Q on board-to-board connectors

[Jonathon_Doran]

I'm thinking of splitting a design into two boards rather than one.  One area I think would be done best as a 4-layer board.  Another is a lot of switches, and I don't see why this shouldn't be done as a two layer board.  I'm thinking:  Why spend money on extra layers when I don't think they will help.

This would require some sort of board-to-board connector, and I have no experience with this.  I would like something that is reliable.  I don't see a lot of connection cycles.  I'd rather not need to buy crimpers etc for a single project.

We are talking low voltage, almost no current.  Some LEDs would need power, but mainly switches.

Any recommendations?

[langwadt]

is it worth that hassle to save a little on the board?

[n4u]

Easiest way is using pin header (if u want one board on another). BTW you don't need crimpers for awg cable / crimp connector - u can make it using pliers (but u will buy crimpers sooner or later).
TIP You can use MUX, or small uC if you have a lot of buttons /switches, and then use serial standard to connect between boards.

[jmw]

Sounds like a false economy unless you're using a certain PCB house known for purple boards and charging by the square inch. In which case, maybe consider somewhere else.

But otherwise, how about flat flex cables? Pre-cut to length, no crimping, and pennies per cable if you order from LCSC.

[Psi]

It's usually better/cheaper to have one PCB rather than two.
So you only use two if you absolutely have to.

One cheap option to join them is a FFC, you can get them made to any length very easily and have an FFC connector on each PCB.

Cheaper than that is a hotbar flex (eg flex without the connectors). But you then need a hotbar jig to solder them.

You can of course just use pin headers to join the PCBs but you have to be careful because that's a rigid connection and you will fracture the solder joints from general vibration unless the boards are held solidly together.

You can always just use a 2.54 wired connector headers. but cost is higher.

Or SMT mount board to board connectors but that is also higher cost especially at high pin counts.

[ajb]

"Board to board" generally means that a connector mounted on one board directly plugs into a connector mounted on another board with no wires in between, is that what you want?  (Also called mezzanine connectors when the two boards are parallel.)  If so pin headers are an easy solution, they can be had in straight vs right angle, 100mil, 2mm, or 50 mil pitch, single/double row, top/bottom entry, etc, and all kinds of pin counts.  Lots of other options, though.

If you need wires in between, then you need "wire to board" connectors.  Lots of options here, how many signals would be the first question since you already mentioned low voltage and very low current.  An FFC jumper with matching connectors would be a good option for high pin counts, as long as you don't mind them being fairly fiddly to mate and unmate.  Jumpers are readily available in a range of lengths and pin counts, usually 0.5mm or 1mm pitch, although you might not find your exact ideal length and pin count at a distributor you can probably find something that will work.  Just make sure to pay attention to which side the contacts are on with both the jumper and the connectors. 

If an FFC doesn't work for you, a ribbon cable with standard IDC connectors could do.  The IDC connectors can be installed onto the cable using a vise or parallel-jaw pliers if the connectors are small enough.  Otherwise, you can find wires with pre-crimped terminals on both ends for many common connector families (IIRC I've seen JST XH and ZH and Molex Mini-Fit pre-crimped leads at Digikey).  You'd need to buy the connector housings separately, and the terminal ends just snap in to the housings with no tools required. These would be good for maybe up to 12 positions per connector, higher pin counts start to get unwieldy. 

[Jonathon_Doran]

Thanks for the feedback everyone.

It may indeed be a false economy.  The first design for this board would be very large, and most of it could be 2 layers.
I may be able to shrink things though.

Wire-to-board is what I was really thinking of.  FFC is an interesting option.   It would allow for a more compact solution

[n4u]

IMo that is not bad decision to split boards (specially with 4 layers layout).
You can make 2layer - interface version by yourself, or buy boards cheaper (or thinner for other impedance etc) , and universal cpu/uC/fpga used in another projects.
Its depend what you want to create instead of pricing. You dont order pricey board to make some adjustments.

[tooki]

BTW you don't need crimpers for awg cable / crimp connector - u can make it using pliers (but u will buy crimpers sooner or later).

I don’t know what you mean by “awg cable”. AWG means “American wire gauge” and is simply one system for measuring wire sizes. It doesn’t say anything about what kind of wire it is or what connectors go with it.

I strongly advise against using anything but good quality crimp tools on any kind of crimp terminal.

The only sort of exception is IDC connectors (like the classic box header type) for ribbon cable, which can be done properly using a bench or machine vice or parallel-action pliers. (DO NOT use standard pliers that close at an angle.) IDC isn’t really a crimp terminal in the strict sense of the word, since the terminal doesn’t squeeze around the wire.

IDC connectors for individual wires require a punch-down tool, though these are far cheaper than crimp tools. (For example, for TE MTA-100, the plastic tool for low volume production is just $40.)