Ribbon cable repair - Page 1
Forum section: electronics / repair

author: NottheDan
author_rank: Frequent Contributor
timestamp: March 30, 2017, 03:30:28 pm
content: This ribbon cable is attached to a 3x8 key membrane keypad fixed to the front panel. No chance of getting that off without destroying it. Is there any sane way of repairing it?

author: SeanB
author_rank: Super Contributor
timestamp: March 30, 2017, 03:39:07 pm
content: Go to the car spares place and get a windscreen demister repair kit, and use the conductive paint to join the traces.

author: Ian.M
author_rank: Super Contributor
timestamp: March 30, 2017, 03:46:45 pm
content: No, but if all the tracks are straight you can probably cut it short, trim the insulation layer to match the original end, remove its connector from the board and add extension wires between the connector and the board.The suggested demister repair (conductive paint) would only work if you could bond the cable to a rigid surface otherwise the cracks in the substrate the tracks are on would cause the repair to fail. However it usually cracks at a bending point and moving the bending point by adding rigid support wiil cause it to crack at the new bending point as it has already become brittle. Its also a royal PITA to apply without it spreading sideways in the cracks and shorting out adjacent tracks.

author: NottheDan
author_rank: Frequent Contributor
timestamp: March 30, 2017, 08:29:01 pm
content: Thanks for the hint. How easy is the substrate to trim without damaging the traces anyway? And is there any good way of getting these delaminating layers back together?

author: Ian.M
author_rank: Super Contributor
timestamp: March 30, 2017, 08:49:51 pm
content: That depends on how far gone it is. If there is a reasonable amount of residual plasticiser left it can be trimmed easily with sharp scissors. However if its totally FUBARed it may be brittle enough to crack if you look at it wrong. Can you manage to get the end out of the connector without breaking the cable and take another photo?Delamination isn't a significant problem - just be careful what you use to stick it back together as incompatible adhesives may leach plasticiser out of the membrane substrate, making it even more brittle.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 30, 2017, 09:00:45 pm
content: Quote from: Ian.M on March 30, 2017, 03:46:45 pmAll true. Don't forget to trim the insulation off of the tops of the traces on the trimmed end. Another tedious task.Often little flexing is required after install. A semi-rigid stiffener and a lot of patience can make a conductive epoxy repair work. Do the work under a microscope. I have had to make my own brushes to get fine enough pitch. I am not sure which is tougher, trimming the cable and moving the connector, or repairing. Either way you really have to want the finished product.

author: jpanhalt
author_rank: Super Contributor
timestamp: March 30, 2017, 09:43:30 pm
content: I have used heat and a spring clip to trim multi-conductor flat cable. Not sure how it would work with your cable. The only insulation I have used it on was PVC. I put the cable in the spring clip and blow hot air through the clip unitl it gets hot enough. Then pull the clip off without opening it, and the insulation is gone.John

author: Ian.M
author_rank: Super Contributor
timestamp: March 30, 2017, 09:54:20 pm
content: Not that sort of ribbon cable - this stuff is thin metallic traces on some sort of low melting point plastic backing. They aren't copper, and applying heat willl FUBAR them.

author: helius
author_rank: Super Contributor
timestamp: March 30, 2017, 10:04:36 pm
content: There is a difference between FFC (flat flex cable, frequently found in floppy drives) and FPC (flexible printed circuit, in the OP's picture). FFC often is soldered, although there are connectors that either type can be inserted into. FPC is never soldered, it must clip into a connector, or in some cases "zebra strips" or z-axis films make the connection.jpanhalt's picture appears to show a .050" ribbon cable, different from either type.

author: jpanhalt
author_rank: Super Contributor
timestamp: March 30, 2017, 10:06:54 pm
content: @Ian.MThat has not been my experience. I have soldered to them when I used an abrasive to remove the insulation. Have you tried it? If not, then your theory doesn't match my experience. BTW, if you only want to strip one side, say the top, then put a thin sliver of balsa wood (or other insulator) on the bottom. Only the hot top will strip.John

author: jpanhalt
author_rank: Super Contributor
timestamp: March 30, 2017, 10:09:50 pm
content: Quote from: helius on March 30, 2017, 10:04:36 pmI have soldered FPC cable and it worked fine (a cable in a Sears Torpedo digital level). Guess it was just imagination.John

author: Ian.M
author_rank: Super Contributor
timestamp: March 30, 2017, 10:34:26 pm
content: Its critically dependent on the plastic used for the substrate and the metal used for the tracks. Some can be soldered if you are fast enough, others just disintegrate or the metallisation dissolves in the solder and cracks away from the rest of the track when it cools. If its got what appear to be very thin printed or plated tracks, briefly touch a hot bit lightly to the edge of the cable in a non-critical area. If the edge melts, don't try to solder to it.Flat flex cables that use copper foil tracks (tinned or untinned) are usually solderable.

author: jpanhalt
author_rank: Super Contributor
timestamp: March 30, 2017, 11:02:17 pm
content: I don't see that anyone has mentioned "Clincher" from Amphenol (https://www.digikey.com/products/en/connectors-interconnects/ffc-fpc-flat-flexible-connectors/399?k=FPC+interconnects&k=&pkeyword=FPC+interconnects&s=3758&FV=fffc0261%2Cffe0018f%2C1f140000&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25).You do not need an expensive crimper to make your own cables. I just use a small (2") machinist's vice. The bendable part does tend to bow out with wider cables, but the contacts stay good. I have used it for GLCD cables with no problems. Smaller or thinner flat cables might look better. You can get male and female varieties to make a splice, if you don't want to or can't solder the splice.John