Powerful Prototypes: An Open-source Adventure

I’m a marketer in my day job, but I’ve fully never let go of the technical aspects of this industry that I have such passion for. I still design electronics in my off-hours. Designing things as a part-time avocation is my version of “gardening to relax.” It also conveniently keeps me connected with customers and gives me some additional perspective on what engineers need when getting boards into an assembly factory and back out in working order.

The other quite handy aspect of me designing boards part-time is that I make mistakes. I've been at this job for nearly 15 years, and I have worked with electronics for considerably longer. I should know how to do this without errors, and yet I still always make some sort of mistake. Perhaps that is part of why I’m a marketing person rather than a full-time engineer, or perhaps not; the world may never know.

On the one hand, those mistakes are frustrating for me and the folks processing my orders. On the other hand, it gives us a close up look at how difficult getting things built really is out in the real world.

The Latest Project

My latest project is a motion-sensitive lapel pin for the Open Source Hardware Summit in New York City on March 13, 2020. The final version uses a Microchip PIC18F22K46 microcontroller (MCU) and an NXP MMA8452 accelerometer (both in small QFN packages) and will be 1.3” x 1.6” powered by a single CR2023 coin cell. I used 0402 LEDs and 0201 passives wherever possible.

Design Compromise

I wanted to make it as small as possible. I picked a CR2032 coin cell because, at 20 mm wide, I could keep the board width to one inch and still have a bit of clearance on each side. Unfortunately, the CR2032 surface mount battery holder uses two little wings out each side for soldering. The dimension with wings dictated the minimum width of the board of 1.3”. On the plus side, I used the extra space created on the front of the board to put my logo and that of Sunstone Circuits, who made the blank PCBs for me (Figure 1).

Duane_Benson_figure1.jpg 

Figure 1: Wider than an inch.

I have used most of this design before. It came from an electronic business card holder I made a while back. The accelerometer senses motion, waking up the MCU. The MCU flashes a pattern on the LEDs and then goes back to sleep. In this case, I decided to add a small MOSFET so that I could brighten and dim all of the LEDs at the same time wiring a pulse width modulation signal from the MCU. That would give me more options with the pattern, and I would be able to dim the 21 LEDs to save on battery power (Figure 2).

Duane_Benson_figure2.jpg 

Figure 2: MOSFET with footprint.

I had used the MCU and accelerometer before, so I had good footprints for both of those. I needed a footprint for the super tiny SMT switch. Sometimes, I make my own footprints, but I try not to if at all possible. It’s too easy to introduce errors in a footprint that will cause problems down the line. In this case, I downloaded the switch symbol and footprint from SnapEDA.

The First Mistake

Remember the MOSFET I mentioned earlier? I set up all of the LEDs with a common cathode and ran the cathode through the MOSFET to ground. As with everything else, I wanted to use the smallest parts possible. I used 5-mA LEDs and would be driving them at a partial duty cycle, so a really small MOSFET would do the trick. At 100% duty cycle, they would only be drawing 105 mA.

I found a logic level part by On Semiconductor—NTNS3193NZT5G with 224 mA drain-to-source current—which was plenty enough to do what I want with more than 50% headroom. It’s a very tiny part at just 0.62 x 0.62 mm in a three-pin land grid array (LGA) package.

Three pads can’t be too difficult, right? Well, it isn't difficult, but I ended up introducing one of those errors I mentioned. Fortunately, it still operates, but any of you who attend the conference and pick one up will be able to see my mistake. You’ll need a high-powered loupe or a microscope (as I said, the part is only 0.62 mm square).

The pads for the gate and source are each about ⅓ the side of the drain pad. Whenever you have a pad that is significantly larger or smaller than the other pad(s), expect trouble—especially at such small geometries. I made the large drain pad too small, so when in the reflow oven, there wasn’t enough area for the solder to spread out. The parts ended up tilted on one side. As I said, it still worked, but it looks ugly in a very miniature way.

The Second Mistake

That was with the 10 prototype boards. It caused even more problems when I had the 400 built to give away. I noticed the problem after I had sent in the higher volume order, but before it had been built. I assumed that since all 10 of the prototypes worked fine, it would be okay in production too. Not a good assumption.

Building 10 is not the same as building 400. On a high-speed, high-volume pick-and-place machine, the solder deposition is even more critical than on a slower prototype pick and place. The boards will all work in the end, but I caused a lot of unnecessary rework by making a mistake on that one small footprint. The smaller the part, the more critical it is to get all of the details right.

Conclusion

Those were not my only misadventures with this board, but the rest are stories for another day. Suffice it to say that, as a maker, I’m pretty lucky to have a professional electronics assembly factory (and a very friendly board fabrication company) at my disposal.

Duane Benson is marketing manager and CTO at Screaming Circuits.

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2020

Powerful Prototypes: An Open-source Adventure

02-26-2020

Duane Benson describes the latest board design project he has been working on in his off-hours—a motion-sensitive lapel pin—including compromises, mistakes, and lessons learned

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Powerful Prototypes: Five Technological Shifts in the New Decade

01-08-2020

Depending on your perspective, we are either starting the last year of the old decade or starting the first year of the new decade. But regardless of what you call the decade, a lot of change is in store. Duane Benson shares five of the more significant technological shifts directly ahead of us and how to respond.

