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APRIL 2024 I DESIGN007 MAGAZINE 65 Instead of relying on the prepreg to fill the etched gaps between layers, we pre-fill those gaps using an epoxy ink that is compatible with the PCB dielectric materials. is method involves screening the ink onto one layer at a time, baking, and then planarizing the ink to insure flatness. You then have to flip the panel over and repeat on the opposing side layer. Not only is this time-consuming but it also requires the use of expensive layer fill ink. at being said, it has allowed us to create some pretty intense heavy copper multilayer products in the past. Our record was 20 layers of 20-ounce copper. Design Considerations Knowing the methods used to create heavy copper PCBs, design considerations should now make more sense. e primary consider- ation will be the overall thickness. If the con- straint is too tight it could result in a much more expensive production method. is the easiest place to start to seek resolution since fabricators should be able to provide stackup consultation with just a proposed layer count, a description of each layer (e.g., power, ground, signal), minimum desired dielectric thickness, and maximum overall desired thickness. An aid to successful stackup design and mul- tilayer lamination will be copper retention— especially on signal layers. A good rule of thumb states that the more copper you retain in the design, the less prepreg you will need to ensure the filling of the etched areas. As such, we rec- ommend filling in all "blank" areas on inner lay- ers with dummy copper (or include a fabrica- tion note allowing the fabricator to do so with a minimum clearance to active areas) and retain- ing "dummy" pads in inner layer clearances. Silkscreen/Nomenclature Another frequent design flaw is the use of silkscreen wherein features traverse across Table 10: Extreme heavy Cu design and fabrication comparison chart