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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

Illustration of PCB-to-PCB connection scenario

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    \$\begingroup\$ Permanent joint, or disconnectable? Rigid, or flexible, or don't care? \$\endgroup\$ – Nick Alexeev Oct 15 '15 at 17:13
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    \$\begingroup\$ This is routinely done with a piece of solid-core ribbon cable. However, for quantities more than a few, stripping of ribbon cable requires a special tool. \$\endgroup\$ – venny Oct 15 '15 at 17:19
  • \$\begingroup\$ @venny that is a good option, if you need a permanent connection and don't mind a flexible joint, a ribbon cable is an excellent option. \$\endgroup\$ – DerStrom8 Oct 15 '15 at 18:03
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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.

Example: https://electronics.stackexchange.com/a/3098/26394

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.

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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.

enter image description here

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.

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You can use a DB-25 plug and a DB-25 socket.

https://en.wikipedia.org/wiki/D-subminiature

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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.

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