PCB007 Magazine

PCB-Feb2018

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76 PCB007 MAGAZINE I FEBRUARY 2018 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 perfect chromate thickness or amount on the surface of the copper foil. Regardless, removing the chromate from the foil surface is the first step to enhancing photoresist adhesion. The chromate conver- sion coating serves as an anti-tarnish to pre- serve it and slow down the copper oxidation process. The chromium phase is actually a contiguous, hydrated Cr(OH) 3 phase, with chromium predominantly at the oxidation state Cr+ 3 , with interspersed zinc. The degree of hydration is critical to the removal of this layer in acid. Typical chromium coverage would be about 5 mg/m 2 . Occasionally, suppliers also apply organic anti-tarnishes such as benzotriazole. It is de- batable why and to what extent these conver- sion coatings should be removed prior to lam- ination. In the case of chromate conversion coatings, there is ample evidence that most dry films do not adhere too well to such a surface. Also, in most innerlayer production processes, prelamination cleaning serves a dual purpose: the removal of chromate serves film adhesion but is also necessary to assure good oxide for- mation for multilayer bonding. In addition, failure to effectively remove chro- mates and other soils will lead to what we call differential etching. Essentially this means that during the surface roughening step (after chro - mate and soil removal) employing a microetch formulation, the roughening will be less than optimum due to incomplete chromate and oth- er soil removal. This in turn negatively impacts adhesion of resists. Understandably, in those ar- eas where organic soils and chromates remain, the microetch will have compromised ability to provide a uniformly structured surface. In-House Evaluation and Data Gathering Due to the importance of surface prepara- tion in printed circuit board manufacturing, the PCB fabricator should embark on an evalu- ation of current chemical and mechanical pro- cesses available. Some of the evaluations will require working closely with the dry film and chemical suppliers. When evaluating the effectiveness of surface preparation for primary imaging, there are sev- eral data points listed below that will influence the success or failure of dry film adhesion: • Surface profilometry with different microetches • Effect of chromate removing chemistry on surface cleanliness and topography • SEM analysis of treated copper surface (utilizing different microetch chemistries) • Film/mask peel tests after chemical treatment • Sidewall SEM analysis after development (comparing differing chemical prep methods) Examples of generic chemistry performance on copper surfaces are shown in Figure 4. Figure 4: Examples of microetched surfaces. Hydrogen peroxide/sulfuric acid microetch (left); oxone (middle); sodium persulfate (right).

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