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I wish to design two separate PCBs (due to mechanical requirements), with 4 interconnections in between them.

Now, I could use standard pin header males and females for this task:

enter image description here

However, PCBs distance is less than 2mm (!).

I couldn't find shorter pin headers, are they existing at all?

I need only 4 pins, and it two PCBs must remain detachable (soldering them together is not an option).

Is there any option for this which I can finetune the 2 PCBs distance at least with 0.5mm precision (but 0.25 or 0.2mm would be even better)

Two boards are needed to be stacked i.e. like a sandwich, one is the bottom board, other is the top board:

---------------- TOP board
 | | | |
---------------- BOTTOM board

4 connectors are used for USB (+5V, GND, D+, D-).

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    \$\begingroup\$ Stacked vertically or side-by-side? \$\endgroup\$
    – Mattman944
    Commented May 9, 2023 at 20:21
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    \$\begingroup\$ Low Profile Mezzanine Conenctors should show you plenty of options. Probably not many with only 4 pins, but you can always double-up and/or ignore extras. surface mount spring battery contact connector might also show some useful options. \$\endgroup\$
    – brhans
    Commented May 9, 2023 at 20:40
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    \$\begingroup\$ Have you considered zebra strips/elastomeric connectors? \$\endgroup\$
    – Hearth
    Commented May 10, 2023 at 0:45
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    \$\begingroup\$ What is the orientation of the two boards at the mating point, top-bottom, bottom-bottom, top-top ? \$\endgroup\$
    – Nedd
    Commented May 10, 2023 at 2:05
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    \$\begingroup\$ I wonder what speaks against having plated through-holes on both boards and screwing a metal spacer between the boards with metal screws. Or soldering an SMT metal spacer on one board and screwing it to a plated hole on the other board with a metal screw. \$\endgroup\$
    – root
    Commented May 11, 2023 at 15:37

10 Answers 10

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Instead of using hard pin connections go flexible. Use a flat flexible cable (FFC) to interconnect the two boards. Two low profile flex cable connectors could be mounted near the edges of each board, then a short flexible cable would be used to connect the boards. With ever shrinking laptops, tablets and cell phones these types of connectors have been getting smaller. If the connector is low enough the bottom board might have it mounted on the same side as the top board. Here is an example of a flex cable connector with a 0.9mm height. Spacers or stand-offs mounted on the bottom board would determine the space between the two boards.

Connectors mounted on opposite sides:

enter image description here

Connectors mounted on same side:

enter image description here If the two boards can be slightly different in size make the bottom board slightly wider. Then you could mount almost any connector type on the bottom board's edge that extends beyond the top board. enter image description here

If the two boards must be the same overall size then consider designing a notch into the edge of the top board. The bottom board would then have a connector placed where the top board's notch is located. enter image description here

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    \$\begingroup\$ You could even run the cable between the boards, so that the boards and cable form a "Z" shape. I see that arrangement in laptops rather frequently. \$\endgroup\$
    – bta
    Commented May 10, 2023 at 18:28
  • \$\begingroup\$ FFC connectors can be a pain in the development process- they only have a few mating cycles and can be fiddly. They do have the advantage that you can lay both boards side by side for debugging. \$\endgroup\$
    – D Duck
    Commented May 12, 2023 at 8:32
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Consider the standard solution: mezzanine board-to-board connectors, specifically dual beam connectors (my site).

For example, Hirose DF40, 2 mm stack height.

Hirose DF40

{Digikey}

I know you said that you only need 4 positions, but, for a stack height of 2 mm, 10 position is the smallest connector that I found.

