Mating Two PCBs together without connector - Page 1
Forum section: electronics / projects

author: Apn15
author_rank: Contributor
timestamp: January 22, 2021, 02:13:09 pm
content: I have a project with a very tight size constraint. I need to mate two PCBs together without a connector.I have considered using castellated holes, but I am not sure if this is the best solution.Are there any better solutions for attaching PCBs together without a connector?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 22, 2021, 04:09:58 pm
content: Your, kind of, four options are:- Castellated holes (parallel)- Land-grid pads (parallel)- Thru-holes with wires (parallel or angled)- Perpendicular in slotI suppose you could do some neat jigsaw work with plated slots connecting edge-wise, kind of making a slot joint but it lays flat. Might not be as strong, it would be supported in shear against solder and edge-plating, and edge plating tears out easily.Guessing wires would be a PITA, so that's out.If you're specifically after parallel mounting, that of course rules out a slot, unless you don't mind the added part, and height, of two slots (or other factors limiting such a design).LGA sucks because you can only do it by reflow soldering, and it's very slow because heat has to conduct through the entire board. Preferably the component board would be a special formulation (higher thermal conductivity?, higher temp rating), and maybe soldered with a higher melting alloy too so its components don't shift around during second soldering.Castellation however can be done with soldering iron, and can be inspected visually. (LGA inspection requires x-ray.)LGA of course can cover a much higher density of pins, whereas castellation is purely around the edge.Tim

author: David Hess
author_rank: Super Contributor
timestamp: January 22, 2021, 05:19:03 pm
content: Any solder only connection like castellation would be too mechanically weak.If a connector is not used, then the common solution I have seen is a through hole SIP with two right angle bends. But a single or dual inline right angle header and pins is not much larger.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 22, 2021, 05:53:30 pm
content: Castellation is traditional for RF modules and such; but those are small. OP hasn't provided dimensions; a large module could indeed be fragile that way.Tim

author: David Hess
author_rank: Super Contributor
timestamp: January 22, 2021, 06:28:07 pm
content: Quote from: T3sl4co1l on January 22, 2021, 05:53:30 pmOr in this case any two boards mounted side by side resulting in maximum bending. Castellation works for small modules because they are completely supported although I question the long term reliability if the main board is not well supported and allowed to flex.Periodically you can find castellation used to connect separate boards in test equipment and consumer gear but that is always the first place I look for failure. Even Tektronix was guilty of making this mistake a couple times.

author: Apn15
author_rank: Contributor
timestamp: January 22, 2021, 09:00:18 pm
content: Thanks for the information. Castellation may not be the best route for me to go.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 23, 2021, 12:03:07 am
content: Quote from: Apn15 on January 22, 2021, 09:00:18 pmDo you want to make a sandwich out of a PCB or insert many small ones like radiator fins into one large PCB?

author: tooki
author_rank: Super Contributor
timestamp: January 23, 2021, 08:37:25 pm
content: Quote from: Apn15 on January 22, 2021, 02:13:09 pmThat drawing doesn't really show what you're trying to do. (I mean, surely you don't mean butt-connecting two PCBs?!? Why wouldn't you just manufacture that as a single PCB to begin with?)But generally speaking, if you're super space-constrained, why not look at the masters of miniaturization and see what they do? One thing you see a LOT in such designs is flat-flex PCBs. Those are no longer reserved for megabucks corporations, since places like pcbway make them rather affordably now.

author: gnuarm
author_rank: Super Contributor
timestamp: January 24, 2021, 02:29:02 am
content: Quote from: tooki on January 23, 2021, 08:37:25 pmYou can get flex cable as a stock item. But unless you use connectors this is not a lot different than soldering wires.I recall dealing with this issue a long time ago when other solutions were not apparent and ended up adding pins to the small board. This does take a bit of height for mounting, but not a lot. It gets the boards separated and provides stress relief from thermal expansion. You ask me where to get such pins? I would not know now. I think I bought mine from Milmax (or Mill-max, not sure), but that was 20 years ago.Where do they get the pins to mount LCDs with? They come in straight pins for through hole mount or bent pins for surface mount. Without some idea of the actual height constraints I have no idea if this is practical.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 24, 2021, 04:09:02 am
content: Mill-Max is still around. And still as expensive as ever.There's also SMT spring contacts or pogo pins, and targets (or just use plain PCB pads, but preferably hard gold plated if so*). Targets range from flat-faced pin arrays (looks like machined pin sockets, but no socket!) to SMT pads that are literally just tiny chits of plated metal.Note that any spring contact of course needs something to bear the spring force -- bolts, nuts and preferably standoffs/spacers. For wide spans with many springs (much total spring force), a strongback may also be necessary (some kind of bracket to stiffen the board -- for example, most PC motherboards have some kind of bracket around the CPU, supporting maybe the socket itself (LGA), or definitely the heatsink).Can also do sockets and pins, which are available discrete (singles) or in arrays. Machined sockets that fit into the board (thru hole) can be very low-profile indeed. Again, some clamping force will be desirable, this time just to restrain it from coming apart -- anything from rails to snaps to screws will do.*Plating: use like with like.- Tin plated pins should be suitable for tin plated or HASL pads. Preferably HASL I suppose, as tin plating may be rather thin.- Don't apply solder paste to pads: likely some rosin crud will bead up on them, and not wash off during cleaning, causing electrical failure (or initial failure until it happens to chip off under flexing -- beware spooky failures during initial testing!) This is probably specific to reflow soldering, and allowing wave solder to touch the pads is probably fine.- ENIG sucks: it wears through very quickly.- To do hard gold, mind that the plater needs electrical connection to all the pads at once -- when doing edge connectors, a shorting bar is added to the artwork (by the manufacturer); this gets milled off when the final outline is formed (including the tapered edge for the plug). Allow for something similar in your design. This will probably restrict what kinds of signals you can connect this way, or where (i.e., not good for signal integrity, around the edge is preferable).- Tin on gold plating causes fretting corrosion. Probably something like, where the two touch, intermetallic compounds form, which are brittle, poorly conductive, and wear away quickly. Prefer tin-tin or gold-gold contacts.Tim

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 24, 2021, 04:39:27 am
content: Not cheap but may meet your needs. Flex boards. A series of boards laminated to a flex interconnect layer. The can be folded one eighty between the boards. Limited vendors.

author: strawberry
author_rank: Super Contributor
timestamp: January 24, 2021, 12:41:06 pm
content: Perpendicular PCB mounting and soldering