Looking for a board-to-board cable

For SMT if you want something reasonably small you could use flat flexible cables (FFC).

• Digi-Key / Cables, Wires / FFC, FPC (Flat Flexible) (spools of the stuff)
• Digi-Key / Cable Assemblies / FFC, FPC (Flat Flexible), Ribbon - Jumpers (pre-cut)

Example:

They mate with, surprise, FFC connectors, like these:

• Digi-Key / Connectors, Interconnects / FFC, FPC (Flat Flexible) - Connectors - Board Mount

You can get them in zero-insertion force (ZIF) with a locking clasp, or LIF without, right-angle, vertical, etc. Mind that "pin 1" to the same connector will be backwards if the cable is stripped on the same side--you can get "upside down" connectors however.

They are very compact and easy to use, but their major downfall (especially for prototype use) is their low cycle life. If you have to disconnect/reconnect it a dozen plus times, they aren't a solution