I'm looking for a smart way to connect two PCB's in a cheap manner. I would prefer not to use any external components for the connection (if possible).
The PCB's are rather small (20 x 20 mm).
Do you have any idea?
Best Regards, Andreas
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If you need a permanent joint, and you need this to be mass-produced, but you care about space (no room for a connector) or cost (connector is too expensive), you could use hot bar soldering.
The idea is that a flat cable (usually a special kind of flex pcb) is pressed down by the hot bar so that it makes all the connections at once.
If you can overlap the boards (ie: they don't absolutely have to connect edge to edge like puzzle pieces, but the edges can overlap like shingles), you could use z-axis adhesive.
What do you consider "External"? Usually this type of connection is called "Card-edge", and it uses a special socket mounted on one of the boards, and the other board has pads located just perfectly on the edge of the PCB. The most common use I have seen is PCI-e connectors in computers. The cards have pads on them and the motherboard has the connectors.
That's really your best option. I wouldn't try connecting bare PCBs together, there would be a lot of stress on the joint and you might rip up pads.
You can use a DB-25 plug and a DB-25 socket.
It looks as though you're designing a jigsaw like electronics kit. In that case try (1) right-angled pin-headers - pins on one side and sockets on the other both lying parallel to the PCBs. They may take up too much PCB area though, given the 20 x 20 mm dimensions.
Any standard edge connector will get in the way of the interlocking bumps on the PCBs.