![]() 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. Here’s a clip from the datasheets for the part: Two-way LED datasheet (Source: Duane Benson) Their part numbers are different by only a few characters. One of these parts has the cathode to the left. ![]() For example, here’s the underside of two nearly identical LEDs from the same manufacturer in the same product line.Ĭontradictory LED markings (Source: Duane Benson) However, doing so only works if you can always use the exact part from the same manufacturer. 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. Backward LED marking (Source: Duane Benson) This is a clip from an actual datasheet for a surface-mount LED. Proper diode marking (Source: Duane Benson)īelow, we see an example of why using just a triangle isn’t always sufficient as a silkscreen indicator. You can also use “A” to indicate the anode. You can also mark the cathode with a “K” (use “K,” not “C,” because “C” can be confused with a capacitor). I’ll show you in a minute why just part of the symbol isn’t good enough. The best way to make things clear is to put the complete diode symbol in the silkscreen to indicate polarity. 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. Standard diode symbol (Source: Duane Benson) The practices used for marking polarity on surface-mount LEDs seem to be purposely designed to introduce error into the process.įor 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. Our industry does have standards, but enough people don’t follow the standards that we have to assume that no one does. 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. Polarity may seem pretty easy when you’re just dealing with a few parts on a schedule that you’ve set yourself. 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. What I didn’t really know until I started working for a manufacturing company is how complex the issue of polarity can become.īefore 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. Who wouldn’t know that? If it wasn’t important, the parts, well, wouldn’t be polarized. I’ve always known (“always” being defined as since roughly the summer of 1974) that polarity on polarized parts is important. 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. Surface-mount LEDs and MOSFETs (Source: Duane Benson) In the picture below, I have some LEDs and MOSFETs. That being the case, I’m going to start this article with a quiz. 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. Surface-Mount,” “ Panelization,” and “ The Bill of Materials”). Hi there, and welcome to Part 5 of my “How to Build a PCB” mini-series of articles (see also “ QFN footprints,” “ Through-Hole vs. The PCB fabrication and assembly industry does have standards, but enough people don’t follow these standards that the manufacturers are obliged to assume that no one does
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