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PCB Design for Clarity: DIODE POLARITY (NO, IT'S NOT OBVIOUS)

The PCB fabrication and assembly industry does have standards, but enough people don’t follow these standards that sometimes the manufacturers are obliged to assume that no one does.

Of all the aspects of printed circuit board (PCB) design, polarity seems to be the one that — in my opinion — is treated with the most contradictory combination of being important and being taken for granted. That being the case, I’m going to start this article with a quiz. In the picture below, I have some LEDs and MOSFETs. You have 30 seconds. Give me the polarity of the LEDs.

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Okay, maybe that’s not really a fair test, but we in the assembly business often feel like this is what we’re up against.

I’ve always known (“always” being defined as since roughly the summer of 1974) that polarity on polarized parts is important. Who wouldn’t know that? If it wasn’t important, the parts, well, wouldn’t be polarized. What I didn’t really know until I started working for a manufacturing company is how complex the issue of polarity can become.

Before getting a job with a manufacturer, I looked at polarity as a simple binary aspect of the part: Either it’s placed properly, or it’s placed backwards. I only wish that things were this simple, but with all of the different ways of marking diodes on the parts, all of the variations in data sheet format, and all of the different ways of marking (or not marking) the PC board, it often seems more of an exercise in code breaking than in assembly.

Polarity may seem pretty easy when you’re just dealing with a few parts on a schedule that you’ve set yourself. But it’s somewhat different when the component is one of 86 parts on a PC board that is one of a set of 75 boards that need to be assembled and sent back to a customer by tomorrow, and this is just one of a few dozen jobs being processed this morning.

For your reference, those standards (IPC-610) state that, on your PCB, a rotation of zero has the diode horizontal with the cathode on the left side.

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What you need to do is design your board with the assumption that polarity will not be clear, and then work to make it clear.

The best way to make things clear is to put the complete diode symbol in the silkscreen to indicate polarity. I’ll show you in a minute why just part of the symbol isn’t good enough. You can also mark the cathode with a “K” (use “K,” not “C,” because “C” can be confused with a capacitor). You can also use “A” to indicate the anode.

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Below, we see an example of why using just a triangle isn’t always sufficient as a silkscreen indicator. This is a clip from an actual datasheet for a surface-mount LED.

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Some engineers think that, with all this uncertainty, why not just duplicate the exact mark that shows up on the component? Even the smallest surface-mount LEDs have a small indicator printed on their underside, so this certainly sounds logical.

However, doing so only works if you can always use the exact part from the same manufacturer. For example, here’s the underside of two nearly identical LEDs from the same manufacturer in the same product line.

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One of these parts has the cathode to the left. The other has the anode to the left. Their part numbers are different by only a few characters. Here’s a clip from the datasheets for the parts:

Contradictory LED markings

The mark on the bottom side of one of the LEDs is the cathode mark; meanwhile, for another part, the EXACT SAME mark is used to indicate the anode. Suffice it to say that the LED manufacturers aren’t doing us any favors.

I first discovered this problem when hand-building a board of my own. I built the first board, and everything was working fine. For the next board, however, I decided to use green LEDs instead of the red that I had used on the original board.

Of course, the green LEDs didn’t work, and I quickly found that they were on backward. I pulled one off, double-checked the polarity marking on the replacement, soldered it on, and, lo and behold, it didn’t work either. I figured it out after a bit more frustration and some close examination of the two datasheets.

The moral of this story is to read the specific datasheet for your parts. With LEDs, “close enough” is not close enough. And, for goodness’ sake, don’t send a substitution to your manufacturer unless you’ve verified that the polarity markings are the same.

Some people will mark diode polarity with a plus sign or a minus sign. This often (once again, “often” does not equal “always”) works for LEDs but may not for other types of diodes. You know your design, so you know what the electrons are up to, but your manufacturer doesn’t.

One of the most obvious cases in which a plus or minus sign won’t work is that of a flyback diode, as shown in the image below.

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With a flyback diode, or a barrier diode, you’ll likely find that the anode (+ on the diode) is connected to ground or negative, while the cathode (– on the diode) is connected to positive in the circuit.

You might ask why a modern manufacturer needs to rely on the physical mark on the part. This a valid question. We have all of this information in electronic form, and in theory, a human should never need to actually look at the component.

Unfortunately, this is one of the many areas in technology where theory and practice don’t match. We’ve found that the polarity and rotation data is wrong so frequently that we just can’t rely on it. It is getting better and the intelligent CAD file formats like ODB++ and IPC-2581 help even more. (hint. Hint. Send us one of those file formats too if you can.)

The IPC standards organization does have standards governing such data, but many times, the CAD information is incorrect. It’s usually the zero-degree orientation in the footprint that doesn’t follow the standard. We’ve found it to be wrong about 30% of the time, which means that we need to see the datasheet, look at the component, and have good silkscreen on the PCB (or a clear assembly drawing) to give us a fighting chance.

Now that you know everything you need about solder mask, hop on over to Screaming Circuits to get a quote and order some PC boards + parts + assembly.

Duane Benson
If you are standing exactly on the North Pole, you can turn in any direction and still be facing: (Anode) or (Cathode)?

 

Comments

Our industry has had standards for this sort of thing for a very long time. It's astounding how loose those standards can be in practice.

This is addition to the question of which pin should be pin 1 on the schematic. I like pin 1 being cathode, as in these examples.

That's a good point about not relying on part markings, which may differ for alternates and may also look sort-of like the diode symbol but in the wrong direction.

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