LED pads’ convention

[Manc]

Hallo all.

I do have a (minor) concern regarding LEDs anode and cathode conventions.
I used to assume that anodes (the long leg) should go into the square pad, however I have noticed that some designers (TH and Fuzzdog for example) decide otherwise. I understand that this should be the correct way - axial diodes' cathodes usually go in the square pad - so it is only natural that this applies to LEDs too.
I just read a note fro pedalpcb.com where they say they are phasing out the anode-in-the-square design in favour of a more correct approach of cathode-in-the-square.
Where does MB sit with regards to LED installs?
Cheers

[storyboardist]

If Brian's Eagle libraries are any indication I think the 2012 version had square pad for cathode, but in more recent versions that's been reversed.

I've always approached LEDs like axial diodes—square pad for cathode. I had one builder I did some custom board design for complain that I had it backwards and despite my explanation he wanted any future boards with the square pad for the anode. I get it, long leg gets the square like with an electrolytic cap, but square pad for cathode makes the most sense to me.

[pickdropper]

Square pad for the cathode makes sense but I find more people use square for the anode so that it's populated like an electrolytic cap.  I use it for the anode because I'm used to it at this point.

[diablochris6]

To be safe, when I design PCBs, I use a part with a silkscreen design to show me where the flat side (cathode) of the diode should go.

[Aleph Null]

Really the solution is to get manufacturers to make the anode longer on LEDs. I haven't designed any PCBs, but I have gotten in the habit of triple checking polarity before populating parts because I've gotten caught by this before. Text indications on the silkscreen are always appreciated.

[BuGG]

I've always approached LEDs like axial diodes—square pad for cathode.

I'm with you on this one.    LEDs are diodes, and the cathode of a diode goes into the square pad.

I also agree that it's helpful if the silkscreen makes note of the proper orientation.    Showing the flat side of the LED is helpful, but quite a few 3mm LEDs don't have a flat side.   

I'm actually marking an A and K beside the pads on my new designs whenever space allows.

[Manc]

Thanks for your replies guys.

I have an old  ZeroPointSDX2 board I'd like to populate, and there are a couple of "discharge" LEDs underneath the two PT2399  chips. Looking at the schematics and at the PCB tracks in the documentation, it looks like the cathode should go in the square pad. Is that correct?
I hate when I second guess myself :/

[aion]

Potentially useful data for other PCB designers:

For 5 years, my Eagle library had square pad for anode - so the same as a polarized capacitor, long leg goes to square.

Last year when I went through and updated all my libraries, I changed the convention to square pad for cathode since as BuGG said, it's a diode and should follow that convention. Also, square and flat go together, so there's the mental hook there.

Since switching to square-cathode, I have had 4-5 people who emailed me saying the LED wasn't working, and in each case they had installed the LED backwards. In the 5 years previous, using square-anode, not one single person ever emailed me with that issue (nor anyone in the past year who was using older boards). In both cases, the silkscreen was also printed showing the flat side to help with orientation, so that wasn't a factor in either configuration. And they were mostly experienced builders, these weren't their first builds.

People who always follow the silkscreen will always be correct regardless of which pad is which shape. But based on my experience, among people who do not pay attention to the silkscreen, there are more people who instinctively put the longer leg to the square pad than people who would note that it's a diode and would instinctively put the shorter leg to the square pad.

So, given this, I switched back to the square-anode convention a few months ago and will be using it on all designs going forward. I think it will almost entirely eliminate the issue of people installing the LEDs backwards.