Designing With Tighter Tolerances


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David Ledger-Thomas is a PCB design engineer with Honeywell Aerospace. He’s spent decades designing PCBs for a variety of applications, including defense, aerospace, computers, and high-performance audio. I asked David to share some of his thoughts on designing high-tech boards with increasingly finer spaces, traces and pitch. 

Andy Shaughnessy: What are the tightest tolerances you are currently designing?

David Ledger-Thomas: Out of the gate, the assumption for “tight tolerances” is referring to trace and space width. That would be 4.5 mil trace width and 4.5 mil trace spacing. But to add for conversation, tightest tolerance may also reference how much trouble is the PCB fabricator going to have with a particular design (i.e., layer count in a specified thickness, drill size aspect ratio, annular ring, cost, schedule, etc.) while still meeting customer requirements. The PCB design team must also take into account some design parameters such as mechanical/electrical performance, manufacturability, reliability and schedule. So all these could be a part of the “tight tolerance” vocabulary.

Shaughnessy: What are the most challenging issues designers face regarding fine spaces, traces, and pitch?

Ledger-Thomas: Signal integrity would weigh heavy on the application of fine spaces, traces and pitch. Reliability would also be a factor, as in, will the board perform over temperature with a very low mean time between failures (MTBF). And of course can the board be fabricated to required specifications repeatedly and in a timely manner. If a fabricator had to manufacture 20 boards to get one good one, then that would be a poor performer for cost, time and reliability (even though the design was great in the PCB design tool).

Shaughnessy: Does your PCB design tool handle tighter tolerances well? What about PCB design tools in general?

Ledger-Thomas: That’s a somewhat “loaded” question. To answer straight out, yes. Here’s the caveat: Some PCB design tools do handle very tight tolerances well and some do not. So the “right tool for the job” rule applies, i.e., expecting a free/web-based PCB tool to design an HDI board would be rather troublesome. But remember that these are “design tools.” Once the “right tool for the job” is selected, then, as with any other tool, it is the applied knowledge of the user that provides the level of results and quality for the design activity.

Now for the fine print. The PCB tool of choice may have issues with handling data of a tight tolerance board with generating outputs for fabrication/assembly. Meaning the as-designed PCB tool board data is not represented correctly in the fabrication/assembly outputs. Due to rounding issues, the Gerber format/ODB++/data output translation from the PCB tool may not have been correlated as expected. Or there may be other not-so-nice nuances of the PCB tool.

Another point would be the preferred/selected fabricator has not been informed/contacted (or received their feedback) as to the specific tight tolerance implementation. This means that the tight design constraint “as designed/specified” in the PCB tool could cause a resultant of fabrication issues.

To read this entire article, which appeared in the June 2016 issue of The PCB Design Magazine, click here.

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