I dig, but SOMEBODY had to push the button, and if the bomb was in the control cabin, all signs would logically point to one of the crew being compromised...
What if it was placed into the drink cart drawer in such a way that as soon as the flight attendants began to get ready for cabin service, and slid the drawer out to count how many of each drink they had, the switch was activated by the motion of sliding the drawer out and the switch clicking against the bottom of the drawer above?
Nobody has to push a button.
The bomb could be triggered by either a simple timer, or an altimeter. Even more sophisticated would be a an altimeter trigger which started a timer so it would go off some time after reaching altitude.
The simple timer could go off on the ground if there are delays. The simple altimeter guarantees everyone is killed, as the aircraft blows up in flight, but most of the time it is still over land and the debris can be recovered, as happened here. The altimeter/timer combination can be set to go off over water which makes recovery and forensics much more difficult.