Thin ribbon cable (LCD backlight) repair - possible for beginner?

[robsoft]

I suspect I already know the answer to this, but if I don't ask I'll never be sure. I'm a beginner, only rudimentary soldering skills. I've got an old Acer Aspire laptop, a little while ago it developed a flickering screen fault and after a little googling we determined it was the backlight, and I put it to one side. The laptop works fine on an external monitor, and the backlight would sometimes appear to be fine for a few minutes until the machine warmed up, sometimes altering the angle between screen and keyboard would cause it to show fine (or go completely black) etc. You can get replacement lcd cables on eBay for a few quid, if it were just the lcd cable I'd have replaced it in a heartbeat.

I opened it up tonight just as a change of project, and after a little gentle exploration (and some googling) I confirmed it was a torn/degenerated backlight ribbon cable within part of the overall LCD display - there's a ribbon that runs from inside the panel, along the outside to a connector that is still on the panel (rather than in the laptop). Putting very gentle pressure with a plastic spudger would cause the display to light up (or go black) etc etc. It looked like the kind of ribbon cable where you just push it into a connector so I gently tried to undo it with a view to trying to reseat it - and predictably it fell apart. :-)

I just want to confirm that this isn't something I can fix by soldering some ribbon cable onto the bit left on the panel. I mean, the iron would probably melt the ribbon anyway, even if I could source some more ribbon, and I guess that introducing even very thin gauge wiring would be almost impossible to do...?

But if there's a slim chance that someone might say, 'oh, you can do this (insert clever trick here) and it might work' then I'll give it a go - I gave the laptop up as a lost cause, it owes me nothing and it really doesn't warrant the £70 or so for a replacement lcd panel, but if it can be repaired then it's something my young daughter can play with until it breaks for good.

(I felt this belonged here rather in the repair forum as my lack of advanced skills is probably more of a factor than any clever repair know-how).

Any thoughts?

The attachments here show the cable, how it was originally bent over itself to make a 90 degree turn, and where it goes into the socket. Also you can see where the other end of it bends off into the innards of the panel.

Thanks in advance (fan of the youtube channel, first time post here, sorry if this is deemed stupid or inappropriate).

[uncle_bob]

Hi

The bit you are looking at is most likely part of a rigid-flex PCB. The only way to confirm that would be to rip open the LCD panel. I would recommend against doing that.

Repair is doable. It's not what I would call easy. You glue the end of the cable to the board and start running very fine gauge wire to hook it up. A good microscope, a steady hand, and a really good temperature controlled soldering iron are the minimum tools for the job.

The other approach is to replace the now defunct connector (you did say it's broken) and plug the cable back into a working connector. That would involve very similar tools and a source for that specific connector. It might (or might not) be easier that the wires.

Bob

[station240]

What's the other end of the flex cable look like ? how long is the cable ?

Some of us have salvaged flex cables from various junk, if both ends of the cable are wired the same, anything with the same pitch and length should work.

[robsoft]

Repair is doable. It's not what I would call easy. You glue the end of the cable to the board and start running very fine gauge wire to hook it up. A good microscope, a steady hand, and a really good temperature controlled soldering iron are the minimum tools for the job.

The other approach is to replace the now defunct connector (you did say it's broken) and plug the cable back into a working connector. That would involve very similar tools and a source for that specific connector. It might (or might not) be easier that the wires.

First off - thanks for the advice!

The connector *isn't* broken, sorry that my ramble wasn't very clear in that respect - the socket appears to be fine and I initially wondered if I might just get the socket off the board, put it on the end of a short run of normal ribbon (putting the other end of that into the pins the socket came out of) and then use it to connect to the end of the rigid-flex 'ribbon'. In other words, half-arse a means of moving the socket so the remaining rigid-flex ribbon can reach it.

However, I can't get at the underside of the connector without taking the panel to bits.... and I note your wisdom about not doing that :-)

[robsoft]

What's the other end of the flex cable look like ? how long is the cable ?

Some of us have salvaged flex cables from various junk, if both ends of the cable are wired the same, anything with the same pitch and length should work.

(Thanks for your reply)

See attachment - without opening the panel I'm not sure what is in the panel itself, but the cable runs for maybe 2-3" from the broken bit to the edge of the panel, where it then takes a sharp turn 'inwards'. If I could get a similar bit of cable with another snap/push type connector, I could see how I could just fit that to the remaining ribbon and bridge the gap that way...

[bktemp]

Most likely the LEDs from the backlight are directly mounted on the flex cable/flexible pcb inside the LCD. The flex cable is typically glued to the metal frame to act as a heatsink for the LEDs. Therefore it can't be replaced without replacing the complete backlight assembly.
Soldering the broken cable is possible, but not easy. The repaired cable needs a really good strain relief or it will break again as soon as it gets bent or someone pulls on the cable.

[imidis]

While it may not be easy to do, the nice thing about even trying this is - it doesn't matter if you fail at it. Which is a great situation to see what you can do with it.

I really enjoy the fixes that well - there is nothing left to lose if it doesn't work out.

It's very satisfying when they do work out!

[Ian.M]

First be aware that any other sharp bends in the ribbon are likely to be in a similar condition, and if access to them and clearance round them is poor, its not likely to have a good outcome.  If you catch a crack before its torn the foil traces, reinforcing it with Kapton tape carefully layered over the outside of the bend may save it.

Its doable but very fiddly due to the thinness of the copper foil and the toughness of the (probably) Kapton insulation layer.   Scraping through the Kapton is going to be a real P.I.T.A, and you may even need to use a large diamond ball burr run at a *VERY* slow speed in a Dremel to abrade through it.

Practice scraping through the insulation and soldering to the copper foil on the wide track on the edge that doesn't go anywhere.  Once you've got a workable technique, trim the ends slightly to take out the bit that's bent at the fold that turned the corner while keeping as much of the fold's overlap as possible, prepare all the tracks you need for soldering (only the six thin ones), and stick the two pieces together in the correct orientation using 3M thin doublesided VHB tape or similar to provide the mechanical support it needs then patch all the tracks with insulated wire, leaving a little slack so the joints don't get tugged, with a dab of glue (e.g. Evostick Impact adhesive) to hold them in place then insulate, preferably with Kapton tape.

[station240]

Oh right, it disappears into the frame, that complicates matters.

[samnmax]

Yes, it can be done, I did it with a similar Aspire laptop. Check the pictures:

http://imgur.com/a/SlQgH

Basically I carefully scratched the ribbon cable exposing the copper, and then run wires to conveniently placed test points near the LED driver IC.
I used a small 15W soldering iron, but the ribbon seemed to resist the heat quite well. In my case the cable had snapped at the 90° turn.

[robsoft]

Thanks everyone, some good points there.

Samnmax (great username) - I take a lot of heart from your picture! Thanks for sharing that, and the tips on how you did it.

Ian.M - I suspect you're absolutely right about the other sharp bends - it looks like a pretty sharp bend as it goes off into the case of the panel, too. That looks like good advice about scraping the track, thanks for that.

Imidis - yeah, I don't really feel like I have anything much to lose - worse case, I put it all back in the box and think 'well if I ever need it, it needs a £60 replacement screen) but best case is... a working usable laptop again :-)

bkemp - I think you're right about the backlight, I googled quite a bit and couldn't find anyone talking about (or selling) a backlight assembly for this, I suspect if the backlight is damaged you have to replace the whole panel. Point taken about the strain relief. :-)

[imidis]

Awesome Samnmax! That's along the approach I would have taken!