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Page 76 of 101

FEBRUARY 2018 I PCB007 MAGAZINE 77 As shown in Figure 4, the various gener- ic microetches impart stark differences in to- pography. One must take these differences into account when evaluating chemical clean processes and adhesion. In a future column, we will explore soldermask 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 preparation processes of the copper surfaces. The engineer will also need to finalize data including sup- porting documentation as to differences in the grain structures of different microetch formula- tions and their effect on the copper foils. Recognizing these differences, the engineer will provide recommendations as to which combination of cleaners and microetches pro- vides optimal surface profiles to meet current industry criteria, including but not limited to: 1. Adhesion performance for sub 5-mil lines and spaces for primary resist (Figure 5) 2. Ability to hold soldermask dams 3. Improve soldermask adhesion under various conditions including the ability to withstand lifting when exposed to ENIG and immersion tin plating processes 4. Documented yield improvements from customers willing to share data In the end, lines and spaces are only getting finer. Sub 4-mil lines and spaces are becom- ing mainstream. Simply relying on laser direct imaging will not mitigate issues related to less than optimal adhesion of film to the surface. Ensuring that the copper foil surface has been properly cleaned of soils and chromates is the first step in ensuring good photoresist adhe- sion. This is then followed by surface roughen- ing with chemistry to provide sufficient topog- raphy for the film to adhere. PCB007 Michael Carano is VP of technology and business development for RBP Chemical Technology. To reach Carano, or read past columns, click here. Figure 5: Sidewall of developed photoresist (20-micron line = 0.8 mil).

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