PCB Assembly Services - Screaming Circuits: And just one more thing...


And just one more thing...

Manufacturing is a complex process. Few people would disagree with that. Quality in manufacturing is all about isolating complexities and reducing big challenges into a lot of little manageable tasks. Much of that reduction must be done at the manufacturer (us, in this case), but a lot of it happens before it ever gets within our walls. Probably the biggest potential risk that an engineer can tackle involves clarity of information.

Building a PC board is really just the manifestation of an idea in your (the designer) head. You have an idea and you draw it out in the form of a schematic, a PCB layout, a bill of materials (BOM) and work instructions. The challenge comes in when a designer assumes knowledge exists in someone else's head. It's fair and logical to assume that a manufacturer will have knowledge about how to build things. It's probably not so logical to assume that the manufacturer has your design knowledge and can easily interpret unclear or incomplete data.

Take the common diode (Schottky, light emitting, TVS or other). It is still way too common to mark diodes with a plus (+) sign for the anode and a minus (-) sign for the cathode. It seems logical and it is electrically correct. However, even in electronics, electrically correct does not mean the same thing as mechanically correct.

If your diode is being used in a flyback circuit, it is there to conduct reverse current spikes and the cathode will be connected to positive voltage and the anode to negative. Zener diodes and TVS diodes are supposed to breakdown and conduct in reverse after a certain threshold voltage is reached. They are also typically installed with the cathode (-) connected to positive (+) voltage. In bridge configurations, you will have anode and cathodes connected together. A plus or minus is not adequate.

There are a few other ways you should not mark your diodes. Don't use a "C" for Cathode. It will be confused with "C" for capacitor. The convention is to use "K" for "Kathode. It is engineer-creative spelling and is universally accepted in the manufacturing world. Some people use "A" for Anode, but that is a lot less common than K. Don't use a vertical line (|) to mimic the cathode line on the diode symbol. It looks like a sidewise minus (-) and puts you right back in the plus/minus ambiguity.

Led marking

Feel free to use the full diode symbol - either between the pads or next to the diode as I have in this image on the left.

And one more thing. Don't assume that the silk screen polarity marks on small surface mount light emitting diodes tell your manufacturer anything. Look at the pattern between the pads in the same image. Figure that one without a guide.

Even worse, it's not just that the symbol is a bit ambiguous, it can mean two different things. Some surface mount LEDs have the markings backwards. Literally, the data sheet will say for X part number, it is an anode mark and for Y part number it is a cathode mark.

If you designed your PC board with X part in mind and then send Y part, the silk screen will be wrong. Going a bit more "2020" on us all, if whomever originally made the CAD library footprint had part X in mind, the footprint itself might be backwards if you decide to use it for part Y. I've done it myself and I've seen plenty of customer get tripped up by it.

Always verify your footprints. Always verify your polarities and always do everything possible to eliminate ambiguity.

Duane Benson
Rocket people will come later



Two LEDs, same manufacturer, two different colors. One is opposite polarity of the other, same package. I put a note on the assembly drawing to have the board house physically check the parts with a meter before starting an assembly run, and put the diode symbol on the board. I also show a graphic of how the part looks from the back, for each type.

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