Two common mistakes engineers make with sensors in PCB layout

Circuit board with passive components
Duane Benson has seen plenty of PCB layouts in his job at Screaming Circuits. Lucy, some designers have got some 'splainin to do. (Pixabay)

Even a brilliant design can run off the rails when it involves sensors deployed on a printed circuit board, as engineers familiar with the process know far too well.

Take Duane Benson, for example, who has been designing embedded systems for manufacturing for 15 years. While his formal title is director of marketing at Milwaukee Electronics, he has spent plenty of time on the manufacturing floor at Screaming Circuits, the company’s online prototype and on-demand manufacturing unit. And here he has pretty much seen it all.

“Dozens of jobs are going through the shop every day,” he said in an interview. “We see everything that’s done right and done wrong, and I’ve learned a lot along the way about what to do to avoid the little traps that are all too easy to fall into.”

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He’s willing to share all that hard-earned knowledge, too. Some of his tips are fodder for a company blog, which is shared with the industry at large: blog.screamsingcircuits.com

He will also present several of the most common mistakes (and how to work around them or avoid them altogether) at the Sensors Expo and Conference in San Jose, June 22-24, 2020.

That open-sharing philosophy hasn’t always been popular at tech companies, but today it is a hallmark of Screaming Circuits. “Our philosophy is that we are better off if everybody knows about the traps,” Benson explained.

His basic premise is that it’s far too easy to make mistakes in a PCB layout. And, while in many cases, the errors are small and often go undetected at the onset, they can have a huge effect on the manufacturability and reliability of a device. “Sensors are some of the most critical components and can be very sensitive to printed circuit board (PCB) errors,” he emphasized.

A big part of the problem is that many of today’s design engineers, while highly skilled at what they do, aren’t always well-versed in the final step of PCB layout. “These are engineers who can put together an incredible design, but who are not as proficient when it comes to the board layout,” he said. “An engineer trained in math and the complexity and details of circuit design may know a little about CAM software, but never actually used it to make a printed circuit board.” 

Another problem is the gradual erosion of specialists highly skilled at doing PCB layout work. In today’s multi-disciplinary, resource-constrained environment, the same person designing the circuit is often the same person doing the PCB layout, even if not fully trained to do it well.

A couple of common mistakes

One of the more common mistakes that engineers make with sensors in a PCB layout has to do with the location of vias (plated holes used to take signals from one PCB layer to another). “When space is limited, it’s pretty tempting to put that via in the middle of the component solder pad,” he said. But when soldered, the result can be a poor solder joint on one side of the board and a pool of solder on the other side. Capillary action of the via will draw the solder from the pad, where you want it, to the other side of the board where you don’t.” 

Fortunately, in small production volumes there are workarounds for this problem. “We may put heat-resistant tape on the backside and fill the via with solder,” he said.

Another solution is to put extra solder on the pad. Yet another fix is to fill the via holes with higher temperature solder. But all those fixes take time, cost money and put reliability at risk. And none of those solutions will work with small components like ball grid array (BGA) packaged parts or in a large-scale, volume production. Try to put vias in tiny BGA pads, and the ancient engineering adage comes into play: “When in doubt, throw it out and start over,” Benson said.

In this example, Benson said a designer probably doesn’t recognize it as a mistake from the get-go and even through error-checking software won’t perceive it to be one. “You can see that mistake with the naked eye and once it’s at an assembly plant like ours, it will get stopped,” he said. When an error is spotted, engineers must find out how to make it work, which can eat up take crucial time and derail the production timeline.

Another common mistake with sensors in a PCB layout relates to the component footprint, also called the land pattern, Benson said. The footprint describes the copper pattern on the board that the component will be soldered on to. It also defines the solder mask, the silk screen and the area where solder paste will be deposited. So many new components are released each year that CAD software can’t keep up and likely doesn’t have the exact footprint in its component library.

Precision sensors tend to have exacting layout requirements and the new or less popular components are often not represented in the CAD library. It’s tempting to pull out a footprint that is close, but not exact, to use. “But in many of these cases ‘close’ is not close enough,” Benson said. “If you do it wrong, the component won’t solder on properly or it just won’t work.”

At the Sensors Expo and Conference, Benson will be sharing his insights on other common mistakes, including layout problems that can induce noise into a circuit or otherwise change the electrical characteristics of a design. “Many sensors have very critical layout requirements,” Benson noted. “For high-frequency devices, keeping noise out is critical and manufacturing has a big hand in that.”

RELATED: The trouble with sensor interfaces

Similar presentations Benson has done in the past have drawn standing-room-only audiences. “When I first started these talks, I was surprised how popular the topic was,” he said. “We’re seeing more and more public sharing, and the open source hardware community is all about sharing. There are millions of components and designs, and there’s a big opportunity for more of this sharing. We’re all better off when we design better boards.”

For more information on Duane Benson’s session and all the others, visit the Sensors Expo & Conference website. The sessions and conference program run June 22-24 in San Jose. 

As a FierceElectronics reader, you are eligible to use the following promotion code to receive $100 off a conference pass or for free entry to the Expo Hall: FE100.

 

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