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Designer's Notebook

The designer’s (large) role to keep projects on track.

The restaurant industry has a saying, “Time to lean is time to clean.” The gist is that there is never a dull moment while the clock is ticking. Bearing in mind the importance of time, the PCB designer is often faced with the prospect of starting a layout before all the necessary data are on the table. A preliminary schematic and a rough outline are a step in the right direction but by no means the whole story.

That beginning may have been delayed while the schematic capture gets to a state where you have enough information to actually start the physical design segment. In the meantime, it’s always good to inquire about any new connectors or other components to get a jump on obtaining or creating the footprint for the library. These kinds of things are often left to the designer. Going to see the cognizant engineer – or at least chatting them up – will let them know you’re on the job and trying to push forward.

Electrical and mechanical engineers have a lot on their plate and can be spread thin. They can seem to have a high tolerance for risk when it comes to the schedule. That is, anything and everything can change at any time. The one exception is the tape-out date. This is the significant difference between a “waterfall schedule” and concurrent design.

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Planning for future updates can save customers a headache.

“Unplanned obsolescence” can happen to a PCB, so the printed circuit board designer must provide a few hooks to give the project a second chance.

Once upon a time, car stereos were almost universally interchangeable, so you could get a new, improved one off the shelf from your local specialty store. Upgrading car sound systems was my “side hustle” as a teenager. There were no how-to videos back then, but there are now.

That’s because it’s necessary! For my car, it starts with vents and the display screen at the top of the center stack and works through the HVAC control module. Then, you can pull the infotainment system out and hope it all clicks back into place afterwards. Who’s got time for that? (Life in the auto repair trade.)

No user-serviceable parts inside? Give me a break! The head unit is tough to get to, but the PCBs within the chassis use active components in quad-flatpack (QFP) packages rather than ball grid array (BGA) packages, which are more difficult, but certainly not impossible, to solder or desolder. The QFP’s perimeter pins are more accessible for the do-it-yourselfer. Functionally, these circuits provide entertainment to the driver, which is unrelated to safety, so a Class 2 PCB should be fine. Still, the BGA package is avoided as it doesn’t fit the high-reliability mindset of the auto industry. QFP devices permit more robust Class 3 circuit boards if the voice navigation aspect is considered a vital system and integrated with the audio.

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PCB designers can have many different career pathways.

The route a PCB designer takes through the job market can lead to a number of different outcomes.

A board designer comes in with a knowledge set that helps transform an abstract schematic into physical electronics. A lot of that has to do with knowing what happens downstream from the day the artwork was created. The steps involved with PCB fabrication and assembly are complex, even for the simplest of jobs.

Take a factory tour and notice the rooms full of different machinery. Material is cut to shape and drilled in a kinetic energy field that makes so much noise. Vast plating lines do serial dunking in different vats of bubbling hazards. These are the CAD data manifested in copper for the first time.

Another room is more Zen, with tons of pressure and high temperature being applied. It’s a slow process, and the presses are very expensive. This is usually a small factory’s bottleneck. Down the hall, a brightly lit room full of automated optical inspection equipment ensures compliance. What we do on the monitors plays out across the factory.

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Four important lessons gleaned over a three-decade design career.

There’s an old saying among test pilots: “Any landing that you can walk away from is a good landing.” They also know that there are old pilots, there are bold pilots, but no old, bold pilots – or so the saying goes. If you want to hang around as a PCB designer, you can only hope to walk away from your mistakes with your career intact. So, this is a chance to learn from my mistakes from 35 years of design work.

Going all the way back to the ’90s finds me in my first PCB design role. I had just taken an internal transfer to the commercial side of the business after a couple of years of feeding from the government trough. My manager on the mil-spec side, Merrill, was a father of a dozen children and was an all-around nice guy, perhaps a bit of a pushover.

Before applying for the transfer, I wanted to talk with Merrill, so I came up behind him and asked if he wanted to go to Armadillo Willy’s, a local barbecue place, for lunch. I didn’t see that he had a sandwich in his hand and was about to take the first bite. Instead, the sandwich hit the desk with a thunk, and we were off to the restaurant. Such was his dedication to his people.

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Accounting for the assembly process will put your design on the fast track.

Thought-provoking questions keep coming my way, and then it’s down the old rabbit hole. So it goes something like this: “How do we integrate so many different parts in such a small PCB area?” The answer is a little deeper than the geometry of Tetris, but that’s a good illustration of packing the available space. This, of course, starts with the CAD symbol library and manifests in the assembly yields at the factory. We have to connect those dots.

Once the PCB logic is sufficiently captured, placement studies can start. Pay attention to the spacing and orientation of components. The interrelationships of neighboring parts can affect the solderability of the overall PCB. The assemblers like to see a consistent rotation of the components and an even distribution across the board.

It’s unlikely that every device on the board will be able to meet that preference. The electrical performance is going to take priority in several cases, particularly with analog designs. That said, you can still pick an orientation that suits most components. The similarity will inform the manufacturing engineer how the board should travel along the placement machinery and guide the soldering process.

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The outline of a PCB can serve as more than a simple perimeter.

The perimeter of a PCB defines the extent of whatever electronics have to be packaged therein. The outline can also serve other functions.

Printed circuit boards come in many shapes and sizes. The first thing the outline gives us is the resulting routable area. The positional variation of each layer in the stackup requires us to compensate with a little pullback of the metal from the edge.

These days, pulling the metal back from the edge by 8 mils (0.2mm) is sufficient for most fabricators. I went to a PCB conference walking from booth to booth and asked all the fabricators what their minimum pull back from the edge would be for production quantities. A few of them, call it 20%, said they could plate the board to within 5 mils (0.127mm) of the edge. In a special case, we used lasers to define the edge and had metal just 2 mils (0.05mm) away.

The next increment is to plate right to the edge and wrap copper around to the other side. Edge plating is used in cases where we want to create a more complete Faraday cage around a circuit. It’s also possible to pass voltage and ground from the top to the bottom around the edge of the board or even using a slot within the outline of a board.

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