Twenty years ago, I sat at a bench at a new job. The products were timers and controllers, and one day, I grabbed some dead units and started to evaluate them. In looking at the schematics, I found a rather bizarre situation -- several 10K resisters were labeled R1. I thought that this might be due to some lazy CAD driver, and that they were probably hand drawn. At Tektronix, we refer to PCB layout drawings as dolly sheets. I looked at the dolly sheets, as well as the silkscreen on the board, and I was horrified. All components with the same value had the same nomenclature.
I talked to the designer, and he told me this was done to make the hand-stuffed-through-the-hole parts easier on the assemblers. I guess he was assuming a high yield. However, reality says human error creeps in on monotonous and repetitious jobs like stuffing PCBs by hand day after day.
I started to work on the boards, but I had to trace every node as if there was no documentation available. Thankfully, these were only two-sided boards. Since 10K is probably the most used value of resistors, R1s were all over the place.
I kept to myself on this, since I was a new guy. I didn't want to ruffle any feathers. Over several months, I did a lot of trace following. The only benefit was that I didn't have to check the bill of materials when a part was missing. It turned out the parts were binned as R1, R2, C1, etc., as well. So all I did was check the silkscreen and fetch the part. Neat, right? No way.
One thing that is missing in our industry is standards. But even with standards, some folks feel autonomous enough to do really goofy things. Stories like this make this field of endeavor rather entertaining.
This entry was submitted by Steve Lindberg and edited by Rob Spiegel.
Tell us your experiences with Monkey-designed products. Send stories to Rob Spiegel for Made by Monkeys.
Sounds like a good example of a non-technical person participating in the design. We would get crazy stuff like that from sales all of the time...what seems logical to others can really play havoc with electronics design. That is why a lot of distributors have an engineering staff - to keep their sales department from making promises they can't keep.
The other downside is that the assemblly line workers learn nothing! So when a decision has to be made it will be the same one over and over again since that is the way it is always done. No thinking. No initiative.
It would be nice to allow the assembly line workers to grow in their craft. Reading resistors and capacitors is a very good start. That way when a mistake on the drawing is made, someone can catch it before 1000 boards have to be reworked or thrown away.
I agree that replacing the name "R1" to "10K" would remove some ambiguity during assembly, but isn't that what a BOM is for? Plus they have 1 letter of silk screening or printing of whatever sort. Not sure if they are paying by the letter. Plus schematics, if drawn properly, have all the info needed per component, right?
When the PCB shrinks and populates the boards to the point where silk screening will not help at all, what do we do? What will human hands do then? I worked at a place where they have one person "who was good at the little parts." They would handle surface mount components. After seeing their not-fantastic job are rebuilding, I just printed and placed another board.
Oh yeah... R1 may change in the future... all the "10K" printed boards would be an issue.
There are two purposes for silkscreening component numbers on a board - to populate the board with the correct components, and to be able to troubleshoot the board at testing. The second is where the system described here failed miserably.
Let's say you have a board with 3 capacitors labeled "C1" and 10 resistors labled "R1". Your schematic shows two different "C1"s connected to five different "R1"s. If one of your "C1" capacitors has popped, how do you find it on the schematic to troubleshoot it?
I'm surprised that most of the comments have missed the point. The assemblers did not have a problem. Putting part A from a bin marked A into every spot marked A on a board is a very reasonable assembly process. If every assembly works and never needs to be repaired, it is a way to possibly save money. But 100% yield and no repairs ever needed is an unlikely situation. Steve discovered the problem when he tried to repair a board with several 10K resistors all marked R1. If you want to measure the voltage on the 10K resistor at the output of a circuit, where do you put your probe? You have several different R1 choices and no way to tell the difference between them. You are then forced to trace the copper connections by hand to figure out which resistor is which, because the labels don't differentiate between them. This is an insanely expesive way to troubleshoot a board which is why component designators on a board are always (except at this crazy company) unique.
As a "greybeard" I've come across many poor design decisions that have much in common with this example. Any design decision is an attempt to solve a problem and is always a compromise between many competing tradeoffs. Poor decisions usually result from a "small-picture" view of the problem due to ignorance of the impact the decision might have somewhere on the "puchasing-manufacturing-end user" chain. In this case a slight improvement in the assembly process made troubleshooting almost impossible. A fact which the designer seemed blissfully unaware of.
Exactly. Every component needs a unique designation, not just a characteristic value. Troubleshooting first cares about WHICH resistor, and then about its value.
The best one I've seen so far is a large board with NO designators or markings of any kind! I've seen this on really old boards, but this was something under 5 years old. I guess they decided to leave the silkscreen off to lower board costs. That one was a pain to troubleshoot from a schematic.
My guess is the lack of silkscreen was not a money saving decision, but one to make reverse engineering more difficult. Working on a board like this is not difficult with the "dolly sheet" in hand.
In fact, it is much easier to generate the "dolly sheet" (wow, after using it a few times I really dislike that term) than place reference designators in locations that are easily read after placement.
I worked at a company that used no silkscreen and only placed part numbers on schematics. The idea was to keep the design proprietary... and it meant you needed the schematic, BOM and "dolly sheet" to do anything.
When your lab is only one scope, one meter, and one homebrew function generator and power supply, the scope is stage center. But this one wasn't working right.
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