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achieving fine lines and spaces: part 3 continues In the past, tarnish resistance was accomplished by immersion of the copper foil into a solution containing chromate ions. Yates and other[1] further improved upon this method with an electrolytic technique to enhance the oxidation resistance of the copper foil. Still, others improved upon this invention further with the introduction of zinc chromate[2]. One should never underestimate the tenacity of the chromate film. This is precisely why I recommend a strong mineral acid cleaning step prior to pumice, aluminum oxide or chemical microetching. It is much more effective to enhance the resist adhesion when a good chromate removal process is online prior to these additional processes. Chemical Cleaning and Micro-Etching First, a review of various chemical cleaning methods is warranted. It is well known that the definition of cleaning is "making the soil soluble in a solvent." I don't remember who is responsible for this quote, but it is something I have not forgotten. Basically, one should under- Table 1: Chemical Cleaners and Microetchants. Source: Idea for table from IPC document 740 (Process Effects) 66 The PCB Magazine • January 2014