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102 PCB007 MAGAZINE I DECEMBER 2018 Figure 2 shows a schematic of poor adhesion leading to resist lifting. As a result, the poor adhesion leads to the etching away of copper. Figure 3 illustrates a typical process for treat- ing the rough and smooth sides of the ED foil. In a continuous reel-to-reel process, the foil un- winds 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 in a second step with more ductile copper. This sequence is re- peated, creating dendrites on top of dendrites. A zinc coating—or a brass coating, alterna- tively—is then deposited, followed by the en- capsulation with silane coupling agent, which forms strong bonds with the resin. On the smooth (drum) side, a very thin zinc (nickel) barrier is depos- ited, followed by a chro- mate and zinc coating. The nickel barrier pre- vents the formation of copper and chrome inter- metallics 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 low yields. Great progress has been made in controlling the conversion coating. The de- sirable stain-proofing properties are balanced against the ability to remove a fair portion of the chromium layer with a simple process step such as treatment with 10% sulfuric acid. One should note that chromate levels may vary for any number of reasons. There is no correct chromate thickness or amount on the surface of the copper foil. Regardless, remov- ing the chromate from the foil surface is the first step to enhancing photoresist adhesion. The chromate conversion coating serves as Figure 2: Inner layer showing poor resist adhesion (L) and result after etching (R). Figure 3: Manufacturing of ED copper foil. (Source: Oak-Mitsui Inc.)

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