LED edgelit strips thermal adhesive importance

[PlayDoh]

So I've been working on a 60” Sony and found a couple dead LEDs in one of the edge light strips. Ordered new strips, reassembled and tv works great for about 2 weeks, then burns out another LED.
I had 2 of the exact TVs both go out within a month of each other so a random LED in 2 strips of 68, go out in 2 TVs at the same time after 5 years seemed unlikely. I noticed on both the power supply boards, the area of the LED connection was getting hot. You can see it's baked the board slightly.
So I just got a new power board and about to reassemble, yet I have concerns about the adhesive that adheres the LED strip to the frame. Factory had a grey thermal ‘gum' type stuff.
I ordered a double sided thermal tape that's designed for LED strips and CPU, and such. Just so happened it's the same width as the strip which is encouraging. Yet it's very thin. I can double it, yet that might be counter productive.
I tested the other tv with the LEDs just laying on the bench, and they got wicked hot in about a min and burnt out several LEDs. Now either the boards are sending too much power, or I underestimated the need for thermal conduction. My bet is on the board, since the Strips just attach to a edge of the frame, that is no thicker than the strip and no heat sink at all.
My options are thermal tape I have, or Arctic Silver epoxy. I had to replace a couple single LEDs so I'm not overly confident the strips will last years, so epoxy might make removal impossible. Yet now that I think of it I do have a spare frame, since the other TVs panel cracked during reassembly.

Just hoping some guru can let me know how important thermal conductivity is.
TIA

[budm]

Re: LED edgelit strips thermal adhesive importance

Each LED is rated between .5 ~1W so add that up with the amount of LED you have and you will see how much power it will have to dissipate, that is why thermal management is very important but the TV manufacturers do not care much about it, as long as it last a year or two it is good enough for them since warranty is already over.
Best to turn down the backlights level (not brightness) to as low as possible on all input settings.
They drive the LED at maximum rating and some time higher than the LED spec.

[RJARRRPCGP]

Re: LED edgelit strips thermal adhesive importance

Best to turn down the backlights level (not brightness) to as low as possible on all input settings.
They drive the LED at maximum rating and some time higher than the LED spec.

Yep, that's what I did to my LG 49-inch'er... I turned the backlight level down from "95" to "45". Does that sound sufficient?

[PlayDoh]

Re: LED edgelit strips thermal adhesive importance

any recommendations as to what to use as an adhesive? I'm guessing I can't go wrong with the Arctic silver, but its not easy to find. I can get it, but since its not manufactured anymore, I'm wondering if theres an alternative?

[RJARRRPCGP]

Re: LED edgelit strips thermal adhesive importance

its not manufactured anymore

What? Did Arctic Silver go under? I know that I could only get that stuff locally, if I was able to go to a Radio Shack! Was Arctic Silver related to Radio Shack the same way that some Windows programs are related to Internet Explorer? So did Arctic Silver terminate, just because of Radio Shacks closing?

[PlayDoh]

Re: LED edgelit strips thermal adhesive importance

What? Did Arctic Silver go under? I know that I could only get that stuff locally, if I was able to go to a Radio Shack! Was Arctic Silver related to Radio Shack the same way that some Windows programs are related to Internet Explorer? So did Arctic Silver terminate, just because of Radio Shacks closing?

Just the Epoxy was dumped

[diif]

Re: LED edgelit strips thermal adhesive importance

The Arctic Silver adhesive is still made and for sale but not suitable for this application. The edge ones I've encountered have been screwed to a larger aluminium heatsink.
Go with the thermal tape and turn down the backlights to reduce their temperature.

[PlayDoh]

Re: LED edgelit strips thermal adhesive importance

The Arctic Silver adhesive is still made and for sale but not suitable for this application. The edge ones I've encountered have been screwed to a larger aluminium heatsink.
Go with the thermal tape and turn down the backlights to reduce their temperature.

http://www.arcticsilver.com/msds.htm

Have a look for yourself. I'm not sure why you would say it's not suitable. It's the Cadillac of thermal adhesives. Maybe it's overkill, but according to LED experts it's the best choice.
https://www.ledsupply.com/blog/#article/1473

[diif]

Re: LED edgelit strips thermal adhesive importance

http://www.arcticsilver.com/msds.htm

Have a look for yourself. I'm not sure why you would say it's not suitable. It's the Cadillac of thermal adhesives. Maybe it's overkill, but according to LED experts it's the best choice.
https://www.ledsupply.com/blog/#article/1473

What is epoxy if not an adhesive ?
It's the application. You're not mounting individual LEDs to a piece of aluminium.
The tape offers exactly what you want. The easiest transfer of heat.
One the page they have showing how to mount LED strips, they use tape.

The temperatures LEDs reach during use and the temperature tolerances allowed during soldering along with what I've read regarding LED failure suggests to me it's over voltage/current and not temperature that is the main element in prematurely killing LEDs.
As long as they are soldered flat to the aluminium strip and that is mounted flat to it's backing that is all that can be done for heat dissipation. Metal to metal is best but there will always be imperfections so the thinnest application of thermal material is required. 0.2mm tape that is able to take the heat seems far less mess than epoxy/adhesive that sets hard and would make further repair even harder. The thickness of the epoxy might move the position of the LEDS that might be critical. It's possible it might provide slightly better thermal properties than the tape but for the reasons above and the mess/benefit ratio, tape wins every time for me. If I am sticking a heatsink on top of a "thing" then I'll use the epoxy/adhesive or fixing something down to a surface that isn't flat or smooth.

