Wednesday, 29 April 2015

SMT LED fail

While making some double-sided PCBs, we had quite a few left-over press-n-peel copies of our PCB design. It seemed a shame to waste them, but we didn't want to go through the trials of making more double-sided PCBs, so thought we could make single-sided versions of our sci-fi scenery walls.

Making single-sided PCBs is a doddle compared to double-sided (though quite why double-sided proved such a headache, we're still uncertain). It didn't take long to get a few boards etched and soldered up with some SMT LEDs.


For this particular design, we're using just a 74HC164 shift register to sink current through the LED to make it light up. Even without the shift register in place, we can test the board so far, by simply applying a low voltage (up to 3v) to the rails of the PCB.

So we did exactly that - applied 3v to the common power rail that is running to one side of every LED, and grounded the opposite sides of the LEDs, though the traces on the board. As each one lights up, we at least know that the LED has been properly soldered to the board, and that there are no breaks in any of the traces.

Except nothing lit up. Not a thing.

We checked our 2xAA batteries and they were working fine - the problem seemed to be with the PCB. Which, given it's only a few LEDs and some traces, seemed unlikely.

Reluctantly, we tried with the polarity reversed.
And every LED lit up, one at a time. Every LED had been mounted backwards!


Now the odd one might be forgivable - but to solder every single LED back to front it just plain crazy! We were quite diligent in checking the markings on the underside, to make sure that they were being soldered the right way around - but somehow, we'd managed to mis-read the markings on every one of the LEDs.
Then we came across this article online:
http://blog.screamingcircuits.com/2013/01/how-not-to-mark-a-diode.html

And it turns out that we're not the first ones to have had this problem. It turns out that some suppliers use the same markings to indicate different things!



In fact, sometimes the same manufacturer uses the same markings to indicate different polarities in the LEDs! So you can't always just assume that the markings on the underside of the LED will always indicate which way around they need to be mounted on the board.

In short, we're going to have to double-check every LED before placing it, to ensure it's the right way around. Our SMT LEDs have been dispensed into little multi-compartment boxes; we've no idea which are which, who made them, whether their markings indicate the anode or the cathode.... even a simple thing like an LED has suddenly just got a whole lot more complicated!