PCB Assembly Services - Screaming Circuits: How Should You Mark Your Diodes?


How Should You Mark Your Diodes?

Current flows through a diode from the anode to the cathode - it will pass current only when the potential on the anode is greater than the potential on the cathode. This is mostly true, but not always.

For the common barrier diode, or rectifier, it's a pretty safe bet. However, with a zener diode, or  TVS, it's not true. And, that is why marking a diode, on your PC board, with the plus sign (+) is not good practice.

Take a look at the schematic clip below.

P-Mosfet and barrier diodes

Once you put this circuit on to a PC board, you could legitimately place a plus sign on the anodes of D3 and D4, and another on their cathodes. In the next schematic clip, you could legitimately place both a plus sign, and a minus sign on the anode of D9.

Flyback diode configuration

We don't know what you had in mind, and, we don't have the schematic. If you use the practice of marking diodes with a (+) on the anode, we don't have any more information than if you didn't mark it at all. The same holds for using a minus (-) sign. It really doesn't give us any information.

Diode markingSo how should you mark your diodes? The best method is to put the diode symbol next to the footprint. on the PC board, as shown below. You can also use "K" to indicate the Cathode, of "A", to indicate the Anode. "K" is used because "C" could be mistaken for "capacitor."

D5, in the illustration on the right would be the preferred method. D7 will work as well. If you don't have enough room on the board due to spacing constraints, you can put the same information in an assembly drawing.

Ambiguity is the enemy of manufacturers everywhere. Read a bit more on the subject here, or here.

Duane Benson
Help stamp out and eliminate redundancy, and maybe ambiguity, or maybe not


Hi Bill - Thanks for your feedback. It makes me very upset when we (or anyone, for that matter) puts a part on backwards, and/or gives poor customer service. We work hard at not letting problems like this out into the field, but when it does happen, the best thing to do is let us know quickly. If you're referring to a specific job (even if it was a while ago) you sent to SC, I'd love to hear more details. You can email me at dbenson@screamingcircuits.com, if you'd like.

Also, I'll write a follow-up post or two to clarify some of the points you called out.

Actually, Bill, electrons flow in both directions in diodes. There is always leakage current and a higher current flow during breakdown.

I recently got burned by an ambiguous silkscreen marking which is why I now prefer the diode symbol (or K if space prohibits) rather than just indicating the cathode with an otherwise ambiguous marking.

I recently designed-in a bi-color LED that had pin 1 marked on the package with a green flag. I naturally placed a similar mark on the silkscreen next to pad 1. The problem is that all other manufacturers of similar parts mark their cathodes and pin 1 on this part happened to be the anode. Needless to say all my LEDs got placed incorrectly. Incidentally, the manufacturer has since changed the marking to conform with the others.

This is not well written and can confuse newbies further. Electron flow is Cathode to Anode. Most designers use a stripe or fill-in on the screening to indicate cathode, which you don't even mention. I've found the real problem with assembly to be a lack of training on the part of assembly house personnel, they often don't know electronics at all. S.C., in addition to poor customer service, often can't find pin 1 of ICs, which are marked with a dot in most of the world...

Bi-colored, and tri-colored can be problematic. I like your very specific approach: "short lead goes in square hole"

Sometimes a very simple solution is the best.

I've had trouble with bi-color LEDs. Had to include a note "short lead goes in square hole".

The biggest problem is with surface mount diodes. In a perfect world, a proper netlist, or assembly file would solve this problem. Unfortunately, too many footprints don't follow the IPC rotation standards. When that happens, the netslist and CAD file essentially tells us the opposite of what the physical part says.

We us a bar on a rectangular silk screen just outside the pads for the Cathode like the thru holes are marked.
A proper netlist carried through the entier process would take care of this too.

Now if only everyone in the world (who's responsible for marking diodes) could just read this, and follow it, that would be greaaat...

Post a comment

If you have a TypeKey or TypePad account, please Sign In.

« Tell The World... | Main | Veterans Day 2015 »