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PCB007-Oct2020

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80 PCB007 MAGAZINE I OCTOBER 2020 Introduction One cannot underestimate the importance of surface preparation of the copper surface and its relationship to dry film adhesion. Further, the quality and dimensions of the circuit as it was designed depend heavily on the surface preparation of the copper foil. Certainly, we have witnessed a step-change in technology that has necessitated a total rethinking of how copper surfaces are cleaned and prepared to enhance photoresist adhesion. Several of these changes are driven by cost pressures leading to substrate construction. The need to handle thin core materials, as well as lower copper foil thicknesses, is driv- ing the move to alternative surface preparation techniques. And while many of these tech- niques are well-established, the drive to 5G, higher frequency transmission, and automo- tive changes—including crash avoidance and A Process Engineer's Guide to Surface Prep and Dry-Film Photoresist Adhesion autonomous driving—higher-density designs with sub-three-mil lines and spaces are becom- ing the order of the day. Thus, the first order of business is to prepare the surface for resist adhesion while reducing the amount of copper removal. In addition, designers prefer lower profile copper to effect improved signal integrity. In this context, low profile refers to the smoother, more uniform topography of the copper foil side facing the prepreg. All this leads to increased emphasis on the adhesion of the photoresist to the cop- per substrate. Further, thinner core materi- als are prone to distortion and cold-working with mechanical surface preparation methods. Cold-working a metal can lead to the creation of internal stress, which then is the cause of copper foil cracking. For this reason, chemical cleaning methods are favored over mechani- Trouble in Your Tank by Michael Carano, RBP CHEMICAL TECHNOLOGY

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