Need help repairing an oscilloscope LCD ribbon cable

[exider]

Hi guys, I need help with a display ribbon cable. I was taking apart my Rigol DS1102E to change the display connector as it broke earlier when I noticed the ribbon cable fell apart.

Has anybody seen anything like it ? If so, any tips on how to fix that would be greatly appreciated as I feel blind without my scope

[exider]

One more pictures of the cable .

I wonder, were those glued together?

[Ian.M]

Its very common for flat-flex cables to break when disturbed where they were sharply creased.  In your case, the break appears to be at the end of the upper layer of Kapton, where the tracks on the lower layer emerge from the 'sandwich' to form the contacts.   That's always a weak spot if the stiffener isn't well bonded with good overlap.

The following is general advice from many years of 'wrangling' flat-flex and similar cables, and is *NOT* specific to your scope:

The first question is, is there enough slack in the ribbon cable to allow the broken end of the main part to be fully inserted in the connector, without straining any existing sharp bends at the back of the display or pulling it round any sharp corners?

If so, you may be able to peel back or abrade away the top layer of the flat-flex 'sandwich' to expose the tracks to form a new connector end.   If you cant peel the 'sandwich' without lifting or otherwise damaging the tracks you'll have to abrade the kapton cover layer away which is extremely tricky to do without damaging the tracks you need to preserve.  Try gently scraping with a sharp scalpel towards the end of each track once the kapton layer starts to break up.   If you've got a scrap cable of similar construction practice on it!

The exposed tracks will be bare copper which is extremely susceptible to corrosion, so if the connector has tinned contacts, you'll need to tin them and wipe them smooth.  If its got gold plated contacts, you are in for trouble as mixed gold and tin contact surfaces fret and are noted for frequently developing a poor connection.  You could try chemically plating the copper with silver, which has better compatibility with gold, but without the ability to electroplate them (because there's no good way of shorting them all together to permit electroplating without damaging the display) you'll only get a thin layer and silver tarnishes extremely easily.

Once you've got usable contact surfaces, trim the end absolutely square with minimal loss of length, rough up the back of the cable and clean and rough up the stiffener, (which you need to peel off the old end and save) and glue the stiffener back onto the cable.  You should then be good to go - 'just reinsert it' and lock the connector, though with less slack in the cable that can be extremely fiddly.

[shakalnokturn]

If you're a little short you could scratch away as previously described then solder a female FFC connector to the end of the LCD flex and use an extra FFC cable to link between the two female connectors.
Apply some epoxy over the solders for insulation and strain relief.

[Ian.M]

N.B. I *think* its Kapton from the color, which is high temperature resistant so you can tin tracks on it, but it would be advisable to test the corner of the broken off end, with your soldering iron bit, after removing and saving the stiffener.  If it melts, its *NOT* kapton, and tinning the tracks with solder would be very risky.  In that case, I suggest electroless tin or silver plating.

[exider]

Yes, there is enough slack. But it looked like the end of the cable was glued on and broke off where the glue was. I will try to take better picture with a microscope tomorrow and post the result.

Thanks for the tips guys.
If nothing else works I will try to scratch the cable to expose the copper. 

[YetAnotherTechie]

I highly recommend using a "fiber brush pen" to expose the tracks, they are so usefull and work much better than scraping with a blade, it's like magic.
Since you are already replacing the connector, you could try to get a tinned one instead of gold-plated and avoid the above mentioned issues.

[perieanuo]

well, 1 hour, insulated copper wire, you're fine
or just buy another cable

[exider]

Just to document  the issue in case someone else run into similar problems.

Initially the display started flickering or sometimes half of it disappeared. But it fixed itself after leaving it on for 10-15 min. It was obvious the problem was heat related as it fixed itself after worming up. I took closeup picture and fond a crack in one of the solder joints. Obviously after worming up the solder expands and was closing the gap.https://ibb.co/0DNzCr6. In short, I've tried to re-solder the joint and damaged the connector in the process.
.
Back to the cable:

I think the two pieces of the cable was attached together by double sided type. The glue is visible on one of the sides of the cable https://ibb.co/JpKrtMZ



This is picture of contact sidehttps://ibb.co/KFY7Y1s

P.S for some reason I can't attach pictures on the right place.

Here they are :

 

[Ian.M]

The cable *IS* the connector.  The plastic stuck on the back is just a stiffener to make it thick enough for the socket to grip and to aid insertion.

(from:  https://blog.epectec.com/how-to-specify-stiffener-requirements-in-flex-pcb-design-drawing )

The 'coverlay' is the top layer I mentioned above, and you need to peel it back and cut it or abrade it away to expose the tracks to make new contacts.

[exider]

I see what you mean, but this cable is different.
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The part where the yellow arrow is goes behind the part where red arrow is. The white part is the stiffner from your image and there is little space between that and the part where the red arrow points. It is hard to explain in writing ... Sorry if it is not clear what I mean.

