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March 12, 2010
Polarity on LED SMT Components

Is there some standard that governs how polarity marks are used to denote orientation on a SMT LED component, PCB silkscreen, and on assembly drawings? Examples: Cathode or Anode in the case of an LED, Pin 1 on an IC, must the mark be visible from the top, etc.

The following is included for background:

I have been an SMT Manufacturing engineer for over 10 years. I program Pick & Place equipment along with SMT process debug. Within the past year I have been struggling with SMT LED orientations. I know how to read a polarity mark and look at a data sheet but I have been having issues with SMT LEDs.

I started running a new assembly and it had two different color LEDs. The LED's were placed on the PCB in the same orientation according to the silkscreen mark on the PCB and the customer assembly drawing. I had not looked up the data sheet or looked at the assembly schematic.

There were no questions with the assembly with regards to the LED orientation during visual and automated inspection until ICT says one LED is backwards on every PCB. I look at all the information and cannot understand why. Based on the data sheet for each LED, the polarity mark denoted the anode on one LED and the cathode on the other LED. Needless to say I was confused. Everything in my training and experience told me both LEDs were placed correctly, until I looked at the data sheets.

Since the above described incident, there have been dozens of cases in which an LED had been placed backwards due to the polarity mark being misread. Historically, I have not had issues with SMT LED polarity. It seems as if LED polarity marks have become ambiguous and highly dependent on application and manufacturer during the past year. Can you offer assistance?

Thanks for you time.

S. B.

Experts Comments

Tradition for many years has made the cathode of diodes the feature to marked with a band or other indicator on the body of the part. LEDs have messed with tradition a bit partly because they are not always thought of as diodes any more but rather as a part with a more positive or plus (anode) contact and a more negative or minus (cathode) connection, like a polarized cap.

And as you know, some caps mark the plus (+++++) like many tantalums and some mark the minus (-----) like many aluminum caps. Somehow we get caps right most of the time.

Whether there is a standard for LEDs or not, if the LED manufactures do not follow it, it leaves it up to us (the contract manufacture) to get it right. I agree, LEDs are harder to know for sure, without opening the spec sheets and in some cases the schematics as well.

It is up to the design engineer to make it clear, based on the LED's spec sheet, how to install the LED on the board based on some visual feature. If there is a question, it is up to the contract manufacture to get a good answer. Hopefully your customer is open to such communications on your part.

Otherwise, you got a 50/50 chance of getting it right, and if you don't it's usually "your fault." Not the best answer, but I understand your issue.
image
Paul Austen
Senior Project Engineer
Electronic Controls Design Inc
Paul Austen is a 30 year veteran Senior Project Engineer with ECD in Milwaukie, Oregon. Paul has seen and worked with the electronic manufacturing industry from many points of view, including: technician, designer, manufacture, and customer.
Additional Comments
Reader Comment

As a contract manufacturer, we too have encountered problems determining correct SMT LED orientations. The majority of LEDs we have seen have the cathode end marked, however we have noticed that even within the same range of LEDs from the same manufacturer, there is sometimes a special note regarding the marking and so far it has always been for a red LED using GaAlAs technology.

Red LEDs using this technology seem to be the exception and as far as we have seen, will always have the mark on the anode end, not the cathode. I have no idea why this is.

When we are unsure of the polarity of a SMT LED, we carry out a simple test using a multimeter: we set the selector to Diode mode and place a probe on each end of the LED. It will only emit light when the negative probe is on the cathode end and the positive probe is on the anode end.

We will usually check any LEDs on the first board off production using this method and then program the board into our AOI. Any boards with incorrectly placed LEDs will be detected automatically thereafter.

P. B.
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