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22 DESIGN007 MAGAZINE I APRIL 2022 Feature Article by Cherie Litson LITSON1 CONSULTING With components getting smaller and elec- tronic devices becoming more compact, we are reaching the physical limits of the typical etched fabrication processes. To address these limits, new additive and semi-additive pro- cesses are being developed to fit into the cur- rent fabricators' production lines without too much disruption or extra cost. at leaves the design engineer with a few questions: Will additive and semi-additive pro- cesses really reduce layer count and sizes? Are there signal integrity and impedance advan- tages and disadvantages? When does it makes sense to switch to additive or semi-additive? Are my DFMs going to be any different? Answers to these questions and many more are still being developed. However, I've found a few answers that I'm happy to share with you. First, let's look at liquid metal ink. LMI is ultra-thin and ultra-dense, conforms to any 3D surface, works with different pure metals and their alloys (copper, gold, silver, palladium, platinum, etc.), and is non-aqueous, which enables low-cost manufacturing. Here are some fundamentals for these very small features. Figure 1 depicts some exam- ples of the additive processes used to create fine copper traces on a printed circuit board. One of the first things you'll notice are the shapes of the traces: ey are not trapezoidal. Figure 2 shows close-ups of traces created with additive processes. Will they really help to reduce layers? Yes, and you still need a good plan for your layer stackup and routing schemes. Everything is just a little different. When the traces get smaller, your dielectric gets thinner. So, it's best to use these either on the outerlayers (foil top designs) or on an inner core pair. ey're great for breakout of fine-pitch parts. Just be Designing Additive and Semi-Additive PCBs Figure 1: A variety of examples of additive processes.

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