# How to daisy-chain boards in a PCB stack

I'm looking for a connector that is both daisychainable and lightweight (and possibly, thin). Currently, I'm using ribbon cables (male pin header and IDC female connector) and using the same cable to connect to other PCBs. My project has 5 PCBs, with the maximum connectors-per-cable being 5. The current solution is pretty heavy, mainly because the connector (cable and PCB). Here is a pic of my current solution: I first thought of using flex cables, but the ones I know arent daisychainable, so I think its not an option

EDIT Some points I forgot to say that are important:

1. I want to be able to disassemble the whole entire thing without desoldering
2. The use of "straight", board to board connections, makes me think that when the device is launched (its a femtosatellite), they might risk the pads of my handmade PCBs, so, I think its better to have something that could easily flex without stressing the PCB pads. Also there is one board (telemetry pcb) that dosent uses one of the connectors coming from the main board but the 4th and 5th boards, do use that connector, so, i would have to loose some space to accomodate a connector/holes that would let me pass through the board to be able to connect to the 4th and 5th board
3. For the moment, we only have acces to single-sided PCBs, so SMD connectors might be an option if they can be soldered to wires and used as standalone wire connectors

Here is a sketch of how boards are laid out and connected:

For the moment, a potential (but ugly) solution, mught be to just use some THT FFC connectors, and when daisy chain needed, just peel the insulation a bit before the end, and just solder them to each other, or, remove the insulation on both sides and plug them onto a single connector. We have yet to test this solution for reliability and possibilities to get disconnected

• All-in all, this stack looks like it was a lot of engineering and proper craft! Congrats :) The wire bridges on top that I see indicate this is probably really self-etched single-sided boards. That's a lot of work, and how "we did it" in the past. These days, with 5€ boards with two sides, solder masks and arbitrary shapes that you can order from oshpark, aisler or JLCPCB, I'd very much encourage you to redesign this for two-sided boards. The much nicer precision these board houses can do mean you can pack much more thightly; for example, the SMPS from the bottom board would fit elsewhere. Aug 24 at 13:45
• rephrased the title of your question to describe the engineering challenge, more than asking for a product recommendation. The latter would explicitly be off-topic here. Aug 24 at 14:11
• I don't know the name of the connector, they look like IDE cables: The board-side connector is just a pin header, specifically a 2.54mm-pitched 2×5 male header, and the cable side is called IDC (insulation-displacement connector). Aug 24 at 14:12
• Is price a factor here? There are tons of solutions ranging from a few  to a few hundreds... Aug 25 at 6:55
• There is a kind of budget (10.000AR$≈ 40U$D). We're trying to respect that budget, but if there is no other option, we might as well spend more money. But it has to be justified
– fpp
Aug 25 at 9:36

The standard solution is to use stackable female PCB headers with long pins.

For example, the PC-104 standard.

{Source: Samtec}

Of course, you can do that with smaller connectors as well. It looks like you may be use 10 circuits on each side.

• Though it works, I do not recommend this approach if you need a longer header more than a few pins. I 20x2 headers like these in a 48 board stack and it was a massive pain in in the ass to disconnect them since if you do not pull them straight out, they will jam and lock up especially with wide connectors. With so many pins, the force to pull them apart is considerable and their long length makes it so being even a little bit off angle will cause them to jam. Aug 24 at 21:20
• This would be an option if i used the pins in all boards, and i actually dont (edited main post). But thanks for the suggestion
– fpp
Aug 25 at 0:37
• @fpp they don't have to be connected to anything on every board. They can just go straight through. Aug 25 at 14:48
• @ffp "This would be an option if i used the pins in all boards, and i actually dont (edited main post). " But what you have in your ribbon cable solution is precisely identical (from an electrical standpoint) to what I proposed. If your solution works electrically, so does mine. The only difference is mechanical: yours is bulkier and mine takes less space. Aug 27 at 15:15

