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56 DESIGN007 MAGAZINE I OCTOBER 2024 is process beefs up the copper thick- ness of the outer layer copper features of the board to create the current capacity specified in the design. It is important to ensure that the channels created by the photoresist are deep enough to accommodate the amount of cop- per required. If they are not, copper will plate over the top of the resist, creating a mushroom at the top of the trace that can be harder to etch out from underneath and create overhanging copper that can break off, form a bridge, and cause shorts. Preparing Surfaces for Pattern Plating We ensure the copper surface is pristine so we can plate brand-new copper atop it. is involves a surface treatment process called micro-etching and running the board through a pattern plate pre-clean. We use a series of sul- furic acid rinses and micro-etches to remove any oxidation and impurities on the surface created during the imaging process. If residual developer solution or photoresist is present, the cleaning process should elimi- nate it and leave a clean copper surface. During this process, we are careful not to disturb the dry film we put in place or overclean the sur- face to where we have etched the copper out of the holes, which we invested time and effort in metallizing during the electroless copper process. Avoiding Overcleaning is balancing act involves ensuring the electroless copper deposit is thick enough to account for how much is removed during the pre-plating, pre-clean process. We use an SPC tool called an etch rate coupon to deter- mine how much copper we remove during the micro-etch process. at measurement quan- tifies the amount of copper being removed. Time in the bath and/or the bath temperature can be adjusted to target the desired amount of removal. However, etch rates are not necessarily con- stant. ey are influenced by temperature, con- centration, and copper loading. If you make a new solution of micro-etch without copper in the bath, it won't etch. Adding copper starts the chemical reaction. Etch rates vary day to day, even shi to shi, depending on how much use the chemicals see. ere are many variables in this process: the chemistry behavior, aggressiveness in etch, and changes in temperature or humidity on the production floor. e etch rate coupon helps manufacturers ensure they are removing cop- per within the appropriate window. Painting A Picture of the Plating Floor e size and layout of plating operations can vary by manufacturer. Our plating floor has six tanks, each holding 205 gallons of blue copper sulfate plating solution. e tanks have cop- per bars (cathodes) running down the mid- dle where the manufacturing panels are con- nected, and baskets filled with copper balls (anodes) on the outside of the cell. ose run to our rectifier—the electricity source that cre- ates the electrolytic plating cell. In the last step before the copper bath, we dip " We ensure the copper surface is pristine so we can plate brand-new copper atop it. "