PCB contract manufacturers like JLCPCB (“CMs”) can not only cut out your circuit board to specified shapes with a router (2-mm endmill), but also score it with V-grooves on one or both sides, so you can break it into panels. Looking at the Boréas Technology evaluation board, I see that it’s scored this way on one side to be able to break it into three boards, but has traces across the scoring on the opposite side of the board, so until you do break it into panels this way, it’s a single board; but once you do, you can reconnect the previously-connected pieces at a distance using a cable, or use them separately. Among other things, this approach is useful for testability.
Other forms of panelization include routing that leaves tabs (“tab-routing”) and drilling a series of holes, called “mouse bites,” most often used to weaken a tab, avoiding the need to score it with a knife, cut with diagonal cutters, or use a depaneling nibbler. Mouse bites in particular can create curved breakaway edges, and they leave a rough edge on the board after breaking, which can be useful or harmful. JLCPCB in particular is willing to do some amount of tab-routing without increasing their price of US$2 for 10 boards of 100 mm × 100 mm. There are established panelization guidelines.
SparkFun has a whole “ProtoSnap” product line based on this idea; their ProtoSnap Pro Mini, for example, had an Arduino Pro Mini SBC already wired up to a USB-to-serial converter, two actuators, two sensors, and a prototyping area, and their LilyPad ProtoSnap Plus has a LilyPad similarly preconnected to several sensors and actuators and conductive-thread connectors, which can then be broken apart at the tabs and reconnected elsewhere, like in your clothes.
LED tape often comes with similar cut points, where you can cut it with scissors to a given length, but if you don’t, the whole tape can be hooked up to from 2–4 wires. (Such tapes might cost US$19 for 5 m, US$43 for 5 m, or US$17 for 4½ m at retail here in Argentina.) “Flat flex PCBs” are printed on thin Kapton and commonly used as connector cables, and can be cut with scissors in a similar fashion.
And, of course, prototyping perfboard is pretty easy to cut along the perforation lines.
There’s also a technique to allow electrical continuity across V-scores cut on both sides of a board: a plated-through hole that the V-score runs through, so the plating in the hole electrically bridges the V-score.
Circuit board “edge connectors” are interesting to mention in this connection: ISA cards, PCI cards, DIMMs, and many USB-A devices have no separate connector, just exposed copper, either on one side of the board or both. (Zebra-strip, Z-tape, and pogo-pin “connectors” on PCBs are also commonly just exposed copper.) Such connectors can be provided at a place that’s exposed by snapping at such V-grooves.
You could imagine designing a circuit board that works as a whole, but can also be broken into smaller circuit boards, or cut with scissors, to get more reconfigurability.
Custom PCB manufacturing is amazingly cheap; TrickyNekro reports €10 for 100 boards of 38 mm × 18 mm each, plus normally US$20 for “shipping”. And if the panel size is exactly 100 mm × 100 mm, they used to not charge anything for V-scoring (though reportedly that was only if all the boards in the panel were identical, and they now charge something like US$8 for V-scoring, while others report problems getting JLC to V-score), although I don’t know if there was a limit to how many lines you could request on your “board outline layer (*.GKO)”. In the same thread, georges80 reports 3–4 day turnaround to the US West Coast, paying:
And blazini36 reports getting 10 100 mm × 75 mm prototyping boards for US$27, including shipping and an extra US$8 for the boards being blue, in 3 days, on several occasions, while OSH Park wanted to charge them US$300 and delay 12 days. Battlecoder reports that they paid US$12 for 10 prototype boards, and had to wait a month, because JLCPCB doesn’t have free shipping to their (unspecified) country.
Nobody in the thread is sure if they can do V-scores at arbitrary angles as well as straight across.
It occurs to me that this sort of thing might also be useful for mechanical construction, although the shapes you can V-score are even more limited than the shapes you can laser-cut — no sharp curves or inside corners. JLCPCB uses a cutting wheel to cut the V-scores so you can’t even do curves. But if you can get some slots routed out in addition to the V-scores, you could get a pretty productive “construction set” pretty cheap. (You’d have to wear gloves to keep the fiberglass out of your fingers.) And then you can solder the joints to hold the assembled pieces in place, like the FR4 mill.
If you could get a 100-mm-square panel panelized with V-scores into a hundred 10-mm-square pieces, which would take 18 V-scores (higher than the usual 5 or so, but not by that much) it seems like you could get a pretty large set of circuit components. With 2.54-mm-spaced holes along the edge (which I think normally costs extra) you could have six breadboard-compatible/Dupont-cable-compatible pins on each such microboard. Backing off a little bit, if you only panelized the panel into 50 10 mm × 20 mm pieces (13 V-scores), you could have 6 breadboard pins along each edge and a 4-pin USB-A connector at one end of the board.
Going to the extreme, SparkFun sells a US$8 perfboard totally crisscrossed with V-scores so you can break it into 1250 pieces.