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    \$\begingroup\$ 4.6 mmX3.38 mm is too much space @Daniel? \$\endgroup\$
    – RussellH
    Commented May 10, 2023 at 0:02
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    \$\begingroup\$ You should look at how insanely small those things are before ruling them out. \$\endgroup\$ Commented May 10, 2023 at 1:01
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    \$\begingroup\$ the pin headers you were proposing and ruled out due to mating height are way wider than 1 mm @Daniel... \$\endgroup\$ Commented May 10, 2023 at 9:10
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    \$\begingroup\$ @SpehroPefhany Or rather, look at how insanely small they are and then rule them out :) Though if some external mechanical strain relief like SMD spacers are used in combination with these, one can get quite a rugged connection. \$\endgroup\$
    – Lundin
    Commented May 10, 2023 at 11:27
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    \$\begingroup\$ @Daniel "I'd prefer having the connector in straight line". Wouldn't have made sense for you to state that in your original question, so that people would focus on finding solutions that best meet your needs? \$\endgroup\$ Commented May 10, 2023 at 13:58
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If the mechanical alignment is taken care of by e.g. screws, a spring loaded contact on one side, and gold plated PCB pads on the other side is an option.

For example Mill-Max has some low-profile spring loaded contacts that work for PCB distance of approx 2.0-2.3 mm:

Mill-Max connector specs

And for even shorter distances, there are individual surface mount spring contacts, such as these from TE Connectivity that are suitable for 1 mm PCB distance:

enter image description here

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Some of the comments and answers mention bottom entry connectors, actually if your boards are orientated back side to back side you might try using two bottom entry sockets, one on each board. Then use a separate double ended header pin at the center to make the board to board connection. See the first diagram.

enter image description here

Using two bottom entry sockets in this way would allow a minimal clearance between the two boards. The plastic center of the header pins would serve as the spacing control. If you need the boards even closer together see the next option below.

.
Several of the bottom entry sockets also allow top entry of the pins. So another alternative would be to use an extra long header pin type and insert that into the top board connector then continue it right through into the bottom board connector. That arrangement could give you the absolute minimum spacing. You would add your own small spacers on one of the boards to obtain the required board to board spacing, or maybe just use a thin sheet of plastic between the boards. See the second diagram:

enter image description here

An example of one such bottom entry socket from GradConn is a 1x4 SMT version that would have a P/N such as: BB02-GS042-XXX
https://www.gradconn.com/News/Connectors-Demystified-Bottom-Entry-Board-to-Board-Connectors https://www.gradconn.com/Products/BoardToBoard/All/BB02-GS

There are even a few 4 pin (2x2) SMT bottom entry sockets in this form that may work for you. One version from Wurth Elektronik:
https://www.we-online.com/components/products/datasheet/610304243021.pdf

.

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  • \$\begingroup\$ Bottom entry is unfortunately no-go, bottom PCB is lying on the "bottomest" level, I can't mount anything onto its bottom. \$\endgroup\$
    – Daniel
    Commented May 10, 2023 at 6:27
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    \$\begingroup\$ @Daniel - What about fixed male headers on the bottom board and a bottom-entry connector on the top board? \$\endgroup\$
    – Rainer P.
    Commented May 10, 2023 at 8:55
  • \$\begingroup\$ @Daniel I was about to suggest this solution too, but not as shown in this picture but just as Rainer says in the comment above: with the top PCB having a bottom entry and the bottom PCB having a header strip. Options for exact mated height 2mm exist because I have used that very solution myself in the past (with a 2x5 2.0mm pitch). It's a good and universal solution with 2nd source available. \$\endgroup\$
    – Lundin
    Commented May 10, 2023 at 11:24
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There exists individual pcb pin sockets that can pass a pin right through. Perhaps they can also be reflowed? Here's some from mill-max:

SOLDER BARRIER PIN RECEPTACLES & SOCKETS

https://www.mill-max.com/products/new/solder-barrier-pin-receptacles-sockets-with-organic-fibre-plug-knockout-bottom

enter image description here enter image description here

I can imagine that you could insert pins though the far side board A and into sockets soldered into board B. If the pins are too long, then they would pass right through. This would allow the two pcb's to be placed as close as you want.

Good luck!

