Need help with custom flex cable

I have an old flex cable that I need to recreate. It’s old and brittle. I will upload an image as soon as I am allowed. The Cable is single sided and connects on one side to three individual 6 pin connectors. On the other side it has 18 pads to solder jumpers to a PCB. Can I do this in KiCad? I’m a beginner to this software and keeping my fingers crossed. Thank you for taking the time to review my request. Any help will be greatly appreciated.

If I understand correctly, you want to make what I would call a flex circuit? Really a flexible pcb? That sounds reasonable for KiCad.

Yes, I guess it would be like a flexible pub. It has a bit of an odd shape.

Someone here ought to be able to enable you to post an image. I am not that person… Have you ever designed any sort of printed circuit using any software?

Hi @Dangerbird

This FAQ explains when you may post multiple images.

It seems you only need a few more minutes of reading 'till you become “basic”.

Thank You for the heads up.

[Converted to jpg and inserted inline - Mod]

I also converted your picture first to a (lossless) .png and it shrunk from 2.2MiB to 830KiB.
I also converted to jpg with Gimp (Quite lossy, with aggressive compression) and it shrunk again, this time to the same 69kiB as your later replacement.

BMP is a quite horrible file format :slight_smile:

But apart from the files…
KiCad is not very good in mechanical CAD. When a PCB outline gets complicated, it’s often easier to draw the PCB outline in e mechanical CAD program, export it as .DXF and then import that into the Edge.Cuts layer in KiCad.

The picture you uploaded is plenty good enough to show us what you want, but it is not a very good picture for reverse-engineering purposes. Small parts are missing from the picture, and the scale is ambiguous. I find that using a graph paper with a known size of squares as the background helps with capturing the size and scale in both X and Y directions.

About half a year ago, KiCad-Nightly gained the possibility to load (and scale) a background image directly into the PCB editor:

KiCad very much likes to have a complete netlist, and a schematic is the easiest way to get this.
It’s also easy to create. Just use some generic connector symbols and draw wires between them. (And there are some tricks to do this quicker, such as repetition with the [Ins] key and moving a connector on top of another (The attachment points must match) and then dragging them apart)

For the PCB I would first design custom footprints for the connector fingers, attach them to the schematic symbols and then Schematic Editor / Tools / Update PCB from Schematic [F8].

Also note that the flex is damaged in both corners where it starts getting wider (And this may well be the reason it failed in the first place). Changes in width of flex cables should be very gradual to prevent stress points.

On the other side…
If it’s for a one-off repair and the cable does not move too much, then I would just solder wires instead of creating a new flex cable. Just soldering wires is quicker than desinging a Flex PCB and having it manufactured.

Wow, thank you so much for your help. Unfortunately this cable moves a lot and is quite brittle with age. And I would like to make several. But I will follow your advice and see if I can come up with something usable.

This is a fairly simple circuit, and I’m curious about that relatively new background image thing. I’ve also got some left over time.

Just curious, If I make a first concept of this thing, what sort of donation are you willing to make to KiCad? (There is a campaign going on right now).

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This post - Most likely not of much interest/helpful… It contains my post re FlexCircuit…

Re your image and my early morning Coffee… I used my home-grown App to measure some items - for scaling, I assumed Pitch between two small edge Pads is 2.54mm. Screenshot worth 1000 words…

I’ll assume you need more coffee, just don’t drink too much or you may green :slight_smile:

If any of the pads are on a 2.54mm pitch (which is already a bold assumption for a soldered flex cable), then it’s quite likely the coarser pads on the bottom. This also comes closer to the background lines, which often are around 7mm (I haven’t measured any) for regular writing paper.

That means BlackCoffee’s assumption is off by a factor of about 2. This indicates how easily you can be off by making such assumptions, and there is no need to make such assumptions when the real thing is available for measurement.

I also use photographs for reverse-engineering sometimes. Below some pictures of measurements on a gear (cog?) wheel.

The messy red blob just above the “25.9” mm radius is a triangle with measurements that can be turned around to overlay the gap between any of the teeth. Here is the same, but the triangle rotated and zoomed in:

With this method you can take quite accurate measurements. Also note that there are two stainless steel rulers in the picture (with mm scale) that can be used to calibrate the picture. To be accurate, the rulers have to be in the same focal plane as the thing you want to measure, as scale changes with distance to the camera. Another way to calibrate is to for example measure the outside diameter of the wheel with a caliper.

For a flex cable like this, pictures are best taken with a flatbed scanner, which has the least parallax error. Second best is to take the picture from far away, and then zoom in the lens. This also reduces distortions in the picture.

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before helping you with a mechanical draw we would need some quotes from the image :smiley:

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