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Does anyone have ideas on how to mate a flex PCB with a screw terminal block?

Some background:

At my workplace, we're using the following Keysight DAQM900A Datalogger MUX cards:

Keysight DAQM900A MUX Cards

In the past we've placed the flying leads of our cables into each slot directly. However, because this requires some labor time, there's been a push to modify the cards. Specifically, we want some circuit designed that can easily be placed inside the MUX card to interface with the screw terminal blocks instead of using flying leads; on the other end, we want to add a connector with a standard pinout so we can easily purchase cables from assembly houses.

What I've done:

So far, I've decided to remove the strain relief slots, drill mounting holes at the entry for a D-Sub PCB connector (for strain relief), and run flex ribbons from the PCB to all 4 screw terminal blocks (see image with basic idea drawn for 2 of the 4 terminals):

Example Flex PCB Layout

However, I can't seem to find any connectors that work between flex ribbons and screw terminals. Originally, I considered using FPC connectors with vertical leads: if done right, the leads could mate with the screw terminals. However, I can't seem to find anything with the right pitch for these terminals. Most FPCs have a 1mm pitch, but I'm looking for something with ~3.25 mm pitch and 5mm lead length.

Does anyone have any other connector suggestions? Or even a suggestion on a different way of going about this?

Thanks in advance for the help!

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  • \$\begingroup\$ I hate DSUB connectors, this is my contribution. When I look at a project with DSUB, I always look the way how to replace them with terminal blocks as you already have. I do think the Keysight engineers had the same opinion. \$\endgroup\$ Jan 15, 2021 at 20:52
  • \$\begingroup\$ flexible printed circuits are available in any configuration that you can draw in a PCB design software \$\endgroup\$
    – Jasen
    Jan 15, 2021 at 21:00
  • \$\begingroup\$ @MarkoBuršič DSUB connectors can be had in industrial, military and NASA specifications, and are used all the time in space hardware - that's the indefatigable Amplimite 109 series from (now) TE Connectivity. Good enough for space is good enough for anyone, I'd think. I usually use that series for internal and T&M gear, because all the pins are machined, and because there are useful variants available (blind mating and improved EMI performance, for example). I hate terminal blocks because they imply manual wiring - cost and mistakes. Nope for me. I guess you never experienced the proper DSUB! \$\endgroup\$ Jan 15, 2021 at 21:56
  • \$\begingroup\$ @Kubahasn'tforgottenMonica As an industrial field engineer, for sure, I had always in hands the best and most expensive DSUB connectors ever. If there is a such need for fast connection for machine transport, then only Harting connectors are used. Now, if you like the DSUB, then use them, me not. There are also lot of quality terminal blocks screw mounting, spring mount, ..somehow you have to fix the wire on the connector and its much better than soldering wires. \$\endgroup\$ Jan 15, 2021 at 23:10
  • \$\begingroup\$ @MarkoBuršič As Kubahasn'tforgottenMonica mentioned, we're trying to not use terminal blocks for the manual wiring costs and mistakes risk. I guess I tend to view DSUBs favorably because they're used for almost all the projects I work on. I haven't really heard of Harting connectors; what makes them better than DSUBS in your opinion? \$\endgroup\$
    – Noah B
    Jan 19, 2021 at 16:54

4 Answers 4

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run flex ribbons from the PCB to all 4 screw terminal block

Why? You're only doing the modification once, so you're probably spending more time thinking about it than will take to assemble it :)

The use of a ribbon from the PCB to the screw terminals is not the best idea, since ribbon cables have too small of a wire to reliably connect to screw terminals. They'll tend to break in presence of any long-term vibration, whether in use or in storage.

Instead, you don't even need a PCB. Get a proper crimper to crimp wires onto pins that fit in a DB-25M shell, and assemble a bunch of such wires, with ferrules crimped on the other end. Then insert the pins into the connector shell, they'll click into place, and screw the other ends of the wires into the screw terminals.

