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achieving fine lines and spaces: part 3 continues stand what composition the soils are and which solvent or solvents are best suited to remove those soils. Chemical compositions designed to remove soils are endless. As an example, Table 1 below provides a summary of those processes. One should also contact the chemical supplier for advice on proper operating parameters, equipment compatibility and costs. There are some suppliers that provide one step chromate remover/micro-etchants. Again, one should consult the supplier's technical data sheet for the proper use and indications. From my standpoint, the chemical cleaning process is more efficient and effective with at least two separate chemical steps—one as a chromate-soil remover and the second as a copper removal/ copper micro-etchant. conformation (Figure 1). In this case, the resist never completely adheres to the copper in these areas. Thus there is a gap that allows for other chemicals to remove copper during the develop-etch-strip process. When other processes are able to remove the copper that was designed to be protected by the resist, the consequence is an open circuit. At the very least one will experience neck downs in the circuit traces. With respect to micro-etchants, 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 processes. Basic Chemical Micro-etching Processes The basic fundamentals of chemical microThe angular grain structure (Figure 2) proetchants are quite simple: Remove oxides from motes sufficient adhesion of the resist to the the surface and restructure the copper foil. The copper surface. In Figure 3, the structure shown latter means to roughen or create a topography represents that of the copper foil that has been for the copper that enhances photoresist adhe- micro-etched by a hydrogen peroxide-sulfuric sion without excessive copper removal. There acid process. Note the differences in the strucare several key points to consider here. First, it ture of each. In each case, the targeted copper is much more effective to create a uniform to- removal was 40–50 micro-inches. The foil is pography without excessive copper removal if the copper foil surface is already devoid of oils, soils and chromates. Thus, the first step in the surface preparation process is to provide virgin surface so that the micro-etch can perform its function. When there are soils and chromates remaining on the surface, the micro-etch will create areas on the surface that, for lack of a better term, are referred to as differential or step etch. The topography will exhibit areas of high peaks and low valleys that can promote resist lockin. Conversely, if there are areas on the foil surface that have deep trenches in the foil due to differential etch, there are concerns with poor resist Figure 1: Poor photoresist conformation. (source: IPC) January 2014 • The PCB Magazine 67