https://www.fischerelektronik.de/web_fischer/en_GB/connectors/H03/Printed%20circuit%20connector/catalogue/fischerData/PR/KK_06_Z/index.xhtml It cannot be unplugged, so you need a loop in the wire for disassembly but it is very robust dealing with vibrations and it is lightweight. You can use it on single layer boards. • this ... That's what we call a "transition header (my site). Aug 24 at 20:57 • @DavideAndrea Thank you, good to know! – Jens Aug 24 at 21:01 • They do look good. They're better than soldered wires. We may use this when we have a finished satellite ready to be launched to replace anything that could get disconnected by vibrations – fpp Aug 25 at 0:39 So, I might have had a bit of a ramble in the comments. I think that adds up to an answer, so let me answer here: These boards look like a lot of work, engineering and generally thought went into them. Nice work! But: the fact that they are single-sided, probably hand-etched is limiting you here, seriously: if you found stackable connectors, you'd end up having to drill very precisely to make the holes in your board stack align. That's why your ribbon cabling is actually a blessing: it allows for pretty arbitrary tolerances. Now, that's not too bad a problem: your tolerances already seem to be pretty good. However, I'd still, for the sake of your own sanity, and because in my experience, it ends up saving cost, re-design your boards to be manufactured by companies like oshpark (USA), aisler (Europe) or JLCPCB (China, probably the cheapest of these), for less than 5€ a piece. You get sufficient tolerances, two-sided boards, where you can basically dedicate the bottom side to proper ground connections and put arbitrary, even SMD, components easily on the top side. And the traces can be much thinner and still reliable, your holes/vias very reliable and plated, and you can put a much wider variety of connectors on and/or through the board. Now, I'm going to suggest that you could simple male/female pin header connections between these boards, if you let go of the idea that the connector has to be daisy-chainable. Instead, do the connection between bottom-side connector and top-side connector of each board on the board itself. if you then always put the top-side connector a fixed amount of degrees clockwise of the bottomside connector, you kind of get a "helix" of connection. Problem solved, no need for special connectors. Same principle applies to just alternating between two different positions for the top and bottom connector. When you get custom PCBs with solder mask made, you can use nice mezzanine connectors such as this receptable/plug things; since they only use one side of the board, you wouldn't even have to do the "Helix" trick. Another common method is to make the connection between the boards a PCB of its own – commonly, even, a flexible PCB, exactly to deal with tolerances and vibrations more gracefully. But solid "backplane" PCBs are just as common! Just requires right-angled connectors on the stack's PCBs; these exist in the shape of pin headers, but also for a lot of other designs. This would also work for your ribbon cable, by the way, but that wouldn't solve the weight issue. If weight is actually an issue (assuming this goes air- or even spaceborne), there's little way around actually leaving the inherently low-density single-sided boards behind, anyways. You're wasting a lot of space and mass on something most likely pretty easy to build, still well-structured, on maybe two of your boards. I'm especially looking at the microcontroller board (a "Blue Pill" if I'm not mistake) there; that's really just a cheap microcontroller with a 3.3V regulator and a USB connector and a button on a PCB, and you could have just dropped the same microcontroller on your PCB yourself, at less than 1 cm² of space, no extra PCB, no extra connectors (if air/spaceborne: vibrations are no friends to your connectors!). • As you said, making the pcb was a lot of work, but this has a reason: Time and costs. You may say, 5UD is pretty cheap, but where i live (argentina) cost multiplies by 230 (aproximate), and also, adding to that shipping prices, and that i'm at highschool (so, i live of my parents), it ends up beaing really expensive (add to that that its the first revision, and I already did some mistakes :)). Also, the other problem, time. It takes a lot of time to get the boards in my hand. they may be in the country in three or five days, but they cant take up to 30 days to be in my hand (idk why)
– fpp
Aug 24 at 14:42
• Something that i have forgotten to say, is that soldering pins or using a through-hole connector type from board to board is not an option. 3 reasons: 1. I want to be able to disassemble the whole entire thing without desoldering, so, direct soldered pins arent an option. 2. The use of "straight" connections between board to board, makes me think that when the device is launched (its a femtosatellite), might risk the pads of my handmade PCBs, so, i think its better to have something that could easily flex without stressing the PCB pads.
– fpp
Aug 24 at 14:51
• 3. there is one board (the 3rd one, telemetry pcb) that dosent uses one of the connectors coming from the 2nd board (main board) but the 4th and 5th, do use that connector, so, i would have to loose some space to accomodate a connector/holes that would let me pass through the board to be able to connect to the 4th and 5th board
– fpp
Aug 24 at 14:51
• the last thing you mention, about pcb-to-pcb connectors, is a good option, but only for two sided pcbs. Though, i could try to buy them, and use one of the connectors on a pcb, and the male version, solder it to a bunch of wires and i could make a daisychainable cable that is both, small, and lightweight (in its own sense, it would be better to use a flex instead of normal wires). But I might consider to use it
– fpp
Aug 24 at 14:54
• By the way, I'm really astonished by your project! This is far beyond high-school level engineering, and you ought to be complimented for it. Aug 24 at 15:21

I recommend using hermaphroditic SMD connectors. You won't find stackable ones so instead what you do is you place one on each side of the board with traces and vias to connect between them. This also lets you do little games like making some signals in board daisy chain, while others only go to the board above and others only go to the board below.

Second to that, I would find SMD male and female headers and use those instead.

Something to keep in mind is whatever SMD header you use, if it has mechanical support posts these cannot be longer than half the thickness of the board or they two connectors on either side will interfere with each other.

I do not recommend stackable headers if you need a longer header more than a few pins. I used a 20x2 headers like these in a 48 board stack and it was a massive pain in in the ass to disconnect them since if you do not pull them straight out, they will jam and lock up especially with wide connectors. With so many pins, the force to pull them apart is considerable and their long length makes it so being even a little bit off angle will cause them to jam. –

The overwhelming thing that was going through my mind when I had to spend what must have been accumulated to be at least an hour each day struggling to plug and unplug my board stack was the ease at which the boards could be unplugged. And that was usually with a stack of 6 boards, not the full 48.

I should have also used edge connectors or castellations for the programming but I got lazy when laying out the board. I paid for that later on by needing to unplug all the boards whenever they needed reprogramming.

• They seem to be good, but in our situation, where weight and space are a concern, the ones i could find seem to be pretty big. If you know of one connector that could allocate 20 pins in a smaller footprint than some normal 2.54mm male connectors, i might consider it as an option. Thnaks for the suggestion
– fpp
Aug 25 at 0:35
• @fpp There are LOTS of male-female connectors with a much higher density and smaller pitch than 2.54mm headers. It's difficult to find ones that take as much room as 2.54mm headers. What are you looking? Samtec? HIrose? Hermaphroditic is scarcer, but they are all much smaller pitch than 2.54mm too. samtec.com/products/lss 2.54mm is really huge and I've never seen any signal B2B connector with that large a pitch. Aug 25 at 0:35

If you want to use flex (flexible printed circuit board) for this bus interconnect, double-sided flex (with vias) can probably do what you want by having the bus on one side and the branches that go to the connectors on the other side.

This would be a custom intwerconnect flexable circuit, you draw it in software like kicad amd order online, it will probably cost less than \$10 each for if you buy 20 pieces. (also cost your time to design the board)

Alternatively you could do a rigid-flex combination with rigid and flexable parts permanently connected, or even put all the components on flexable PCB.

If you you want to stay with 1970s technology you could use a rigid PCB for the bus and put edge-connectors on it.