Do you check your PCBs?

During my summer vacation in 1973, the Yom Kippur war broke out in the Middle East. I was at the end of 3rd year at the Technion, Israel Institute of Technology, and being a temporary resident meant that I was not called up to the army. Almost everyone I knew was – students and lecturers alike were all taken out of circulation. The next academic term was put on hold, pending everyone’s return. The military call-up also meant a depletion of the workforce as a whole, so I got a job in the documentation department of a subsidiary of Control Data located in Haifa. Control Data was renowned for their supercomputers, but we were working on a minicomputer.

All drawings were sketched and then transferred to the drafting department to hand drawn on vellum (a version of tracing paper). Stencils were used for the electronic symbols. Then a PCB layout specialist would lay out the board using dot and tape on a transparent mylar base. One of my jobs was to check that the resulting PCB matched the schematic exactly. Of course we didn’t work directly with originals. They were copied on a blueprint copying machine, a machine that involved ultraviolet light, lots of rollers and treated paper that needed ammonia to develop. Aah, the memories of that smell! It was pretty harsh on the actual layout because repeated copies would result in the fatigue of the physical design. Late in the development cycle there would be problems with tape lifting and resulting in shorts and open circuits. But I digress.

With a copy of both the schematic and the PCB I would check each and every line to track. It proved quite educational, as only the real world can. It remained a habit with me ever since then. Even when I designed and laid out my own boards I still checked them scrupulously. A one-man business making small volume production runs cannot afford too many mistakes.

Today, despite the layout tools check of netlists, a CAD design can include errors- normally as a result of human failing. But this is not the only thing I am checking for. I check the placement of the decoupling capacitors, clearances, heat-sinking, earth planes and more. I find the most important part is that it gives me the opportunity for sober second thought – is this the best way realise the design? Why aren’t these two nets connected? Should we be using two smaller boards rather than one big one? Potential cross coupling of signals and on and on. Can I reduce the number of gates by changing the layout, or simplify the layout by adding some gates or shuffling the I/O pins on the micro? Some of these were taken into consideration at the start or the process, but when it has made it onto paper (or a screen) you can evaluate the wisdom of your choices.

Some engineers simply plug the design into an autorouter and don’t even check for a trace that meanders around the board. Especially with multilayer boards when you get to the physical product, it is impossible check visually on the inner layers. In addition correcting those errors with jumpers is much more difficult. I maintain that a full check gives great benefit that increases the probability of getting it “right the first time”. The time lost in checking is recouped manifold when you don’t have to spend hours debugging.

Who’s with me?

19 thoughts on “Do you check your PCBs?

  1. “Layout is progressing to a “weird phase” right now, by that I mean seeing amazing products (like the I-phone), yet when I try to find a contract layout firm in the Northeast US (where I live and work), the overall skill level except for digital only des

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  2. “There are other aspects to this.nnSometimes it's necessary to check that the PCB fab has put the layers in the correct order. I remember a colleague spending several days trying to work out why a 2.4GHz radio design wasn't working as expected. Despite

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  3. “I agree. I do all the things that you do and I spend less time debugging. I also check part availability before I commit to using a part. I used to check net by net long ago from schematic to the PCB but it has been several years that I just compare th

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  4. “Gregorion”I also check part availability”. nYes. absolutely. Because sometimes it takes a while between the design and getting the PCB made/populated, I sometimes even purchase stock of critical components.”

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  5. “@Sensatechn “It is just do we have the time “nnUnfortunately it is true that many projects are subject to a time deadline. It is my contention that time spent before committing to a PCB design saves time spent afterwards, but a lot of engineers aren'

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  6. “Even on small boards, I've found layout checks pay off. Management sometimes pushes to get the prototype boards out before doing rigorous checks.nnIn the past, I've taken various colored highlighters to print outs of the schematic and all the pcb layer

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  7. “@Modaln ” I'd look at the footprints and insure they matched what I was expecting.” nnYES! I am often guilty of failing to do this, and have suffered the resulting problems. Never assume!nnAlso I often find Pin 1 markings of connectors and some oth

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  8. “The design went to production. When new boards arrived, we found that some wires were crossed, some signals had polarity reversed. Firmware came to the rescue.”

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  9. “Garbage in, Garbage out. Always start the review with the on controlling connectivity.nSchematics – review for obvious errorsnNetlist – English readable. Single node nets can be a dead give-away for things like LED0 and LEDO not being connected (and de

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  10. “When I was involved with making PCBs (I did schematics but not layouts) I always wrote a short program to convert the layout's net list into a sorted text file. I would always cross-check this text file against the schematic, marking each schematic pin w

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  11. “Whenever possible, I'll have a board review with the engineers and PCB layout to go over the board. The main things we check for are layout of Switching supplies, inadvertent “slots” in ground and power planes due to via placement, traces that meander a

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  12. “When do a pcb revision, I load both the old and the new gerbers into a viewer and overlay them one layer at a time. By assigning complementary colors to old and new, any differences pop out as a third color. It is easy to check that no unintentional chang

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  13. “We have to do this as well in another circumstance. For reasons that I don't understand, some of or Chinese subcontractors take it into their heads that they need to modify the board. Is it a track that offends their sensibilities or some perceived improv

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  14. “@ BetajetnGiven my history of working with Excel, the thought occurs that maybe you could work a similar approach using it. Perhaps it would easier to manipulate and order the text. nnI once made a spreadsheet that loaded a microcomputer listing and th

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  15. “@Elizabethn”Better to find that you need a series resistor before you build the board. “nnToo true.nnI must apologise that all my stuff is written from a perspective of a small organization. It must be nice to have others around who understand and

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  16. “Working in an assembly shop, I regularly see justification for extra time spent checking the PC board. In addition to nets and component location, footprints are very important too.nnMake sure that the part ordered matches the part in the BOM, which mat

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