if to add few things:
CPU is pretty difficult to install, and I will call it most difficult part in system manufacturing as you got to be very very careful while installation of CPU
plus it requires efficent use of thermal pad (though boxed cpu's comes with pad already applied with their fans)
plus a little im-balance of the heatsink while adjusting in on CPU die, it can crack the die, and even Manufacturer will not be able to exchange for physical destruction.
you can read the guide lines on the boxed processors, (AMD gives out very good processor installation guideline with their boxed procesors, colored photographs)