The columns and rows names k* and ki* aren't just because, but Those are the names labeled in the calculator board, so I kept them to make things easier.

For each of them one must solder a cable, like the picture:

And then, after opening a hole on the calc case, pass the cables (flat cable was my choice) and solder them to a connector. Care must be taken with the names, or you'll have to redo all the config_key.sh file to match the code and the atmega board.