Tag Archives: LED

LED & Diode Markation Guidelines for PCBs

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Have you ever had an LED or other diode placed backwards? PCB assemblers work hard to place every component from the largest, highest pin-count logic chip down to the smallest passive components and micro wafer-scale BGAs correctly every single time. A key element of that accuracy is our understanding of your board and the component markings.

If you use surface mount diodes or LEDs, you probably understand the challenges involved in correctly and consistently indicating diode polarity. LEDs are usually cathode negative, while zeners and uni-directional TVS diodes can be cathode positive. Barrier diodes can be either orientation. It all depends on whether the diode is a rectifier, an LED, a uni-directional TVS, part of a daisy-chain and a host of other considerations.

When you start looking at the CAD libraries, you not only have all the differences from that manufacturer, you may also have different markation schemes from each CAD package developer and from each library builder.

Guidelines for diode polarity mark silk-screening — the diode symbol, “K” for cathode or “A” for anode. To ensure the best accuracy, we recommend extra care in marking diodes to remove any ambiguity.

The preferred method is to place the diode schematic symbol in the silkscreen. You may also place a “K” for cathode adjacent to the cathode. “K” is used because “C” could imply that the spot wants a capacitor. An “A” adjacent to the anode on the board works too, though it’s less common. If you are producing a board without silkscreen, put the mark in the copper layer or submit a clear assembly drawing with the other board files.

Relying on +, – or _ are not definitive in what they indicate and are not recommended. For example, a “+” or “-“ sign isn’t good enough, because it’s not always true that current flows through a diode from the anode to the cathode. For the common barrier diode or rectifier, it’s a pretty safe bet. However, with a zener diode or TVS, it’s not necessarily true. That is why marking a diode on your PCB with the plus sign (+) is not good practice.

Accursed Diode Marking

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Am I a broken record? Pretty much — especially when it comes to confusing diode marking.

For example, everyone knows what the diode symbol looks like, and pretty much everyone knows which side is the anode and which is the cathode. Right? It’s just like in the following picture:

Is that big enough?

Normally, the clearest way to indicate polarity on an LED is to put something like this diode symbol in silk screen next to, or between, the copper pads. In theory, that should remove ambiguity.

Ambiguity in marking is the enemy of polarized parts. Unfortunately, as I cover in this, and many other blog articles, LED manufacturers seem to conspire against us all when marking is concerned.

We recently ran across a case of built-in ambiguity. The PCB had, what looked like, a very clear marking. The image on the right is from the assembly drawing, which is just a blow-up of the board silk screen and documentation layer.

With that marking, I’d quickly come to the conclusion that the anode is on the right and the cathode is on the left. I’d even confidently state that it’s a sure thing and extremely unlikely to cause any problems. But …

Here’s where I’d be very wrong, and why it’s so important to always check the datasheet when dealing with diodes. Take a look at the following clip from the component’s datasheet. Scroll down to the bottom of the image for the punch line.

Wow. I can’t even …,

The board designer was just following the datasheet. That’s a perfectly proper thing to do, except when the manufacturer flips a coin, as it appears to have happened here. In this case, dispense with the symbol altogether and use “A” for anode and/or “K” for cathode in the silk screen. (Use “K” because “C” looks too much like a reference designator for a capacitor.)

Duane Benson
In the land of the insane, only the sane are crazy

http://blog.screamingcircuits.com/

Push-me Pull-you LEDs

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I may never get tired of talking about LED and diode polarities. It’s so much fun. Not long ago, I wrote about two LEDs from the same manufacturer, marked with opposite polarities. I recently ran into another one, but at least this one tells you on the same datasheet.

The image at right is an actual unmodified clip from the datasheet.

I can’t for the life of me understand why this would be done on purpose. I could maybe understand is one was designed in a different building, but it couldn’t have been too hard for someone to say: “Hey – wait a minute…” before sending these things off to manufacturing.

Of course, maybe they built a million before noticing and then just decided it would be easier to change the datasheet. Regardless, it’s kind of nuts in my opinion. The other thing here is that, while you can generally get away with the indicators “+/-” on an LED, you can’t with all diodes. Thin Zener and TVS.

Duane Benson

Matter + antimatter makes what?

Does it really matter?

Does anybody really know what time it is?

http://blog.screamingcircuits.com/

 

LEDs: Seeing Double

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Like I do so often, I’m being a bit redundant. While I’m all for stamping out and eliminating redundancy, this is redundancy with a purpose (not a porpoise). Not long ago, in a galaxy not far away, I blogged about annoyances in surface mount diode polarity markings. You can read that here.

I’ll wait.

Messy isn’t it? Well, after reading that blog, someone asked me about dual diodes. For some reason, I can’t seem to find the page covering dual diodes in my IPC book, but that’s not the important part. What is important is the way the diodes are marked on the PC board.

We do ask for centroid data which, in theory, contains the component rotation. That would be cool except that we find that far too often, the zero degree orientation (and the rotation from that) differs from the standard. That, and there are seemingly half a dozen or so standards.

Since LEDs don’t work too well backwards, we really would like to see everything marked in a non-ambiguous way in silk screen (or in an assembly drawing if you don’t have silk screen). A “cathode bar” won’t work because it could be a bar indicating the cathode or negative. The cathode isn’t always negative, especially when looking at TVS or Zener diodes.

Mimicking the diode markation pattern printed on the part may not be secure either. Read that article I linked to right at the start of this blog. What if you put silkscreen down to match one of those LEDs but ended up buying the other one? That’s exactly what I did myself. Trust me. It just leads to disappointment and possible soldering iron induced finger burns.