[Ian.M]

Possible, but very unlikely.  The layup I've described above is the industry standard way of making bare flat-flex connectors.  Peel the contacts and their backing layer off the stiffener, flip them over and see if there's any sign of a solder joint.  If not,  I'm correct.  You can also try examining the two ends either side of the break under a microscope.  If you are correct, they'll be dead straight factory cut.  If I'm correct they'll be slightly irregular as it cracked and broke, and the irregularities in the two ends will match up.

[exider]

This is how it should look like when put together. I have to try when I change the connector. It might actually work. Will see ...

[exider]

"If you are correct, they'll be dead straight factory cut."

Yes, they are factory cut. They are straight ...

[exider]

Thanks for the tips guys.

I will post the results after changing the connector.

[Ian.M]

I'm not doubting your word, just how closely you examined the ends

"If you are correct, they'll be dead straight factory cut."

Yes, they are factory cut. They are straight ...

Under a microscope butted up against an engineer's steel rule in good condition or other reference edge?

It cracked along a fold or stress line so to the naked eye the edge *may* appear straight.

Hwever, *IF* you are correct it may be a little easier as it may be possible to expose enough of the tracks inside the flat-flex to solder them to the contacts after gluing the flatflex back onto the stiffener.  It wont work if the joint would be under the connector latch, or if you cant get  *REALLY* good glue bond.  The *ONLY* way I can see it being made that way is to save process time hard gold plating the contacts by buying them in and simply bonding them on.

[exider]

Ian, I know exactly what you mean.

This is the type of cable from your picture " alt="" class="bbc_img" /> and it has stiffner on the back side
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But the cable from the scope is different. It seems that pressure is applied to the point of contact when the cable is inserted into the connector. I am waiting on a delivery. I had to order 10 of the connectors as they weren't selling single ones.
I will try after changing it and will post the results.

Thanks for your time and help.

[shakalnokturn]

If the last two closeups in post #8 are the pictures of your flex I think you're mislead, that definitely looks broken from excessive bending.

[exider]

Yes they are, and there is good chance you might be right about that, even though I haven't bend the cable much. Now when I took a closer look I am starting to doubt myself.
Also the reason I think you might be right is because I cannot see any exposed copper wire or any contact point at all. I have no idea what I am going to do in this case. Probably try to disassemble the display and see if I can take the cable off and find a substitute . I don't know yet, will see in 2 days when the connectors arrive. Maybe will try the earlier suggested methods.  Thanks for pointing that out.

[exider]

It's very strange though. The breaking point is very straight. If it's a cut I really have no idea how it came to be...

[Ian.M]

It's very strange though. The breaking point is very straight. If it's a cut I really have no idea how it came to be...

That's simple.  It failed along the stress line at the end of the cover layer, where it goes from a two layer Kapton sandwich with copper in the middle to a single layer of kapton with copper in what was the middle but is now the contact face.  As the cover layer has a square-cut end, it was like bending the layer carrying the tracks over a knife edge.  The stiffener unbonding didn't help as one of the jobs of the stiffener is to prevent bending at the transition.

I have no idea what I am going to do in this case. Probably try to disassemble the display and see if I can take the cable off and find a substitute.

Odds are the other end of the cable is bonded to the display glass.  Disturb *that* joint and it will be truly unrepairable.

[exider]

Ian, It looks you have experience with those cable so, I thought It will be good idea to post before I proceed in case I made mistake somewhere.

I managed to "shave" off the cable and expose the copper.
I also managed to damage the corner of the cable by cutting it a bit, but this shouldn't be a problem if I use and glue the old piece of plastic to the backside of the newly exposed pins so the width stays the same ( any suggestions here?).
Then I used melted solder paste on top of it to ensure every bit of the transparent cover layer had been removed. I will clean it later with the solderwick.
The connector I have ordered has different closing mechanism, but that shouldn't be a problem as the pinout is the same.

I will cross my fingers and try put it together   

[james_s]

Wouldn't it be easier to just replace the cable? Many LCD panels use one of just a few standard pinouts so in many cases you can buy off the shelf cables that will fit.

[exider]

I thought about it but it was suggested that I wont be able to detach the cable from the LCD panel.

[Ian.M]

It would be worth cautiously investigating the panel end if you can get to it without too much trouble - if you are lucky its another connector the same and James's suggestion to replace the whole cable would be preferable.  However I fear that it may turn out to be directly bonded to the glass with a chip on glass or flat-flex mounted controller, or it may transition from flat-flex to rigid without a connector.

*IF* the damaged corner still has full thickness Kapton under the actual track/pad, that should do once you've wicked off the excess solder, solvent cleaned it, and reattached the stiffener.   If not, you'll need to start over.   

Before attaching the stiffener, the end *MUST* be cut accurately square so the pads go right to the end and the cable sits square in the connector.  You only need to skim a sliver off so wont loose much length.  Its probably easiest to use sharp heavy scissors or a guillotine.