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    \$\begingroup\$ I also have two PCBs that must be very close to each other. I agree the MillMax cups do the female part with almost zero spacing. On the male side I use the pin headers but mount the plastic on the other side of the board. The long pins pass through the top board and them into the bottom board. \$\endgroup\$
    – user338146
    Commented May 9, 2023 at 22:11
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This is not a low-cost solution, but Cinch makes a product called CIN::APSE, which is a Z-axis compression technology. It uses what looks like small Brillo pads (randomly wound gold plated molybdenum wire, formed into a cylindrical shape)in a holder that compress when the two assemblies are mated.

This page, from the Cinch CIN::APS brochure shows how this works to mate two PCBs together. From top to bottom, we have the bare wires assembly, then the holding assembly, which is then compressed between the two PCBs.

enter image description here

Here's the link to where I found the brochure.

https://www.belfuse.com/product-detail/cin-apse

Note, we used this successfully on a hi-rel satellite application, so it is robust. But again, like I said at the outset, this is not cheap.

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I'm not really fond of soldered-together un-inspectable unrepairable assemblies, but you could use 2mm or even 1.27mm pitch header pins.

enter image description here

Maybe you can find a bottom-entry socket that would allow the boards to be plugged together using one of the finer pitch headers (or with the header on the other side, but that is problematic- can't be machine soldered and if the solder wicks up the pins you'll have problems).

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  • \$\begingroup\$ How can I go below 2mm with these? Like 1.8mm distance between the sandwiches? \$\endgroup\$
    – Daniel
    Commented May 9, 2023 at 21:02
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    \$\begingroup\$ Smaller ones! eg. Samtec FTS 1.27mm SMT or through hole male and bottom entry female. digikey.sg/zh/htmldatasheets/production/183877/0/0/1/… These can go down to 0.9mm through-hole or 1.27mm SMT \$\endgroup\$ Commented May 9, 2023 at 21:35
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Hirose makes some good connectors the ds-13 series is great if you need a lot of conductors and smt. But really just go to digikey and look in the rectangular connectors and you can search by height and mated height. Set mated height to 2 millimeter and you can find many connectors that meet your needs

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If these are USB, you can't do it with simple interconnects - and your PCB layout probably needs fixing too

The data lines for USB don't just need any random connections. They need to be treated like transmission lines, even for USB1, because you've got signal harmonics in the hundreds of MHz to deal with. If you're even contemplating this kind of interconnect, then it's basically certain that you haven't got your boards laid out for this

You could use a separate coaxial connector for the data lines. At work we've used low-profile clip-on connectors for running signals to a panel connector, so those could be an option. I can get you details if you want.

But the fact you're looking for something which has a cast-iron guarantee of failing EMC - and more than that, simply might not work at all if you're trying for faster than USB1 - tells me that you have a much deeper problem than just your interconnects.

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    \$\begingroup\$ USB2 is run over headers inside PCs all the time (just look at how a front panel USB port is wired to the motherboard) \$\endgroup\$ Commented May 12, 2023 at 2:20
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    \$\begingroup\$ re inside PCs: OP may not have the PC metal case \$\endgroup\$ Commented May 12, 2023 at 4:36
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    \$\begingroup\$ @FlorianCastellane I may be wrong, but I kind of assumed that the metal PC case is to keep the noise IN not OUT. \$\endgroup\$
    – Pelle
    Commented May 12, 2023 at 7:57
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The photo shows a technique I have tested out. These are standard through-hole 0.1in headers mounted on their sides, but I gather there are SMD variants in smaller sizes (eg 1.27mm) available.

The RHS PCB is rotated (as though turning over a book page) and the pins slide into the sockets as shown. There is some flex possible in the vertical plane and along the pins.The sliding arrangement might not be possible depending on what is present on the boards etc. It also doesn't feel 'intuitive' as you expect to be able to simply pull the boards away from each other instead of sliding.

This has the advantage of being contained within the gap between the boards.

A PCB showing how headers can be mounted horizontally for low height mating.

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