How to neatly fold FFC cables - Page 1
Forum section: electronics / general

author: kuon
author_rank: Regular Contributor
timestamp: February 09, 2021, 04:04:07 pm
content: I have to assemble some boards that are connected together with FFC cables.I need to fold the cables in a Z shape. I did it manually with no tools, but it is surprisingly hard to get it perfect within the tolerance which is <1mm.I found some suppliers which can supply custom folded cables, but only for >5000 units, and I need only a few hundreds.Do you know a technique/tool to do it relatively quickly and properly? Or a supplier that accept small quantities order?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 09, 2021, 05:43:58 pm
content: So, two tight bends? Easy enough to make a jig by drilling two holes in a plate, then insert dowels. Set cable edgewise against the plate, bend around dowels, done. You'll have to test diameters to get the right bend angle: too tight creases the cable, too loose and it doesn't take a set.Maybe even something corny like with cardboard glued/taped together, though your requested tolerance and quantity would seem to preclude that. I don't see exactly how such a tight tolerance comes to be, with material as flexible as that. But without a drawing, model, or anything, I'll have to assume you know best.Also, I'm assuming these are simple broadside-facing bends, say to take up extra length with a short accordion section? I think of 'Z' as a flat plane curve, which would be an edge-wise view of the final cable. Maybe it's that the connectors are off axis, so some angled lap folds are necessary? Which would be less flexible, hence the tighter tolerance? If that's the case, a bending jig with rods should still do, it'll just have to be supported oddly (maybe not on a plate, but a frame to fit better around the final shape, keeping itself rigid). Or maybe made out of stacked sheets, cut to make a corner, slot or pocket to align the cable against, and radiused edges for bending around. Hmm yeah, I can see making something like that out of like, I'd probably fiberglass sheet, I have plenty of that handy. Hard fiberboard would be fine too, even heavy cardstock probably, if the edges are reinforced with a dab of superglue or something. Or 3D printing of course, if you don't mind taking the time to design and print the thing.Tim

author: kuon
author_rank: Regular Contributor
timestamp: February 09, 2021, 07:40:21 pm
content: I guess designing a folding rig and 3d printing it or have some delrin sheet cut would do the job.But that seemed like a common problem, I was wondering if some "standard hack" existed.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 09, 2021, 07:51:58 pm
content: Hm, not aware of any standard tool for that. I guess, like, maybe you could send your cables to a sheetmetal fab shop, and have them bend them up with a CNC brake? Might not get the angles required, might tear them up, dunno. Doubt the cost would be worth it. Can't really think what else a standard method could be?Next closest thing I can think of, would be some kind of crimping pliers, basically regular pliers or shears, or if not a simple lever then a parallelogram drive or something, with sculpted jaws to clamp down on the material and set a bend in it. Basically a handheld press brake. But that's useless without angle and position references, for which you need... some kind of jig.I guess there might be something the cable harness guys use. Don't know what all kinds of tooling they have. You'd have to call one up and ask. Ah, but again, the cost might not be worth it -- it would be worth asking though. Even if they're just doing it by hand anyway, they might have a lower overall labor rate than you would get (doing it personally, I guess?).Tim

author: kuon
author_rank: Regular Contributor
timestamp: February 09, 2021, 08:00:23 pm
content: I'll try different methods and see what I can come up with.As for the tolerance, they are not "that tight", but if both folds have like 1-3° error and are of by 1mm, it can be off by quite a bit at the end, of course, the cable is flexible and it "can" be connected, but it then unfold a bit and it's not "neat" and there is some stress on the connectors.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: February 11, 2021, 09:10:02 am
content: Indeed, a curved one, even with a small FFC margin of error, is very bad at hitting the right spot.When I had such a task, I drew in CAD, taking into account the relative position of the sockets. Specifying the ends that cut and bend lines, printed it on self-adhesive paper, glued it to plywood, laid the tape, starting from one end, pressed it with a ruler along the bend line and bent it. Didn't have the drawing saved, but it looked something like the picture. It turned out from the first time to bend very smoothly, so that the train entered smoothly and did not experience torsion.

author: KL27x
author_rank: Super Contributor
timestamp: February 12, 2021, 12:35:20 am
content: I have faced this problem. Ordinarily, folding cables is not a big deal. But in this case, the two bends were really close together. The vertical section in say that pic ^ above by S.Petrukhin was only about 3mm, not 15mm.I bought a pair of smooth jaw parallel pliers and ground the jaws down to 3mm wide. Clamp over the ribbon at 45 degrees, then bend one side up, the other side down. Done.Actually, if that give a bit too much offset, you can adjust your angle rather than going back to the grinder. Less angle gives less offset, but shortens the overall cable length a wee bit. (the cable will fold back on itself more than vertical). No matter what angle you do, the cable will remain straight/parallel, as long as the jaws are ground straight and parallel on the sides.

author: Berni
author_rank: Super Contributor
timestamp: February 12, 2021, 06:25:07 am
content: You can make a custom FPC cable using flex PCB.For examplehttps://www.pcbway.com/does them in small quantity and gets pretty reasonable prices at quantity of about 100 or 200 when compered to buying a actual FPC cable from a distributor like Farnell/Digikey/Mouser etc. You just give them gerber files like you would for a normal PCB, so it can be any shape you want with any pin arrangement you like.We use these a lot when we need weird snaky FPC cables or we want a cable with better high speed signal integrity or EMI since they also make these double sided so a ground plane can be used on the bottom, allowing for GHz range signals to run over it just fine.

author: kuon
author_rank: Regular Contributor
timestamp: February 13, 2021, 02:05:58 am
content: The flex PCB way is a very good idea, I have a complex path and I think it will be a good way to have something precise.