It is not a secret that flexible PC boards can bend. But despite the substrate’s flexibility, you can’t really fold them completely over. That bothered [Carl] so he developed a hinge design so that he can fold a board completely in half. You can watch a video showing an example, FlexBox, below.
Normal boards can fold over, but the copper traces can’t tolerate a very tight bend radius. [Carl’s] trick is to make the folding part have no traces at all. Only a small bridge carries traces between the two halves and it is allowed to bend almost like an interconnecting cable.
It’s true that a board-level interconnect between two PCBs could also be used but there is something attractive about having the entire assembly as one piece, especially if you are using a flex PCB anyway. We don’t have any data, but we imagine the flexible PCB is more reliable than a socket and plug and adds no additional cost.
[Carl] is the master of flexible boards. He’s done videos on cheaper prototyping for hybrid flexible/rigid PCBs using stiffeners. He’s used them for jumping robots and sort of holographic optical displays. If you want more ideas about flexible PCBs, check out the results of last year’s flex PCB contest.