[sdiy] PCB extenders with a flexible ribbon cable - looking for suggestions on construction method

[Ben Stuyts]

You don’t mention the length of the cable, but perhaps some other possibilities are:

- rigid/flex pcb (but could be expensive depending on length of the tail, panelising will help here)
- FPC cables (e.g. https://www.we-online.com/en/components/products/em/connectors/fpc_connector_and_ffc_cab/wr-ffc_flat_flexible_cable_0_50mm)

Consider 3d-printing the case.

Ben

> On 3 Feb 2024, at 12:26, cheater cheater via Synth-diy <synth-diy at synth-diy.org> wrote:
> 
> Hi all,
> I need to build a bunch of PCB extenders. Basically, they're two PCBs,
> connected with a ribbon cable. I have to build like a 100 or so. Both
> sides have gold finger PCB edge connectors, there is no slot on either
> side.
> 
> I was wondering if anyone had suggestions on how to best do this to
> keep the cost, assembly time, and rejects to a minimum.
> 
> The specs are:
> - The signals carried are up to 35Vpp, tiny current, sub-500kHz, balanced
> - Each pcb is 25mm wide (but can be a little wider) and can be
> anywhere between 10 and 80mm long. No logic, purely 1-1
> straight-through connection. They can have vias if needed, but
> hopefully not needed. Produced in a pcb house, not at home.
> - The card edge is on the 25mm long edge and the ribbon would be going
> out of the opposite edge
> - 32-36 pins need to be carried by ribbon. 32 is the minimum, but more
> is fine (I'd just carry ground)
> - The ribbon needs to be as flexible as possible. what is the most
> flexible ribbon you've ever seen? I'm looking to keep BOM cost down
> but if the thing is tough to bend like a stick then that doesn't help.
> Are more core strands better? Mouser lists a few stranding options:
> ones that start with 7 and ones that start with 19. The options seem
> to be: 7, 7x34, 7x36, 7x38, 19x40, solid. I assume the left number is
> the number of strands and the right number is the AWG. What's better
> here for flexibility? The cable isn't going to be moved a lot or
> strained, but up to 2 meters of it are going to hang off the
> connector. I assume higher AWG (=smaller strand diameter) is better,
> and more strands is better, but is 19x40 not going to be, in
> aggregate, thicker than 7x38, and therefore less flexible?
> - What sort of material should the insulation be made out of to be
> (and remain over a few, say 5, years) the most flexible? The options
> are: EPDM, PO, EVA, FEP, PFA, Polyester, PE, PP, PUR, PVC, TPE. Am I
> right to assume that PVC and Polyester are going to be the least
> flexible?
> - Are there tools that can strip all wires of a ribbon at once? Are
> they good? Are they expensive? Can someone suggest such a tool?
> 
> Here's are some construction methods I was considering:
> 
> Method 1. Take a ribbon that's been stripped and pre-tinned in a
> solder pot with 7mm bare wire sticking out. using a press, pre-bend
> the bare wires so if the ribbon is placed flat on a surface, the wires
> aren't "up in the air", and clip the leads to size (think something
> like the surface-mount leads of a QFP or something). The pcb has
> surface-soldering lands on it that will mate those 3mm bare wires.
> Take the pcb and lay the ribbon parallel with it, so the tips of the
> wires align with the pcb. However, the ribbon should go from that edge
> *towards the card edge connector side* which is "the other way" than
> you would expect. Then, you stick the whole thing in a solder pot, and
> after a few seconds, it's all soldered in place. Add a mechanical
> brace around the pcb+ribbon for retention. Finally, bend the ribbon
> 180 degrees so it's going in the right direction.
> 
> Method 2. similar to above, but instead of using a pcb ribbon, use a
> hot bar soldering iron tip. Mount the soldering iron in something that
> looks like an arbor press, put the pcb and ribbon in, press down while
> feeding solder.
> 
> Method 3. Everything's through hole, put the wires through the holes,
> and put the thing in a solder pot; add a mechanical brace.
> 
> One thing that's kind of questionable is whether the pcb is wide
> enough to comfortably solder a 32-wire ribbon without microsoldering
> needed for bridge removal. If not, I considered putting the ribbon on
> a diagonal across the PCB.
> 
> Do you think any of that would work? Which method is the best? If
> yes/no, what's necessary to make it work better? Does anyone have
> better suggestions?
> 
> Finally, I'd have to find some sort of enclosure. Basically, I'm
> trying to make something that will look like the "cartridge" in this:
> 
> http://devkits.handheldmuseum.com/IS-CGB-EMU/index.htm
> 
> ...but obviously I'd need some sort of cartridge casing. I'd prefer to
> buy something that's ubiquitous and can be repurposed for what I'm
> doing. Sadly gameboy cartridges are just too wide for my use, at 60mm
> (vs 28-30mm being perfect), and I'd like to find something thicker
> because I'd like to fit the ribbon inside them for mechanical
> retention. I considered DB-15 connector back shells, but they have
> those "wings" on the side that the mounting screws go through which I
> guess I would have to cut off or something, plus idk if these cases
> would be easy to mount a pcb in. 3d printing is an option, but I'd
> really like to avoid it, given the cost + maybe they're not as
> mechanically durable (it's a connector after all). Essentially i'm
> kind of looking for something shaped like the rectangular container of
> a PEZ dispenser, so I'd welcome suggestions. It's fine if I have to
> lop off one of the sides to make an opening.
> 
> Also, after assembling the extender, I guess it would be nice to have
> a test harness (pass/fail on all signals indicated by LEDs plus
> per-wire LEDs).
> 
> PS does anyone know why PCIE slot connectors (like on motherboards)
> have those slanted "rasps" on one side? Always found that weird. What
> are they for? Some sort of retention mechanism? Curious.
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