So what is the answer, and why am I talking about single LEDs and TVS diodes when the blog is about dual LEDs? Well, the answer is the same. The best way to communicate the desired polarity of an LED or any kind of diode is with a mini version of the schematic symbol. It doesn’t matter if it’s a single LED, dual LED, Schottky, Zener or whatever kind of diode. The schematic symbol is the clearest way to go.

Led marking

The diode footprint has the manufacturer’s polarity marking, but I don’t care. I still put the diode schematic symbol next to it. If you don’t have room for silk screen, put it in an assembly drawing. You won’t regret it.

Duane Benson
And they called him Flipper…

http://blog.screamingcircuits.com/

More Cautionary Tails

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I recently wrote about the horrors of LED marking variations. Unfortunately, LEDs aren’t the only place to find inconsistencies in our world. Another part to keep a close eye on is the ubiquitous three-terminal voltage regulator. For just short of a million years, pretty much all three-terminal voltage regulators followed the 78XX convention. Lm7805 convention

It is not completely universal, though. (Is saying “completely universal” repetitive and redundant?) There are some regulators that divert from convention in thru-hole and in SMT form-factors. Despite the overwhelming prevalence of the 74XX pin-out, you may find some parts that dispense with convention and can bite.

Take the LM1085, low drop out (LDO) regulator, for example. It looks, for all intents and purposes, to be a standard TO-220 or TO-263 three-pin regulator. You’d look at it and assume that it’s a direct replacement for any old 75XX series. But, rather than In-Out-Ground, it’s pinned as Ground-Out-In. The LM1117T is the same.

Mismatched SOT-223You might think: “Of course, it’s different, the part numbering doesn’t follow the 74XX number scheme.” That sounds logical until you look at the LM2940. It follows the 74XX pin convention, as does the MIC39100. It’s not the LDO specification that justifies change the pin-out either. The LM2940 is also an LDO.

Unlike the LED polarity issue, this one isn’t as likely to bite you during assembly. The SMT regulators can only go onto the board one way. If your CAD library footprint is correct, it will be assembled correctly. The through-hole can be easily reversed though if your silkscreen isn’t clear. Marking pin 1 on the board (and checking the CAD footprint) is the recommended approach.

Duane Benson
In the land of the insane, only the sane are crazy.

http://blog.screamingcircuits.com/

‘The Magic One’

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It was 50 years ago today when a then 33-year-old scientist at General Electric invented the first practical visible-spectrum light-emitting diode, a device that GE colleagues at the time called “the magic one” because its light, unlike infrared lasers, was visible to the human eye.

Dr. Nick Holonyak, Jr. could not have known then the door he was opening, but today LEDs are not only the foundation of a massive government-driven push to eliminate the popular (but higher energy consuming) incandescent, but also a potentially enormous market for the manufacturers of LED and driver printed circuit boards at the core of LEDs.

 

LEDs contain metal or graphite core bare boards, generally with lead-free solder paste. They are not easy to rework, on account of their heat-sinking core. The LED components are typically surface-mounted, although some versions have radial-leaded parts, explains Scott Mauldin of LEDnovation, an OEM of residential and industrial LED lighting.

Many PCB and EMS companies have an opportunity to play in this market today, thanks to one man’s bright idea 50 years ago. We owe Holonyak a big round of applause.

Image courtesy Scott Mauldin, LEDnovation

Fun with Electrolytics

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I was fiddling with one of my robot boards the other day — popping some passives on and off and checking out subs and alternate values. I was doing this on a couple of boards at the same time. Everything was going along fine until I started to do a power-on test. The first board was fine. The second one would briefly light the power indicator LED. It would start a full brightness and then fairly quickly fade out.

My first thought was that I had been too aggressive with my soldering iron and had burned something out. (Who has already guessed what really happened?) Turns out, that wasn’t the case. I put it aside and came back to it a few days later. This time, I gave it the finger test and discovered that my regulator was hot. Darn. Next, I found a hot tantalum cap. Nothing looked out of the ordinary/ I stared at it for a while. The + side was on the left in both parts and… The plus side was on the left in both parts. One was supposed to be on the right. Oops. The cap had a high enough voltage rating that it didn’t blow up. It just pulled down the supply until the over-current protection in the regulator shut it down.

I’ve heard a number of folks recommend that you keep all your polarized parts facing the same way. It’s not always possible, but it can certainly reduce opportunities for errors like I made here.

Duane Benson
Left, right. Left, right. Left, right. Left, left. Left, right …

http://blog.screamingcircuits.com/

My Mistake — Naturally

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I received my PCBs for this project from Sunstone yesterday at about 10:10. I quickly dropped them into my box of parts and delivered it to the receiving pile-of-boxes in our shipping.receiving department. At 11:40 a.m. yesterday, I received an email from our auditing department informing me of a BoM mismatch.

BOM mismatch Yes. I had made a mistake in my bill of materials. The board has a bunch of yellow LEDs and one red LED. I had mistakenly only packed up yellows. Our audit department caught my mistake and sent me a quick email. If I hadn’t have responded yesterday, I would have received another email at midnight. I could have just told the Downsized_0421110938a manufacturing folks to put a yellow LED in that spot, but the yellows are for a display and the red is a power-on indicator so I got my red LEDs and delivered them to receiving.

This morning I got my assembled boards all nicely wrapped in anti-static bags along with all my leftover parts in their original packaging. Next step, get some batteries and power them up.

Duane Benson
Thor, Dog of Thunder, is not allowed

http://blog.screamingcircuits.com/