PCB Assembly Services - Screaming Circuits: LED Polarity - Still

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LED Polarity - Still

Some things never cease to astound me. A lot can go wrong in a product design cycle. Creating a good layout from a complex schematic is a difficult enough job as is, but when component marking logic leaves the building, it can seem like the deck is very much stacked in the other direction. I've written about LED polarity challenges for many years and the problem hasn't gone away.

Yesterday morning, I spotted a Twitter thread about the same issue. @GregDavill showed some photos of yet another example. This is from the datasheet he posted on Twitter.
Anode and Cathode mark G
Not only are similar parts marked opposite, they are marked in a way that, if the silk screen in the CAD footprint mimics the LED marking, which is not a rare practice... Well, you get the idea.

Diode polarityOn the other hand, you might think that, with newer intelligent CAD data, it wouldn't matter. The machine will read the files and know how to place the part with 100% accuracy. Theoretically, that is true. However, it's also not uncommon for CAD software to reuse footprints for similar parts. It's also not uncommon to have a last minute sub with a different part that is "pretty much the same." 

Intelligent CAD data won't help you there if you're swapping from the anode marked to the cathode marked. Cases like that often lead the assembly house to refer to the silk screen. And, there you go again. That's why we always download a datasheet for LEDs and get our a DMM. The message is never get complacent with LEDs - always double check everything.

Duane Benson
It's even true on opposite day

And don't forget, Screaming Circuits is always here to build your prototypes and on-demand electronics manufacturing - even if your marking is ambiguous. Although, we much prefer that it is not. Visit www.screamingcircuits.com and get a pcb assembly quote.

 

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