depends on what socket sempron it is, since with something like am2 or even am1(perhaps even sooner, I kind of skipped the 754 and 939) they introduced cpu throttling and thermal monitoring which makes it practically impossible since the system would shut down long before it got way to hot for damage to occur, if it's an older sempron(socket a or 7xx something it was ), just remove the heatsink completely, though it will fry in seconds then, not sure what is your actual goal to see there, i burned one of my socket A athlons just by getting some dust in the paste back then(which i did not notice at first a friend gave it to me, didn't check if the stuff was clean) it fired even before the post screen