What makes an LED "bad"?

I work at a company where they hand-solder LED leads onto wire. All the LEDs are tested before they are soldered and again after they are soldered. From a lot of 25 pieces, 8 of them do not light up under a 20mV DC power supply after they have been soldered. I've looked at them under a 40x microscope and the wire that connects is still intact, exactly like the LEDs that light up. I think the heat from the soldering iron (500�F) may have ruined them, but what exactly have they ruined to make 20% of the product fail and 80% of the product p?