Designer’s Notebook

John Burkhert

Lessons learned the hard way.

I still remember a day back in the late ’80s when an electrical engineer invited me into his office and showed me a CAD PCB layout on his monitor. How cool would it be to do that? Well, now I know. Pretty cool, but frustrating at times as well. Placing and routing are the meat and potatoes of PCB design. (If you don’t like “meat,” think of your own metaphor.) There are other things to do, but this is what holds it all together.

The basic framework is built around two disciplines: mechanical and electrical engineering. The two main features are the components and, of course, the board. An intelligent set of library parts is essential to getting the placement off to a good start. Over the years, schematic capture has shifted from the PCB designer’s hands to those of the EE.

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Read more: Place and Route: The Art of PCB Design

John Burkhert

Or why aspect ratio rules all.

The humble via comes in many flavors. By connecting one layer of a conductor pattern to another, vias have connected the world. My career has depended on them as part of the hardware I used to design for others to use. A foundational innovation in electronics brought plated through-holes to the masses. As a leap forward from wire-wrap technology, multilayer printed circuit boards put a “mainframe” in each of our pockets almost overnight.

From the first plated through-hole to the latest, the trend is to support higher-density interconnect. The key driver in plating holes is the aspect ratio, the hole’s width relative to its depth. For a through-hole, the depth is the thickness of the PCB. Most reputable fabricators can handle a 10:1 ratio, such that a common 0.062" board thickness will require a minimum finished hole size 0.006" (FIGURE 1).

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John Burkhert

Design reviews can easily go off on tangents. Make sure you’re the one sharing your screen.

Every job eventually gets to tape-out day. But before that day comes, a lot of moving parts are wrangled into place. Even the simplest layout will require deliverables for assembly, including custom paste stencil and a bill of materials to associate the correct component for each location on the board. Along the way, a set of physical and electrical properties will be used to gauge line width and length, among other parameters (FIGURE 1). Getting the responsible parties to give guidance on the many assumptions made during layout is the point of the design review.

 

 

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DfM means design for money. If it can’t be built, that’s a waste.

Great ideas come together with great timing; what’s left is great execution. Flipping the switch that sets the factory in motion causes a few pain points. These “opportunities for improvement” will dictate your agenda down to the minute with all the little things that go wrong. Let’s say there is a factory downstairs from you. Further, the factory is doing slow and laborious rework on old printed circuit boards, aka PCBs. There is a solution! New PCBs. We’re going from P0 (zero) to P1 (one). That’s where we, the designers, come in.

At new PCB time, the first order of business is improving the electronics in some way. The fix could be better performance, lower cost, higher reliability and, in some cases, all the above. Venturing into wireless technology and gaining FCC approval to play in its allocated spectrum is no slam dunk. Beefing up the power grid is a typical step. A good power distribution network has been known to cover for otherwise iffy routing. Every engineer will have some considerations carried forward.

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