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I need to plug an OpenCM9.04 A-type board into an OpenCM485 expansion board and connect some cables to the gpio sockets. From the manufacturer, the gpio sockets on the OpenCM 9.04 board are through holes and the 485 expansion board has the PCB socket pre-soldered on and I do not intend to remove it unless I have to.

My current solution is shown in the pictures. I soldered some long pin PCB headers with soldering pads on the other end onto the OpenCM 9.04 so that I can plug the board onto the expansion board. Then I solder wires to the soldering pads. However, it is not a pleasure to solder the wires, and the wires may come loose if the board is not carefully handled. What would be the recommended solution to this?

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  • \$\begingroup\$ At worst you could just solder it onto an identical header and plug that header into the header on the PCB. Then it would be removable, albeit with difficulty. Not sure why you would use round machine pin though when your baseboard looks like a square pin socket. Because you could have just used square headers which have compatible male crimp pins and housings for them. \$\endgroup\$
    – DKNguyen
    Commented May 18, 2021 at 21:03
  • \$\begingroup\$ @DKNguyen I didn't known they were machine pin... I thought they were meant for easy soldering... I also do not have much headroom for long square pin sockets. \$\endgroup\$
    – Anthony
    Commented May 18, 2021 at 21:17
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    \$\begingroup\$ Get right angle ones then with long tails. I recommend you go male headers for the PCB and female for the wire if you have enough clearance in your baseboard. \$\endgroup\$
    – DKNguyen
    Commented May 18, 2021 at 21:22
  • \$\begingroup\$ @DKNguyen Thanks a lot. I thought about using right-angle headers but I found dupont connectors come loose easily. What would you recommend for female connectors? I am thinking about adding dupont crimp to the wire and then solder the crimp with the wire on the male pin and heat shrink to cover it - not sure if this is considered as a good idea. \$\endgroup\$
    – Anthony
    Commented May 18, 2021 at 23:54
  • \$\begingroup\$ Are you planning on yanking these or what? Some manufacturers have normal-force and hi-force variations of their female crimp contacts. There are also positive locking versions of header and crimp housing. They can have a surprising number of pins (25 max on the brand I am looking at), but because of the extra width on the sides you can't put them side by side without skipping over pads. \$\endgroup\$
    – DKNguyen
    Commented May 18, 2021 at 23:56

2 Answers 2

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Don't go with round machine pins. Yes they are higher quality but are available with fewer options and right now you need those options. Go with square headers instead.

Get right-angle long-tailed male headers to go on the board (don't go female on the board because they are available in fewer options).

Then get crimp housing with female crimp contacts.

The lowest profile is probably this set up. If you are concerned about pull out, it might not be as bad as you think when the crimp housing is so long. Have you ever tried unplugging a crimp housing 20 pins long? It might be more difficult than you think; Not like with fewer pins. The forces from each pin adds up and when the connector so long, unless it is pulled straight out it likes to rotate instead and jam. If you're still concerned you can also just loop and tie a piece of thread or dental floss between the header pin and crimp housing wire on each end of the mated pair.

Some manufacturers also produce high-force versions of their female crimp contacts. In addition, you can get headers and crimp housings with a postive locking latch. These take up a bit more space and width (the width means you can't stack them side-by-side without skipping over a 0.100" space. However, you need long-tailed headers and I'm not sure if latching headers are available with that.

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This is not a good idea overall.

  • Avoid soldering straight to a PCB in case it needs to be replaced. Use connectors instead.
  • In case you do solder straight onto a PCB, then do so directly in the vias. Remove the socket strip, it fills no purpose. The solder job on the picture is poor and the presence of the awkward socket strip is likely partially to blame for that. Some cold joints, some burnt isolation, poor wetting etc. You'll need to remake this from scratch.
  • You need mechanical strain relief on those wires regardless of how you attach them to the PCB. Your current solution will easily break above the solder joints even if they are done properly. This means at a bare hobbyist minimum: apply some hot glue. Though real professional solutions integrate strain relief with the connectors themselves.

Instead you could use a header strip, then you'll have more options when it comes to some suitable board-to-wire connectors. This does become somewhat problematic if you need to connect all of these pins - if it was just a select few you could go with some connector family with built-in strain relief. There exist lots of 2,54mm board-to-wire solutions, many far from ideal. Most require some sort of tool.

Search terms: 2.54mm/0.1'' board-to-wire, single row header strip, IDC. I'd generally recommend to start looking at the well-known premium brands such as Samtec, TE Connectivity, Molex, JST. Find something you like there and try it out. If you need to optimize cost for volumes in the future, then you can go look at cheap second sources from there.

As for general soldering advise: avoid bending the wires at the point where the joint is - in case you need to bend them 90 degrees (not recommended), then do it advance before soldering. Carefully pre-apply solder to the wires before attaching them to the board. Measure exact peel lengths. Easy on the heat, you should only keep the solder iron on the joint for a second or two, just until the exact moment when it wets. If you are a beginner at soldering then get a flux pen and apply some extra in advance, makes everything much easier.

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