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APRIL 2022 I DESIGN007 MAGAZINE 25 control of the trace/space and can offer tighter control of impedance variations. Now we just need the calculators out there to tackle these new geometries. When does it makes sense to switch to additive or semi-additive? When you can't get the signal trace density with the current process, or when you can't break out from that 3.5-mm or 3-mm pitch BGA. Don't go there if you only have one compo- nent at that pitch. Get a bigger com- ponent package instead. For DFM's sake, don't mix small-pitch parts with large power parts on the same side of the board. at will give any assembler nightmares and cost you extra. Also, consider switching when you have very flexible products, need a circuit on a different type of mate- rial (like aluminum), if you need to add copper onto a 3D-printed surface, or create an ultra-smooth conduction surface. Are my DFMs any different? Yes and no. ere are a few things that need to be consid- ered. Normal DFMs still apply. All the typical output files can be generated the same as you would for a subtractive process. You will need more notes and direction to your fabricator. You will need to work with a fabricator who has experience with additive or semi-additive processes, and you will need to look deeper at the layer stackup and component placement planning. I'm compiling a set of DFM guidelines now, and it's not quite done yet. I'll have more infor- mation about additive processes in my next article for Design007 Magazine. Get involved with additive design. As min- iaturization continues past the practical limits of subtractive etch, additive and semi-additive processes are likely to become more main- stream. DESIGN007 All images used in this article courtesy of Averatek. Cherie Litson, CID+, is the founder of Litson1 Consulting and an instructor at EPTAC and Everett Community College. Cherie has more than 30 years of design experience, and has been an instructor since 2003. Figure 4: A cross-section of an additive PCB. Figure 5: Designing additive and semi-additive PCBs is much like designing traditional PCBs, but with a few important twists.