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PCB007-Oct2020

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82 PCB007 MAGAZINE I OCTOBER 2020 cal methods, as long as copper removal rates are reduced and excessive surface roughness is avoided. Getting to Optimum Resist Adhesion Let's review the task at hand. The objec- tives are simple. First, a clean, virgin copper surface is the first step to ensuring optimum adhesion. In addition, some surface restructur- ing is needed to provide anchoring points for the resist. The ideal surface should include: • Oxide-free • Moisture-free (moisture causes resist to flow into holes, leading to lock-in) • No pumice residues • No fingerprints • Complete chromate removal • High surface area but somewhat uniform • Water break test (holds film in water for 20–25 seconds) Figure 1 shows an example of fingerprints, and Figure 2 demonstrates severe oxidation. The oxidation can be blamed on allowing mois- ture to remain on the boards without proper drying, or a heated dry was too hot. Oxida- tion leads to resist lock-in, making developing, resist stripping, and final etching problematic. Fingerprints are simply caused by poor han- dling procedures. Keep in mind that rinsing after chemical pro- cessing steps is as important as the chemical processes themselves. Using counter-flow rins- es and maintaining the cleanliness of the rinse water will minimize ionic contaminants from negatively impacting resist adhesion. As stated earlier, oxidation will lead to resist lock-in. Fingerprints will prevent uniform ad- hesion of the film and will negatively impact surface preparation and final etching (Figure 3). The improper cleaning of panels, result- ing in residues (i.e., fingerprints) remain- ing on the panel surface after precleaning before resist application, leads to the issue seen in Figure 3. Improper handling is also a potential root cause. Figure 1: Example of fingerprints on copper. Figure 3: Example of oily fingerprint preventing complete etching of the copper. (Source: RBP Chemical Technology) Figure 2: Oxidation. (Source: RBP Chemical Technology)

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