Backlight screen only works when I put pressure on the keyboard

All, 

Since a couple of weeks I have problems with the backlight of the screen on my Latitude E5550 laptop.

The backlight works only when I put pressure on the keyboard (I do this in the middle of the keyboard), so I was thinking about a loose connection or something like that. I took the whole laptop apart (including removing the mainboard) and took every connection loose and reconnected it. 

Unfortunality no improvement. 

This problem suddenly appeared some weeks ago.

Does anybody has any suggestions?