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78 PCB007 MAGAZINE I JANUARY 2022 Surface Preparation—The Foundation of the Photoresist Imaging Process Introduction e photoimaging process is one of the first steps in the PCB fabrication process. To ensure that the image of the circuitry conforms as close to the desired design as possible (i.e., lines and spaces), surface preparation of the copper foil surface is one of the most critical success factors. Employing the optimum mix of surface cleaners and microetchants will pro- vide a clean surface with sufficient surface area to promote dry film adhesion. e fabricator has numerous options and should determine the optimum process by accounting for the type of copper foil used as well as the classes of soils to be removed. More on copper foil types in a future column. One may call the sur- face preparation pro- c e s s t h e f o u n d a - tion of the imag- i n g o p e r a t i o n . Surface prepara- tion is critical for resist performance and increases the pro- cess latitude of the resist lam- ination, exposure, and development pro- cesses yet to come. Reasonable yields at 4 mil lines and spaces are no longer the norm in today's high density and ultra-high density intercon- nect technology. e need to support advanced packaging and IC substrate production is push- ing the envelope on the imaging process. The Options ere are several options available. In addi- tion to pumice and aluminum oxide surface preparation, chemical cleaning as a means to ensure optimum photoresist adhesion has gained significant popularity. In this case, only chemical processes such as acid cleaners and microetchants are employed. However, chem- ical cleaning is more than simply utilizing microetchants to restructure the copper sur- face. First the chromate conversion coating must be dealt with. Chromate Conversion Coating All copper foil and/or laminate produc- ers process the foil through an anti-tarnish treatment to prevent oxi- dation of the copper surface. is treat- ment is based on c h r o m i c a c i d . e chromic acid treatment pro - vides a hydrated c h r o m a t e f i l m o n t h e c o p p e r that prevents cop- per oxidation. While preventing oxidation is necessary during storage, the chromate must be removed prior to microetching. Failure to remove the film completely will lead to dif- ferential or step-etch during the microetch- ing process. e step etch will leave the cop- Trouble in Your Tank by Michael Carano, RBP CHEMICAL TECHNOLOGY

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