PCB007 Magazine

PCB007-Dec2018

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DECEMBER 2018 I PCB007 MAGAZINE 103 an anti-tarnish to preserve it and slow down the copper oxidation process. The chromium phase is a contiguous, hydrated Cr(OH)3 phase with chromium predominantly at the oxida- tion state Cr+++ 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/m2. 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 before lam- ination. In the case of chromate conversion coatings, there is ample evidence that most dry films do not adhere well to such a surface. Also, in most inner layer production process- es, prelamination cleaning serves a dual pur- pose: to remove chromate to help film adhe- sion and ensure good oxide formation for mul- tilayer bonding. In addition, failure to remove chromates and other soils effectively will lead to differ- ential etching. This means that during the sur- face roughening step (after chromate and soil removal), by employing a microetch formu- lation, the roughening will be less than opti- mum due to incomplete chromate and other soil removal. This negatively impacts the adhe- sion of resists. In areas where organic soils and chromates remain, the microetchant will have a compromised ability to provide a uniformly structured surface. In-house Evaluation and Data Gathering Due to the importance of surface preparation in PCB manufacturing, the fabricator should embark on an evaluation of current chemical and mechanical processes 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, at a mini- mum, the following data points must be mea- sured: 1. Surface profiliometry with different microetchants 2. Effect of chromate removing chemistry on surface cleanliness and topography 3. SEM analysis of treated copper surface utilizing different microetch chemistries 4. Film and mask peel tests after chemical treatment 5. Sidewall SEM analysis after development comparing differing chemical preparation methods Examples of generic chemistry performance on copper surfaces are shown in Figure 4. The different generic microetches impart stark dif- ferences in topography. One must consider these differences when evaluating chemical clean processes and adhesion. In a future column, I will explore solder mask adhesion issues. This is critical due to the aggressive nature of several final finishes including ENIG and immersion tin. Desired Outcomes The overarching task for the fabricator is to optimize the cleaning and surface prepara- tion processes of the copper surfaces. The en- gineer will also need to finalize data including supporting documentation as to differences in grain structures of different microetch formula- tions and their effect on the copper foils. Figure 4: Hydrogen peroxide-sulfuric (L), oxone (middle), and sodium persulphate (R).

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