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50 DESIGN007 MAGAZINE I DECEMBER 2024 Roughening the Copper Once the board has passed inspection, we begin the solder mask process by slightly roughing up the board's copper. is is to promote adhesion and create a good bond between the copper on the sur- face and the solder mask. We use three different methods to rough up the copper, each with distinct advantages and disadvantages. Designers should consider which is best for their designs. A mechanical scrub, depending on how aggressively it is performed, can create gouges, troughs, and rivers that impact performance for boards requiring high-speed signals. For most boards, a mechanical scrub will not cre- ate issues and is a common, cost-effective method for roughening copper. e second method involves running the panels through a slurry of a so pumice mate- rial that can also create pits and gouges. is process does not have as much impact on the trace performance as a mechanical scrub, and it creates great adhesion. is method has a downside for man- ufacturers because the pum- ice becomes a hazardous waste that requires extra care upon disposal. e last method is an adhesion promoter that chemically roughens the surface. is is the least likely to create perfor- mance-impacting abnor- malities on the PCBs. Designers can communi- cate their method preferences by consulting with their CAM engineer and calling it out with thor- ough design notes in the design specifications. Solder Mask Application ere are two common methods for apply- ing solder mask. Most of us think of the clas- sic flood-coating the board with liquid solder mask and removing it in the areas we want to remain solderable. However, additive printing works like an inkjet printer. It is cool, but for this discussion, we will focus on the tried-and- true methodology. ere are a couple of different ways to apply solder mask to panels. Screening of the sol- der mask involves placing the boards in a tight mesh screen contraption—a rectangle with a wood or metal frame with the mesh stretched across it. We use a squeegee to apply the solder mask by pushing it through the mesh onto the board's surface with a uniform thickness. With this process, we coat every square inch of the panel that can contain a circuit board, effectively flood-coating the entire side. To ensure we leave holes where the solder goes, we use a liquid photo-imageable solder mask (LPI) that allows us to transfer the solder mask file onto the board and make the solderable areas visible. Aer thermally drying (a fancy way of saying baking) the panel to remove some solvents used, we transfer the image onto the board. We have two methods for doing this: photolithography and laser direct imaging (LDI). e first method invol- ves creating a film with all the board compo- nents such as solder- able surfaces, through- holes, pads, and traces blacked out. We then run the boards through a developing process that removes the solder mask everywhere else. LDI achi- eves the same result by "mark- ing" the LPI that needs to be removed during exposure to the laser and protecting the solderable areas from it. Aer running through the chemical develop- ment process, we have a board that is the famil- iar green (or other color of choice) with the pads exposed. At this point, we have applied the solder mask and exposed it, but before we