactured in a reel-to-reel continuous electrodeposition process (Figure 3). After the copper foil is formed the foil unwinds and passes through several treatments in a treater line. Copper dendrites are first grown on the rough side. These dendrites are brittle and need to be encapsulated with more ductile copper in a second step. This sequence is re- peated, creating dendrites on top of dendrites. A zinc coating, or alternatively a brass coating, is then deposited, followed by the encapsula- tion with silane coupling agent, which forms strong bonds with the resin. On the smooth (drum) side, a very thin zinc (nickel) barrier is deposited, followed by a chro- mate/zinc coating. The nickel barrier prevents the formation of copper/chr ome intermetal- lics that are difficult to remove with acid clean- ers. In the past, thickness and chemical com- position of chromium layers were not well-con- trolled and could lead to poor resist adhesion and lo w yields. Great progress has been made in controlling the conversion coating. The de - Figure 2: Innerlayer showing poor resist adhesion (left) and result after etching (right). Figure 3: Manufacturing of electrodeposited copper foil. (Source: Oak-Mitsui Copper Company)