Flat flex contact repair - Page 1
Forum section: electronics / repair

author: Rooster Cogburn
author_rank: Regular Contributor
timestamp: February 02, 2025, 12:18:00 am
content: Is there any sensible way to fix this:It's a big custom flat flex with those metal membrane buttons glued on and some pads for a press-fit LCD, so nothing I can just replace with a standard cable. Some of the pins have corroded away. I could rescue a couple by cleaning / scraping, others are just gone.

author: pqass
author_rank: Super Contributor
timestamp: February 02, 2025, 03:46:47 am
content: QuoteI would:a.find a scrap PCB with straight traces in my junk bin, or if none are found, etch a few strips from a blank PCB,b.use an Exacto knife to lift up on the donor traces and pull a length off the PCB,c.scrape the broken soldered pad off,d.scrape a bit of the green trace just before the removed pad,e.solder the donor trace to the existing scraped green trace,f.use superglue behind the donor trace (apply with a pin) and pull taut and push down with a card to flatten then wait to dry,g.scrape off any green mask on the new pad and apply fresh solder; wick-up any high spots with solder wick,h.repeat for each broken pad.If the substrate can't handle much heat, do the tin+wick-up (stepg) as part of stepaand be very quick in stepe(with extra flux).

author: benj38
author_rank: Frequent Contributor
timestamp: February 02, 2025, 08:47:29 am
content: Instead of etching a PCB to get the copper you need, Just use 3M EMI Copper Foil Shielding Tape 1181, 1245, or 1126. The copper is the exact same thickness as on a standard 1oz PCB.It may be a bit softer, but I am not sure, and in any case this is hardly an issue here in my opinion.BTW, while these tapes have conductive glue (or embossed copper in the case of 1245), I would not necessarily trust it on its own for such a small area. Try to solder and reinforce with CA glue as @pqass suggested.

author: Rooster Cogburn
author_rank: Regular Contributor
timestamp: February 02, 2025, 09:57:13 am
content: I don't think I have the dexterity to cut out these tiny strips and glue them in place with CA glue :/ These seem to be standard 0.5mm, maybe I could use the copper strips from a standard flex cable as donors? Got plenty of those. Also, I was hoping there might be anything I can just paint on maybe, is there a marker with conductive paint? Probably nothing that is abrasion resistant enough and that I can solder to to make a connection with the green covered traces on the flex.

author: pqass
author_rank: Super Contributor
timestamp: February 02, 2025, 02:04:16 pm
content: Quote from: Rooster Cogburn on February 02, 2025, 09:57:13 amIf you keep the donor strip long (many cm) then you can hold with one hand and tack-down with your iron in the other hand. Or just tape-down one end of the donor strip to your bench and slide the flat flex underneath until aligned, solder, lift end to apply CA, flatten and hold with a card, then trim the excess. You should practice on a spare flex first.Copper strips from standard flex should work too; wherever you can get it. Of course you can buy rolls of copper tape of any width. I'm just impatient and look for expedient solutions.I don't think paint would last even a few cycles and would be more demanding to apply (for me).

author: benj38
author_rank: Frequent Contributor
timestamp: February 02, 2025, 04:48:50 pm
content: I know of no conductive paint that would survive in this application: the paint would in all probability be (at least partially) scraped off as you insert the cable, and you run a serious risk of loose paint shorting things out. Even if the connector is the type where you insert the cable with no friction, and then clamp it down, there is still the risk of dislocating conductive particles. I wouldn't do it.A microscope, patience, and a steady hand following @pqass advice is the way to go.

