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Typically, through-hole LEDs seem to have the cathode (-) side connected to the "anvil" and the annode (+) connected to the "post" (see diagram below).

LED Diagram

...But I now have in front of me an LED that does not follow that convention.

MPN: C4SMF-RJF-CT0W0BB1

backward LED anvil

The light is coming from the annode (+) side.

Note that this LED does not have a "flat spot" as seen in the diagram. The lens is actually ovular instead of round.
The cathode is still the shorter leg, so I assume that convention that is more universal.

As for the anvil/post, why would this one LED be different, and is there any purpose to either convention?

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    \$\begingroup\$ Never assume anything should follow convention. That's what datasheets are for. \$\endgroup\$
    – Tom Carpenter
    Commented Jun 16, 2017 at 20:04
  • \$\begingroup\$ @TomCarpenter - Conventionally, datasheets are correct. But I've come across several that were not. Assuming anything is quite dangerous indeed. \$\endgroup\$
    – Bort
    Commented Jun 16, 2017 at 20:14
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    \$\begingroup\$ As with any IC, if you depend on the "typical" you're going to get screwed eventually. FWIW, I can't understand why anyone would care where the anvil is, so long as the cathode is short, and if there's a flat, it's on the cathode. \$\endgroup\$
    – Scott Seidman
    Commented Jun 16, 2017 at 21:23
  • \$\begingroup\$ The longer lead and the package flat are conventions for denoting the anode vs. cathode. Which side of the die faces up is a process parameter that you shouldn't expect to be constant across different types of LEDs. If your LED doesn't follow one of those two conventions, you need to check the datasheet to determine the difference. \$\endgroup\$
    – Evan
    Commented Jun 16, 2017 at 21:32
  • \$\begingroup\$ So, there goes my alternative mnemonics for identifying terminals from cut LEDs. (Minus has a minus shape as seen from above). Good to know. \$\endgroup\$
    – Sredni Vashtar
    Commented Jun 16, 2017 at 23:23

3 Answers 3

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The LED chip can be made to bond in a cathode up or anode up configuration. The pole that is down is typically bonded with a thermal epoxy that transports the heat from the die to the lead frame. The top pole is typically gold wire ball or wedge bonded to the other part of the frame although aluminum wedge bonding is also used.

Regardless of the bonding orientation, manufacturer's should follow the convention that the shorter lead is the cathode, the longer lead is the anode, and the flat side of the package indicates the cathode.

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  • \$\begingroup\$ Glenn, what do you mean by "wire gold ball"? \$\endgroup\$
    – Mike Waters
    Commented Jun 16, 2017 at 21:27
  • \$\begingroup\$ Mike - That should say gold wire ball bond. Thanks for catching that. \$\endgroup\$
    – Glenn W9IQ
    Commented Jun 16, 2017 at 21:31
  • \$\begingroup\$ @GlennW9IQ - I think something is still not right in there.... "wire gold wire ball" \$\endgroup\$
    – Bort
    Commented Jun 21, 2017 at 17:50
  • \$\begingroup\$ Thanks, I guess I needed to slow down and read it again. Hopefully I have it right now. \$\endgroup\$
    – Glenn W9IQ
    Commented Jun 21, 2017 at 17:57
  • \$\begingroup\$ Do you have any opinion as to why some manufacturers (inc. Cree) wouldn't follow the flat side convention? \$\endgroup\$
    – Paul Uszak
    Commented Jun 21, 2017 at 21:17
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Was your LED essentially free from China? Are they really Crees? I have a bag of similar LEDs from eBay in pretty blue. They came without any information, just a ziploc bag that was made of the thinnest plastic I've ever seen.

Cathode is still the short lead. Your statement regarding that standard might be true. But the flat is on the anode(+) side. They work satisfactorily. This is the first experience I've ever had with atypically formed LEDs, so perhaps it's not all that common. We could do a poll if SE were into that kind of thing...

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  • \$\begingroup\$ These came from a reputable distributor, the one that rhymes with Kigi Dee. I would certainly hope they're Crees as stated! \$\endgroup\$
    – Bort
    Commented Jun 19, 2017 at 15:03
  • \$\begingroup\$ Completely OT, have you tried the grocery bags you get there? Unbelievable thinn. Seller knows it's crap and tripple bag it for you so you have a fighting chance it get it home. \$\endgroup\$
    – winny
    Commented Jun 21, 2017 at 18:01
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When making 5mm LEDs you can choose;

  • beam angle , round or oval, lambertian or batwing, diffused or not, colored or clear; ESD protected or not. THe leads may be have stoppers or not and the case may be smooth or D coded for cathode. ( the flat edge being the bar on the diode symbol)e

THe flange is useful as a stopper when paired with the lead stoppers when a soft edge is avalable. But a hard edge causes interference, so flangeless in a matching hole allows more height variation without interference.

So no D and no flange. By convention, Anode (+) has the longer lead. THese are conventional flangeless parts with lead stoppers. (not wrong)

Only once have I seen D cases on the wrong side and that was the 1st and last time I ordered 10k of those parts by mistake 12 years ago.

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