Why does applying light onto a mylar ribbon "fix" my TV?

[dyril]

My backlit Samsung UN32EH500 LED TV got crippled, probably due to excess indoor humidity from El Nino late last year.
One third of one half of the screen was glitchy.

Visuals explain it quicker:
https://youtu.be/z-3m7UqoIBg

Pictures of me trying to fix it: http://imgur.com/a/lOCeS

So shining a light on the chip on a rotted ribbon leading to the panel corrects glitchiness enough to call it a fix.

How and why does this correct the picture?
And can I expect this to only be a matter of time before it fails?

[Brumby]

Neat trick.

No idea - but I'd be interested to find out.

[cdev]

Does your TV adjust its brightness based on the ambient light level?

[Monkeh]

There is actually a chip on that FFC, which explains it being photosensitive. Couldn't tell you why it behaves that way, though.

[helius]

Very interesting question that requires a dissertation on quantum physics to fully answer.
The short of it is that light is affecting some device it is shining on, that is not designed as a photocell. This is possible because the device is not encapsulated but is mounted as a CSP (chip scale package). See how light can upset the behavior of CSP causing a product to crash:
https://www.eevblog.com/forum/blog/eevblog-716-raspberry-pi-2-xenon-flash-problem-explained/

Now, in that thread the problem is that bright light upsets the chip and causes a crash. In your problem light is required for correct operation. These are both consistent with the behavior of silicon ICs: light is trapped in the silicon and creates charge carriers (electrons and holes) which then diffuse through the silicon. In a very bright flash, the amount of charges is huge, and can upset the state of the device making it lock up from sudden voltage fluctuation. In a dim light, the amount of charges is low, but they do move around in the silicon and interact with other charge carriers. If a transistor is injecting a lot of charge because it is damaged, the charges generated by the light can recombine with them before they upset something else.

You can look at it as another way of diagnosing a faulty chip, similar to using freeze spray. You're creating a condition that makes a damaged chip work a little bit better by reducing the amount of effective leakage current that is screwing it up. The chip will need to be replaced.

[Rerouter]

I also agree on a photosensitive chip, I would just bodge a white smd led next to the ribbon and call it a day,

[voltz]

You could try using a much smaller point of light, say an LED (white if possible, but others may work) and really home in on the exact point where the fault (fix) occurs. You have a great opportunity while its in this state to locate the exact component and fix it.

If it turns out to be the ribbon cable then look for printed components on the cable itself and just replace the cable assembly. If its not populated with any components then that is a very strange photoelectric effect worthy of physics interest but still just replace the cable.

[EPTech]

I also agree on a photosensitive chip, I would just bodge a white smd led next to the ribbon and call it a day,

This has made me laugh out into tears.  Who dares to say us techs do not have a sense of humor. But then again only us techs would laugh at a joke like that.

Anyway. It would be nice to know what kind of light it reacts to best. I would expect the higher the energy, the better. Maybe try some UV and some infrared if you are up for an experiment of course.

[Fred27]

Maybe try some UV and some infrared if you are up for an experiment of course.

Genius. If the light isn't visible nobody will know that you bodged it!

[dyril]

Thanks for the insights!
I can only add that it seems 5 of those 10 broken traces seemed to be common (visually at least) and there are a couple of smd caps where they originally lead to.

I had already bodged it in a most conspicuous manner (see album link in OP); really I was just curious as to why this was happening.

The ribbons are integrated with the panel so they are unfortunately not user-replaceable. They are also pressed onto the controller PCB in a permanent, one-time fashion.

Aside from better climate control I also wonder if one could preemptively protect mylar ribbons of this nature by say, slathering them in mineral oil to prohibit oxidation. Probably not worth the effort in most cases to go against planned obsolescence where entertainment hardware is concerned.

[helius]

If the device has multiple bond-outs for its common net, and most of them are no longer conducting because of cut traces on the FPC, then the light could be increasing the conduction to the remaining pins by supplying more charge carriers.

[AndyC_772]

Light shining on a silicon device tends to increase leakage current, by generating more electron-hole pairs. Perhaps what's happened is that one half of a differential pair has become flaky, but by changing the threshold voltage slightly on the other half, a correct signal is still being transferred via the working half alone?

[Rasz]

brilliant bodge, this is hackaday worthy

[EkriirkE]

brilliant bodge, this is hackaday worthy

You got your wish

[Rasz]

I did the leg work

Iv been doing LCDs for hmm 7 years now, and never seen anything like it. Heating, squeezing, reflowing (very gently), but never light 
even if its fake, it would be a brilliant prank

[Delta]

Superb work!

It shows the difference between an engineer and a technician 

(proud technician here BTW)