Saturday 13 August 2011

Soldering 0.025" pitch cable

Our miniature guitar project uses a double-sided PCB for the fretboard which needs to be connected to the main board using a ribbon cable. We've tried "regular" IDE cable, but had trouble with the multi-stranded cores (they spread during soldering and can bridge with the wires/pads next to them).

Also, using 0.5" pitch IDE cable, the base of the neck-PCB needs to be quite wide to accommodate all the pads. Then we discovered some 0.025" pitch, solid core ribbon cable which seems perfect for the job.

Except, of course, that it's an absolute nightmare to work with.
Just look at how small this strip of 8-way cable is!



Although we have managed to make some workable guitars, each one took a loooong time to complete, and wouldn't be suitable for making on a regular basis. We've pretty much got assembling of the SMT boards down to quite a quick and simple method, using the solder-paste-and-hot-gun approach of assembly. Although fiddly, it's much quicker than using through-hole components.

We're still looking for a quick and simple way to connect our guitar's "body" and "neck" PCBs together. If there's any way we can quickly and repeatably solder this tiny cable to the PCB edge connectors, the electronic design will be complete, and we can put an order in with either quick-tech.co.uk or maybe PCBCart

We made up some test boards, to practice using different techniques for soldering the ribbon cable to the edge connectors. These boards don't actually do anything but allow us to practice connecting ribbon cables



With previous attempts at soldering this tiny pitch cable, we've had problems with it moving around as the solder paste melts and pulls the cable cores onto the traces. Holding or taping the cable to the bench causes it to lift up at the end touching the PCB which makes soldering particularly different. So before soldering, we prepare each ribbon cable:


the tiny bit of cable that will be connected to the board is trapped under a spare piece of PCB/copper clad board

After stripping and trapping one end of the ribbon cable, the remainder is bent vertically upwards and pinched between a second board to make a sharp crease...



...then bent along a second piece of board, away from the trapped end



The result is a "kink" in the cable, which allows it to lie flat along the bench, while the exposed fingers of wire sit perfectly along the PCB edge

Those horrible dirty fingernails are because they've been scraping solder paste off the kitchen table before anyone else noticed!

Using a small soldering iron we managed to solder an example bit of ribbon cable by applying solder paste onto the board and heating it with the soldering iron tip. The result was a technique that worked, but was very fiddly and difficult to do well. The solder paste bridged across the traces and was difficult to remove. Touching the soldering iron between the traces did remove the bridging, but in a lot of cases, also disturbed the other cores and bent them so they went out of alignment with the traces on the board.

So we prepared the board to try the hot air gun method of soldering



This was less successful than the first attempt!
The hot air caused the cable casing to melt. As the plastic melted, it caused the cable to buckle and lift off the PCB edge. With a bit of re-working, we got this bit of ribbon cable connected, but it too was tricky to achieve.



The final method of soldering was the "traditional" approach and worked the best of all (the latest cable is the one on the right with hardly any solder visible). This involved tinning the solid-core strands, placing them onto the PCB traces, then lightly touching each with a soldering iron tip. In this case, a larger tip works well as it joins two or three cores at a time.

The end result is a cable that is attached with very little excess solder and no chance of bridging. Soldering is as simple as holding the tip in place for a second or two. It's by far the easiest method so far for connecting the ribbon cable to the PCB.



The only downside is the preparation of the ribbon cable and tinning the individual strands. Using even a small soldering iron tip, this was fiddly and it took a few goes to get all strands tinned without any bridging between cores.

One solution may be to use a "solder pot" for tinning the wires.



Videos on YouTube suggest this may be the case:


So now we've got a solder pot on order from dealextreme.com. It'll be a few weeks before it arrives, but with a bit of luck, we'll be able to quickly tin/solder all 13 cores with a single dip - the plan being to batch-prepare a load of ribbon connectors and tin them all in one sitting. The solder takes about 10 minutes to heat up to a usable temperature so we don't want to be doing this for just one cable at a time!

3 comments:

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  2. Man I'm commenting here for an advice as you seem to know much about these topics, the thing is that I have a texas instruments ti nspire cas with touchpad calculator, and I accidentally broke the display on it, now i have to replace it but the display's pcb is connected to the main pcb by one of those orange flat cables I think it's called a flex cable or something, but I can't apply heat to it because I know it will melt and everything will become a mess, what I was wondering is to ask you if you consider possible and pertinent to replace that impossible-to-solder cable with a ribbon cable, like one of those IDE hard drives, and gently and patiently solder pin by pin of the cable to both sides of the pcb(s), do you think this will work, please give me a light here as this calculator was very very expensive and it's now out of warranty and texas instruments won't repair it, I would have to pay $80 to get a new one, but I don't want to give up without trying first. Thank you in advance.

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  3. The smallest cable I've managed by hand is this quarter pitch (0.025" or 0.625mm) which I got from a load of old floppy drives. The actual cores are solid copper, which makes them easier to work with than the IDE cables you get from hard disk drives - the HDD cables have multi-stranded cores and it's really easy to create bridges and get little stray strands.

    So stick with multi-core cable that has solid core strands. If your connector is smaller than 0.025" (or 6.25mm) then I think you might need to find someone who has a hot bar or similar.

    I've changed how I solder these a little bit - I flood the pads with flux, put a bit of solder paste across the pads (in a squiggle, don't try to do each pad individually) then touch the tip of the iron onto each pad. The solder paste jumps to the nearest pad, and all the pads are now tinned, ready to go.

    Dip the end of the cable into a solder pot (or carefully dip into a pot of solder paste if you don't have a solder pot - but remove any bridging, you only want a tiny amount of solder on each strand).

    Line up the strands to the pads and dab the end of the soldering iron tip onto them. If you've a sharp point on your tip, you can drag it between two strands and solder two in one go.

    Good luck!

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