author: Rooster Cogburn
author_rank: Regular Contributor
timestamp: February 02, 2025, 09:29:17 pm
content: I'll just solder some enamel wire to the flat flex and hardwire them to the socket on the mainboard then. Using CA glue (I can never use that stuff without it running everywhere and gluing random things together, it always runs down the bottle and glues it to whatever, etc. etc.) to adhere sub-millimeter copper strips to a flat flex cable is just one of these things I know 100% I can't do. I was hoping there was shortcut like putting conductive paint on the cable with a stencil like solder paste or something like this, but if there's not I'd rather do something dumb I know I can do first try. Sorry, I'm a fraud I'm afraid

author: benj38
author_rank: Frequent Contributor
timestamp: February 02, 2025, 11:11:14 pm
content: Sticking to what you are confident with is not a bad strategyAs for the CA glue, as @pqass suggested, use a sewing pin:hold the glue bottle slightly pointing down and press it gently until you see a little drop forming at the outlet;dip the tip of the pin into it and pull the pin away --- it will carry with it a nice little drop you can apply very precisely.If the drop is too big, just wipe it off the pin and try again. Completely foolproof.

author: Smokey
author_rank: Super Contributor
timestamp: February 02, 2025, 11:42:29 pm
content: I had to deal with a very similar situation on the LCR meter I attempted to repair a while back. The contacts on the flex stuck to the display zebra stripe and I pulled some contacts off before I realize what was happening. It actually left an interesting transparent spots on the flex where the pads used to be.I saw no reasonable way of fixing it apart from replacing the damaged parts entirely. My only thought was to have a really short flex cable made and graft it to the end of the damaged flex and try to get that to make contact with the zebra pads. I'm glad I didn't have to do that though since it sound like a lot of work for a low probability fix.

author: Rooster Cogburn
author_rank: Regular Contributor
timestamp: February 03, 2025, 11:56:17 am
content: I recently had some success bulding up a few layers of CA glue to replace some worn down nubs on a plastic part. I put a bead of glue on a surface and the dipped a toothpick in and then kept depositing a few layers. For the .5mm flat flex the sewing needle seems like a good idea, can make finer droplets this way.The connector main board side is one of those lever clamps, so I'm not too worried about damaging my repaired cable. Those press-fit cable sockets are much worse and frequently damage normals cables afters a few insertions and removals, so at least that's good.In unfortunately can't source a replacement part and it wouldn't be economically feasibly anyways. Worst case this will just become a parts unit.The damaged pins are all for buttons and I confirmed it works when the wires are connected manually, so I think I can probably solder 3-4 little magnet wires. One question for me with this approach is that this flex seems like the conductors inside the cable are made out of a material more like graphite / carbon than copper. I've seen that before on flex cables with button membranes glued to it, it's a black/grey surface when exposed, not the usual tinned copper. I don't know if I can even solder to that.

author: Smokey
author_rank: Super Contributor
timestamp: February 03, 2025, 09:36:34 pm
content: Quote from: Rooster Cogburn on February 03, 2025, 11:56:17 amUsing baking soda (sodium bicarbonate) to both accelerate and supply structural bulk to super glue works really well in situations like this. Not sure I would trust it to be conductive though.

author: StrayElectrons
author_rank: Contributor
timestamp: February 04, 2025, 07:12:28 pm
content: This flex is made of silkscreened conductive paint.Use conductive paint to make new traces and contacts.Cheers,

author: benj38
author_rank: Frequent Contributor
timestamp: February 04, 2025, 08:41:06 pm
content: Quote from: StrayElectrons on February 04, 2025, 07:12:28 pmLooking carefully at the image, it does indeed look like it may not be solid copper. Also, the dimples indicate a clamp down mechanism, so no scraping motion pushing it in.Can you recommend a conductive paint available to us mere mortals that will hold as well as the original paint without the risk of pealing off?I would love to get my hands on some!I have never been able to find such paint, the conductive paints I ever laid my hands on would separate from the underlying flex substrate if I would just bend it a few times.

author: Rooster Cogburn
author_rank: Regular Contributor
timestamp: February 07, 2025, 06:01:19 pm
content: Would love to know what to buy or how to do this. My initial though was that I could get a stencil like for solder paste application or have one made with the right pitch and then just paint over it to lay down tracks. I'm not terribly concerned about force / debris due to the clamp connector with no abrasion during insertion and because at least in that half of flex it's all buttons and shorting things out would cause no lasting issues.