Are There Standards Governing Polarity Marks?

Phil
Welcome to Board Talk. This is Phil Zarrow and Jim Hall of ITM Consulting coming to you from ITM intergalactic headquarters here on Mount Rialto in New Hampshire. We're here to talk about processes and components and materials and surface mount, and all kinds of other exciting things that our listeners send into us.

Jim
Today's question comes from S.B. Is there some standard that governs the polarity marks which are used to denote the orientation of an SMT LED component, PCB silkscreen, and on assembly drawings? Examples, cathode or anode in the case of an LED, Pin 1 in an IC. Must the mark be visible from the top, etc.?

They sent us a very detailed explanation, and I'll just summarize it for you.

They've been building equipment for a while and have not had a problem, but they're building a board that had two different color LEDs on it. There were no questions with the assembly with regards to LED orientation during visual and automated inspection, until ICT. In circuit test says one LED is backwards on every PCB.

I look at all the information and cannot understand why. Based on the data sheet from each LED, the polarity mark denotes the anode on one LED and the cathode on the other LED. Needless to say, I was confused.

Phil
All right, well, where to begin. Well S.B., welcome to the wide and wonderful world of surface mount technology and electronic manufacturing in general, because whether we're talking about LEDs, QFPs, QFNs, even PLCCs, SOICs, there is no standard marking for denoting Pin 1 on active components, I mean ICs, and certainly on polarized components. There's a lack of synerization there.

As you have found out the hard way in paying tuition for that lesson. Yes, on some polarized capacitors and diodes, you will see a band or a color dot typically denoting the anode end.

Believe it or not, surface mount having been around for about 30 years now in this wonderful industries, there is no standard on demarcation of polarization. What can you do about it? Well, if you really feel so inclined, start a committee.

Why JEDEC has never really thoroughly addressed this? But the problem is exactly what you discovered, the fact that these could be arranged in different ways on your feed mechanisms, whether it's tape or matrix tray. And there's really no practical way for the pick and place machine to pre-detect it.

Jim
Think about the capability that is in your pick and place machine with the cameras, the vision recognition, the lighting, and everything else it can do to look at physical attributes of a component. The industry can't come up with the standardization that will allow it to accurately and repeatably denote the proper orientation.

Phil
All it would take is something like standardizing a dot placed in the vicinity of Pin 1 for ICs, or a dot will denote the anode end or cathode end, but let's standardize on it, and let's have some consistency, and therefore, repeatability. But alas, nobody's gotten around to doing it. And that's probably one of the biggest mysteries I think I've encountered in this industry. Something as simple and basic as this has been so severely neglected.

Jim
Phil, you've been talking about it as long as I've known you, which is well over 20 years now, and one would think that something would have happened. It just doesn't seem that illogical.

Phil
Well, this industry ponders on. There's a lot of other illogical, it certainly gives us lots of other topics coming up. Yeah, cause we don't do committees real well. That's why we work for ITM.

But anyway, in the meantime, we hope that somebody will champion this cause, and I think this type of thing is a little bit more important than some of the other things we see championed out there, but I won't get into those soapboxes just now.

Jim
So we've wasted more than enough of your time, or you've wasted it listening to us. We do thank you for your support.

Why hasn't this been handeled already by I.S.O. Isn't that partly why they came into being?

Our primary LED supplier changed their specs to show their pin-1 from Cathode to Anode a few years ago. Thus, older designs have cathodes as pin 1 and newer ones have anodes. We set up our build per the vendors' specified definition of pin 1, then direct our operators to assume pin 1 is the cathode, unless otherwise noted in our work instructions.

Our SMT placement machines have a diode-polarity check capability, which we have activated on the first 3 placements out of each reel to confirm setup.

We've also experienced the oddball red LED that swaps the physical anode and cathode (with the die in the same location) from at least 2 separate suppliers (so a common-cathode layout with 4 green indicators next to 1 red indicator will appear visually to an inspector as if the red part is reversed). I suspect this has something to do with the physical characteristics of red LED's, but have never seen a good explanation why this is.

In short, we'd also love to see a polarity standard introduced, especially for LED's!

Anode Cathode pin1 these should not be a issue. There are rules for thru hole and you can carry them over to SMT. As in when learning about thru hole Diodes you learn a saying Cathy lives at the bar, meaning the bar shown on the Diode is the Cathode. When building this part I always make the cathode pin one. SMT or thru hole. The parts 0 orientation always matches the tape and reel. This is not rocket since but it does take formal requirements to be followed. Keep your standard rules and you should always be placed correctly.

I'm currently chairman of the Automated Component Handling Committee of the ECA/EIA, and we publish the EIA-481 standard for carrier tape. This issue of non-standard or no-standard marking for polarity or Pin 1 location has caused nothing but headaches for us.

We've done our best to establish standard rules for component orientation in tape, but our efforts have been hampered by the inconsistent markings by manufacturers and legacy orientation practices dating to the beginnings of SMT. Would one of you, Jim or Phil, be willing to contribute your thoughts on this matter to our committee?