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2019

Powerful Prototypes: Cost Reduction in Design

12-11-2019

Getting custom electronics manufactured is not cheap, fast, or easy. Fortunately, there are ways to keep costs down and yields up without adding cost. Duane Benson shares six ways to keep costs down and yields up that you can do without a lot of effort.

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Powerful Prototypes: New PCB Fab Technology—What You Need to Know

11-20-2019

Exotic materials have been around for a while, but being “exotic,” most of us could safely ignore them. However, as clock speeds increase, and board sizes decrease, some of those exotic materials are getting close to mainstream. Duane Benson shares some of the newest terminology you might see in your daily electronics adventures and will need to be familiar with when venturing beyond a standard, rigid FR-4 PCB.

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Powerful Prototypes: Panelization—What Is It and Why Would You Want It?

10-30-2019

We see orders for a single board, and we see orders for thousands. “A few thousand” falls way outside the realm of “prototype,” but in the startup and open-source worlds, the lines are blurred. Once you order more than about 50 boards, a few things change; for example, you should consider ordering your boards in a panel, also called arrays or a palette, of multiple boards.

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Powerful Prototypes: Never Assume—A DFM Story

09-19-2019

I write a lot of words about DFM and best practices for PCB layout. Working for a manufacturer, I regularly see the results of not taking DFM seriously. DFM is something never to be taken for granted at any point in the design cycle, and I mean at any point in the process.

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Powerful Prototypes: 5 Common Myths About Solder Mask

08-14-2019

Before parts are added, a typical PCB has four ingredients: substrate, metal, solder mask, and silkscreen. Solder mask, in particular, seems to be looked at as a great place to cut when costs are tight, but Duane Benson disagrees. Read on as he dispels five common myths about solder mask.

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Powerful Prototypes: The Ideal Bill of Materials

07-10-2019

A good portion of a quality electronics build is simply the result of clear information. Not long ago, I wrote about the set of files containing the information required by your manufacturing partner to ensure a quality build.

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Powerful Prototypes: Moisture Sensitivity—What’s the Risk, and What Can You Do About It?

04-18-2019

I recently traveled to New Orleans, Louisiana, for a week of beignets, fried food, and urban exploration. It’s a good thing that parts of the exploration came in the form of walking as some level of exercise was needed to compensate for the lack of greens and heavy emphasis on the word “fried” that went along with most of the food.

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Proper PCB Storage: Top Three Hazards

03-28-2019

Overall, our modern world could not exist without PCBs; they are everywhere, but they aren’t items to be taken for granted. Like most technology, PCBs need proper handling and storage. PCBs don’t last forever and are even more vulnerable before the parts are soldered on. The solderable metal surface is very thin and subject to a number of potential problems, especially if not stored properly.

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Seven PCB Cost-reduction Design Tips

02-11-2019

Like everything else in the modern world, design decisions can have a pretty big impact on your manufacturing cost. Here's a list of seven design decisions that can make your manufacturing more affordable.

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Eight PCB Assembly Tips for 2019

01-17-2019

It's now 2019, and all I'll say on the coming year is that we are in for a wild ride. The last few years have been pretty crazy, and 2019 looks to continue that trend but amped up. While predictions might be fun to muse upon, they really won't help you get your job done. So, here's my top eight pieces of PCB assembly advice for the coming year to make up for that.

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2018

What Is Your Supply Chain Telling You About Components?

12-24-2018

Right now, many, many parts are in short supply, or unavailable with extraordinarily long lead times. Allocation is the word of the day and substitutions are your friend. Sure, electronics components shortage happens every now and then in this industry. It's a periodic nuisance, but what should you do for the long term? Read on.

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Electronic Manufacturing Files: What We Need for PCB Assembly

12-07-2018

As PCB assemblers, manufacturing is all about taking data and delivering good working circuit boards. It can be just data, as in full turn-key, data plus some parts, or a partial turn-key or a kitted job. Regardless of whether you're sending parts and boards or having us buy everything, PCB assemblers need good data, and a lot of it.

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The Future of PCB Designs

11-28-2018

Duane Benson designed his first PCB using tape and etch-resist pens from RadioShack. He penciled the schematic on graph paper, drew the layout directly onto the single-sided copper-plated board, and then etched it. At the time, commercial PCB design wasn’t too different. In his column, he talks about the advancements in PCB design and the key considerations when designing boards.

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Top 5 Things to Know When Moving from Hand Assembly to Robotic Assembly

11-14-2018

A lot of factors go into the decision to hand-build or outsource circuit boards. When the decision is to outsource, there are a few important things to consider. Some things that work fine when hand soldering may stand in the way of quality, repeatability, and reliability when machine assembling. Here are some of the most important considerations when changing from hand-build to outsourced.

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Top 5 Ways to Mitigate PCB Component Availability Problems

11-07-2018

The electronics design world is by now aware that we're in a very serious period of components shortages. Allocation and shortages hit every few years, but this one seems to be the worst in recent memory. It could be a problem until 2020 and the supply chain and world of components manufactures will likely be a different animal coming out of it. Here are five things you can do to minimize the effects.

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