Issue link: https://iconnect007.uberflip.com/i/1481876
68 PCB007 MAGAZINE I OCTOBER 2022 to resist lamination. A closer look at the cop- per surface will find several things that need attention: • Chromate conversion coating • Release agents • Fingerprints • Oils It goes without saying that surface prepara- tion is the cornerstone of the imaging process. (is is not to shortchange the importance of the photolithography process.) However, the optimum surface prior to photoresist lamina- tion provides several critical attributes: • Increases process latitude for the resist • Provides a stain-free surface • Removes organic surface contamination as well as the chromate conversion coating e overarching goal of surface preparation is two-fold: • Produce a surface free of soils, fingerprints, etc. • Provide a microroughened surface topography to enhance the adhesion of the resist In general, surface preparation is done to assure good adhesion of metal, dielectric, photoresist, or solder mask to the prepared surface, although avoiding excessive adhesion could also be the object. Take the example of surface preparation before dry film photoresist lamination and potential failure modes (Figure 1): • Failure to achieve good adhesion in a print-and-etch process will cause etchant attack under the resist and ultimately an "open" defect • Failure to achieve good adhesion in a plating process will cause tin/lead or tin underplating, ultimately lead- ing to "shorting" defects ("shorts") • Failure to achieve good release of unex- posed resist during development can cause etch retardation in a print-and-etch process, ultimately leading to shorts • Failure to achieve good release of unex- posed resist during development in a plating process can cause poor adhesion of the plated copper—causing peelers • Failure to achieve good release of the exposed resist during the print-and-etch process of inner layers can inhibit the uniform formation of the subsequent oxide or oxide alternative bond enhance- ment coating • Failure to achieve good release of exposed resist in the plating process can reduce the effectiveness of the final etchant—leading to excess copper The Importance of Uniform Surface Topography Why should we be concerned about a non- uniform topography? Quite simply, today's dry film resist technology relies on a clean and adequately roughened surface to promote the Figure 1: Improper surface preparation leading to excessive or insufficient adhesion of resist.