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JANUARY 2022 I PCB007 MAGAZINE 81 Again, one should consult the supplier's tech- nical datasheet for the proper use and indi- cations. From this writer's standpoint, the chemical cleaning process is more efficient and effective with at least two separate chem- ical steps—one as a chromate/soil remover and the second as a copper removal/copper microetchant. Basic Chemical Microetching Processes e fundamentals of chemical micro- etchants are quite simple: remove oxides from the surface and restructure the copper foil. e latter means to roughen or create a topography for the copper that enhances photoresist adhe- sion without excessive copper removal. ere are several key points to consider here. First, it is much more effective to create a uniform topography without excessive copper removal if the copper foil surface is already devoid of oils, soils, and chromates. us, the first step in the surface preparation process is to pro- vide a virgin surface so that the microetch 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 differen- tial or step-etch. e topography will exhibit areas of high peaks and low valleys that can promote resist lock-in. 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 conformation (Fig- ure 1). In this case, the resist never completely adheres to the copper in these areas. ere is a gap that allows for other chemicals to remove copper during the develop-etch-strip process. When other processes can 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 neckdowns in the cir- cuit traces. Figure 1: Poor photoresist conformation. (Source: IPC)