I just got to ask, why were you putting enough stress on the screw to snap the head off? If the head was snaped off why didn't you just remove the motherboard then remove the damaged screw?
P.S. not wood, think its some kind of plastics. PCB is made up of layers, so even if you don't see damage to the first layer.... well you get the idea here.
P.S. not wood, think its some kind of plastics. PCB is made up of layers, so even if you don't see damage to the first layer.... well you get the idea here.