I've done just that and it works fine. I don't like the cheap sheet metal stamped pins, so we used the machined "mil style" ones - Amplimite 109 series (catalog here). When perusing the catalog and looking for deals, note that most parts have multiple and different TE part numbers for military, industrial and NASA qualified parts - because of different internal bureaucracy and QC needed for each target market, even if the physical part is otherwise identical. Sometimes there's so much overstock available that even prime distributors have NASA or milspec parts for cheaper than industrial parts.

This is a high-reliability solution - we got the wire as well as the pins from overstock, so they were cheaper than usually, but still was well worth it. The pins are crimped with an adjustable 4-point crimper:

When you look inside the mux then, it looks like a million dollars :) I generally like my production test tools to be reliable, and connectors and connections are the primary source of trouble, so using good ones helps. You won't do too badly looking for this stuff on eBay, since the parts are niche and it's easy to tell whether they are genuine. You have to apply some engineering common sense to it to balance your time vs. cost savings.

If you want to mess with it a bit, those HP mux boards aren't too magical and can be reverse engineered to make a bespoke variant with the D-SUB connector footprint on the board.

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  • \$\begingroup\$ "ribbon cables have too small of a wire to reliably connect to screw terminals" -- can't one use crimped on wire ferrules for that? The wire will still be fragile, but screw connection should be ok. With some creative strain relief / a cover to protect it, it should be safe. After this no need to access the card all the time. \$\endgroup\$
    – Pete W
    Jan 15, 2021 at 23:57
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    \$\begingroup\$ @PeteW It may work OK, but it looks like crap, and the DB25-IDC-to-ribbon connection while not needing a PCB still needs a vise and steady hand and an eye for trouble, ideally you’d make a 3D-printed fixture for that. It just is not very convincing. Learning how the D-Sub pin crimping works will pay off many times over, since it’s basically the way D-Sub was meant to interface connectors with wires. There’s a single gas-free connection in the crimp zone, and it’s rather resilient with 22 or 20AWG wire. Half the time the wire will break away from the pin if you try to pull it out. \$\endgroup\$ Jan 17, 2021 at 19:35
  • \$\begingroup\$ "Why? You're only doing the modification once, so you're probably spending more time thinking about it than will take to assemble it :)" @Kubahasn'tforgottenMonica While with every PCB I design, I use this wiring layout, there's several projects in my lab that don't. Occasionally, we need to reuse the cards for other projects, so there's some labor that ends up being repeated. I guess because my lab wants to reuse the cards instead of just purchasing more for the other purposes means I want a setup that can easily be set up and removed repeatedly reliably. \$\endgroup\$
    – Noah B
    Jan 19, 2021 at 21:16
  • \$\begingroup\$ Also it appears my management is okay with me working on this for a bit now because due to COVID, I'm a bit less busy. My thinking is I can use this time to reduce future costs and schedule delays. \$\endgroup\$
    – Noah B
    Jan 19, 2021 at 21:25
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Your question is tagged "pcb design" - how about designing a PCB for it? :)

I mean a normal rigid PCB, with edge milled to shape to fit the connector pitch. Something like this:

Adapter PCB mockup

One end will mate with the screw connector, and on the other half you can put whatever connector suits your purpose.

For good contact, make sure to have copper on both sides of the "pins", use gold plating and select board thickness to be suitable for the screw connectors.

To avoid trouble with the PCB manufacturer, check what is their minimum "non-plated slot size". Usually it is about 1 mm. Put rounding arcs with equal or larger diameter on all corners to make it easy to manufacture.

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For test setups, I've used pin headers soldered into a bespoke PCB.

If you can't find pin headers with the required pitch and length you might resort to using pieces of wire.

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If you have the possibility to drill holes into a metal frame of the equipment, then I would suggest you to use Harting HAN, D, DD, EEE, EE... series of connectors. In similar way as Kubahasn'tforgottenMonica suggested you to use wires with crimped ferules at the trminal block, and then to use pins that you crimp on the new connector end. You do insert those pins into a connector and then you screw on the connector holder.

enter image description here

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