LED Fault in PCB - Some strings of LEDs intermittently dim

Question

I have a university project that involves working with surface mount LEDs.

For some reason, some of the rows of LEDs, when lit up, are dimmer than they should be. This problem is only intermittent however, and sometimes when the boards are switched off and then back on, they light up fine. This is the issue I need to resolve.

The circuit is 24V constant voltage, made up of several parallel strings of LEDs. Each LED string has 4 resistors and 2 transistors to control current flow.

The PCB they are mounted on has been scored, so that the circuit can be broken into smaller sections if need be. My first thought was that when the boards are snapped along the score line, the copper tracks are damaged, but the LEDs are only dysfunctional on an intermittent basis.

Sometimes, when the LEDs are dimmed, pressing down on the board causes them to light up properly.

I know its a lot of information but if anyone has any idea as to what the problem is, it would be very much appreciated.

Answer 1

Sometimes, when the LEDs are dimmed, pressing down on the board causes them to light up properly.

This would seem to indicate that you have intermittent connections in the circuit.
It's certainly possible that tracks may be damaged, but it seems more likely the problem is cracked resistors or stressed solder joints on your components.

Check the soldering quality and you might be able to find broken resistors and component connections with either a cold spray or heat gun.

Answer 2

These types of problems are not uncommon with LED PCBs. Without a schematic, photos, and LED footprint I can only offer generalized information.

Two high probability issues would be thermal runaway and more likely cracked ceramic substrate when the score lines were broke. A third issue may be the solder process. Some LED footprints are subject to solder stencil and assembly problems. Even the rotational position of the LED and the direction of how the solder paste is applied can cause issues.

Thermal runaway can be a slow process that takes many hours to manifest. Then when powered down and allowed to cool the problem goes away. When powered again the problem will repeat. This is generally associated with a constant current source and unbalanced parallel strings. You are using constant voltage and current limiting resistors so not likely. But if there are any parallel strings getting current from exactly the same path then it is a likely source of a problem.

Keep in mind you could have multiple problems.

Most likely is caused by different coefficients of thermal expansion on the layers of the PCB causing warpage. The warpage puts stress on the LED package and solder joints.

Section 3.6 (FR4 Board Handling) of this app note explains the warpage and cracked ceramic packages and copper patterns that can help.

LUXEON Rebel Plaform Assembly and handling information

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The following app note goes into detail regarding the solder paste and assembly process recommendations.

See Section 3 Electrical Layout Consideration and
Section 4. Assembly Process Guidelines

LUXEONC Color Line Assembly and Handling Information

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Many do not believe thermal runaway is real.
This Texas Instruments app note provides some details.
See Section 3 Pitfalls of Parallel LED Arrays

Design Challenges of Switching LED Drivers

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This OSRAM app note explains how to prevent thermal runaway. Many times the unbalanced strings will reach thermal equilibrium before one of the strings fails and the current flow is such that some strings are dimmer than others. Again this may take hours to progress.

Current distribution in parallel LED strings

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sometimes when the boards are switched off and then back on, they light up fine.

If the intermittent nature of the problem is not mechanical warpage, the way you describe it, it sounds just like how how I've seen unbalanced parallel strings symptoms manifest. I would need a schematic to verify.

I had some strips that were unbalanced the forward voltage were 43.29V (yellow) and 43.85V (blue). I powered them with a CC (HLG) and CV (HEP) with 5Ω resistor in series with each strip.

Unbalanced strings is a bigger problem than most designers realize. Most of the time it is undetectable. I have done a lot of experimentation with driving parallel stings and the unbalanced nature of the problem. Sometimes economics dictates running multiple strips with a single CC driver. What I do is add shunt resistors to the design so I can measure how unbalanced the string are. They are ALWAYS unbalanced and I want to know by how much. When I receive strips the first thing I do is measure the forward voltage of each strip and try to pair the parallel strings by V_f. No matter how close the V_f is matched, the current is still unbalanced.

Answer 3

Your described problem would typically be a result of heat. Either a bad soldering job, bad solder, overheating from lack of accurate Heat sinking or active cooling, or running the leds at a higher current leading to heat damage. As they get turned off, they can cool down and the issue goes away until turned back on and the temperature rises.

Heat can cause the solder to soften and disconnect, or cold joints to crack. Worse, it can lead to internal failure of the led layers. You can tell by looking at the led while off for burn marks internally.

Make sure you are meeting or exceeding the manufacturer recommended heat sinking, temperature, current and voltage requirements.