[RDC55]

Re: LED edgelit strips thermal adhesive importance

I had to replace a couple single LEDs so I’m not overly confident the strips will last years.
TIA

Not sure how the LED's are wired on the Sony, but I am working a Sharp 60 inch that sounds similar; two LED strips with 68 6 volt LED's each. They are then divided into two sections of 34 that are driven in parallel of two rows of 17 – 4 separate circuits on the power supply.

So, what happens on this model is when you lose one LED from normal operation you lose one group of the parallel 17. Since the power supply is regulated for a fixed current based on DIMM command, the remaining group of 17 will have to supply all the current. For the Sharp, that results in a normal voltage of approximately 107 volts going up to approximately 120 volts, or stated as current the 250ma of current that was being supplied by two parallel paths is now being supplied by just one bank of LED's. The result is the working group of 17 is extremely over driven until one LED overheats and opens.

So, when models use a parallel configuration of LED strips and one particular section has faulted you could assume a part of that section has been exposed to double the current rating. And, since you can’t know which group was the original failure then at a minimum that entire section (34) should be replaced, or new strips installed.

[PlayDoh]

Re: LED edgelit strips thermal adhesive importance

What is epoxy if not an adhesive ?
It's the application. You're not mounting individual LEDs to a piece of aluminium.
The tape offers exactly what you want. The easiest transfer of heat.
One the page they have showing how to mount LED strips, they use tape.

The temperatures LEDs reach during use and the temperature tolerances allowed during soldering along with what I've read regarding LED failure suggests to me it's over voltage/current and not temperature that is the main element in prematurely killing LEDs.
As long as they are soldered flat to the aluminium strip and that is mounted flat to it's backing that is all that can be done for heat dissipation. Metal to metal is best but there will always be imperfections so the thinnest application of thermal material is required. 0.2mm tape that is able to take the heat seems far less mess than epoxy/adhesive that sets hard and would make further repair even harder. The thickness of the epoxy might move the position of the LEDS that might be critical. It's possible it might provide slightly better thermal properties than the tape but for the reasons above and the mess/benefit ratio, tape wins every time for me. If I am sticking a heatsink on top of a "thing" then I'll use the epoxy/adhesive or fixing something down to a surface that isn't flat or smooth.

I agree with the notion that it's current that kills LEDs most often, and that tape is more practical. In fact that's what I used. Yet I don't see how epoxy would do anything to the position of the strips. CPU's can use compound, epoxy or tape. Compound and epoxy being the thinnest and most convective. The thermal transfer ratings are way higher than tape.
Yet if thermal issues were that important, the LEDs would have been dead years ago. They seen in the ballpark of 20,000 hours.

[PlayDoh]

Re: LED edgelit strips thermal adhesive importance

Not sure how the LED's are wired on the Sony, but I am working a Sharp 60 inch that sounds similar; two LED strips with 68 6 volt LED's each. They are then divided into two sections of 34 that are driven in parallel of two rows of 17 – 4 separate circuits on the power supply.

So, what happens on this model is when you lose one LED from normal operation you lose one group of the parallel 17. Since the power supply is regulated for a fixed current based on DIMM command, the remaining group of 17 will have to supply all the current. For the Sharp, that results in a normal voltage of approximately 107 volts going up to approximately 120 volts, or stated as current the 250ma of current that was being supplied by two parallel paths is now being supplied by just one bank of LED's. The result is the working group of 17 is extremely over driven until one LED overheats and opens.

So, when models use a parallel configuration of LED strips and one particular section has faulted you could assume a part of that section has been exposed to double the current rating. And, since you can't know which group was the original failure then at a minimum that entire section (34) should be replaced, or new strips installed.

I agree that an LED failure causes the others to be overdriven and Receive more current. Yet I don't follow your math to be honest. If a bank of 17 receive 107v and one fails, then 16 receive the 107v. The resistance of the LED strip being now altered, could effect the supply components, to the point of damage. Yet my guess is the other way around. It's the supply that initiates the failure.

My unique situation is having 2 TVs bought at the same time and used in the same house. For 5.5 years they worked, then both developed a dead LED within weeks of each other and then both died a week apart. That fact alone speaks volumes to the cause. It's far more likely 2 of 2 power boards fail at the same time, both of which show signs of overheating. My guess is a cap or resistor begins to short. Or something.
Vs 2 out of 272 LEDs fail simultaneously. 1 in each bank of 136. Possible but far less likely.
I replaced all 4 LED strips, only to have them fail in a week. Again, the power board is the cause of the LED failure, not the LED strips or individual diode. The lifespan for LEDs is in the decades, but the components that drive them don't nearly last as long.
I advise you to swap the power board along with the LEDs. I've opted to replace the 2 individual LEDs this go, with the new board. If it lasts more than a couple months I'll consider another pair of strips. Yet I'd bet they will last longer then that.

[budm]

Re: LED edgelit strips thermal adhesive importance

When you measure the 107V, is that ref to the chassis GND? The Vf of the LED will change with the current being forced through the LED and also by temperature, LED is the current device.

[mmartell]

Re: LED edgelit strips thermal adhesive importance

I agree that an LED failure causes the others to be overdriven and Receive more current. Yet I don't follow your math to be honest. If a bank of 17 receive 107v and one fails, then 16 receive the 107v. The resistance of the LED strip being now altered, could effect the supply components, to the point of damage. Yet my guess is the other way around. It's the supply that initiates the failure.

Not if the led fails open and there is a parallel strip taking the excess current.