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The Shaughnessy Report: Additive Design—The Same, but Different

When we decided to focus on designing for additive and semi-additive processes this month, we ran into a challenge much like the one we faced with the February issue of PCB007 Magazine: There aren’t too many experts on this subject, not to mention educational resources or agreed-upon best practices.

Additive and semi-additive design and manufacturing are in flux, and PCB designers in the additive world are basically making the rules as they go. As instructor Cherie Litson says in her feature article this month, when it comes to additive processes, there are far more questions than answers. Cherie is currently working on an additive design curriculum which she hopes to have ready to go this year. The industry certainly needs more additive design resources.

At recent trade shows, there was plenty of chatter about additive and semi-additive design techniques. Much of it is speculation, but the interest is there. If your company’s high-density boards have pushed traditional subtractive etch fabrication to its practical limits, you’re probably already researching additive and semi-additive processes.

Defense and aerospace contractors are very interested in additive. NASA is doing an additive happy dance, and so are consumer device makers who are on the leading edge of miniaturization, in the arena of ultra-HDI. If you need spaces and traces below 1 mil, additive is an attractive option.

It’s not that subtractive etch can’t go below 1 mil spaces and traces. Traditional fabrication processes can go below 1 mil, but the boards become burdensomely complex, which is going to cost you—in extra routing and via layers, and increased lamination cycles, which drive up cost and drive down reliability and yield.

Additive and semi-additive processes seem to be a better way, at least so far. Additive can produce spaces and traces 0.5 mils wide and smaller, and the laminate itself can be thinner, too. And the additive traces are beautiful, with straight walls—not trapezoidal. For 60 years, fabricators and designers have wondered what it would be like to have perfectly straight-walled traces, and now we can find out.

But there are plenty of potential pitfalls. In fact, we don’t even know what we don’t know, as Pentagon spokesmen like to say. How does a designer get involved in designing additive PCBs? What resources are available? When does it makes sense to use additive or semi-additive vs. traditional? How are signal integrity, crosstalk, and impedance affected by these new constructions? Where is the sweet spot? Do today’s EDA tools support additive and semi-additive?

This month’s issue of Design007 Magazine focuses on how readers can get started designing additive and semi-additive PCBs, and how the design process for additive technology differs from that of traditional subtractive processes.

In a conversation with Dave Torp, he discusses additive design techniques and why the design cycle is similar to traditional design, but in a different order. Cherie Litson walks us through the steps in additive and semi-additive design, including DFM and signal integrity considerations. Luca Gautero explains how solder mask is applied during additive processes, and how this affects the design rule check guidelines. Columnist Tara Dunn answers some of the many questions designers have about designing boards made with semi-additive (SAP) processes. Tomas Chester addresses designing PCBs with additive traces.

Columnist Barry Olney discusses a typical SAP design process, its benefits and challenges, and why SAP may be part of a natural manufacturing evolution as lines and spaces grow increasingly denser. Dave Wiens explains how today’s EDA tools will have to adapt to optimize the design cycle for additive and semi-additive PCBs, and Calumet’s Todd Brassard and Meredith LaBeau detail some of the hurdles facing designers who move into additive PCBs, and why a little competition here would help get this technology standardized.

We also have columns from our regular contributors Matt Stevenson, Martyn Gaudion, John Coonrod, Vern Solberg, Kelly Dack, Beth Turner, and Joe Fjelstad. Finally, we have a book review by Skyler Sopp of The Printed Circuit Designer’s Guide to… Stackups: The Design within the Design, and an article by Anaya Vardya.

Additive and semi-additive processes are microcosms of the industry—there’s a lot of innovation going on. And we’ll keep bringing you the design information you need to know. See you next month!

Click here to download the preview of this magazine. Download the PDF of the April 2022 issue of Design007 Magazine to your library.