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82 PCB007 MAGAZINE I JANUARY 2022 With respect to microetchants, the two most commonly used are: • Persulfate based (sodium or potassium) • Hydrogen peroxide-sulfuric acid Persulfate-based processes tend to create a much more roughened topography than does hydrogen peroxide/sulfuric acid-based pro- cesses. However, when the chromate con- version coating is thoroughly removed, both etchants are effective. As shown in Figure 2, the different generic microetches impart stark differences in topography. One must take these differences into account when evaluating chemical clean processes and adhesion. e angular grain structure promotes suffi- cient adhesion of the resist to the copper sur- face. It is important to recognize that an overly roughened surface is detrimental to good resist adhesion as well. In general, a more uni- form surface roughness is beneficial for resist adhesion. Extremely rough and non-uniform surface profile leads to areas on the surface where the resists does not contact the copper surface. Summary "Making a soil soluble in a solvent." at is a simple but accurate definition of cleaning. In the case of copper foil surfaces, this suggests that organic soils, chromate anti-tarnish coat- ings, and oxides must be removed from the copper prior to microetching the foil. e for- mer is accomplished with acid cleaners con- taining mineral acids, surfactants, and other functional materials. Once a clean virgin cop- per surface is obtained, the fabricator is then able to increase the surface area of the foil with a chemical microetch. PCB007 References 1. U.S. Patent 3,853,716 2. U.S. patent 4,387,006 Michael Carano is VP of technol- ogy and business development for RBP Chemical Technology. To read past columns or contact Carano, click here. Figure 2: Hydrogen peroxide-sulfuric (top), oxone (middle), and sodium persulfate (bottom).