EOS Damage
The thermal damage is the result of the excessive heat generated during the EOS event.

The heat is a result of resistive heating in the connections within the device. The high currents experienced during the EOS event can generate very localized high temperatures even in the normally low resistance paths. The high  temperature causes destructive damage to the materials used in the device’s construction. 

The destructive damage was found at the metal oxide film resistors where affected resistors turned out of tolerance in a short periods, sometimes few days late, the resistors recovered after the tension of the stress released. The recovery of the resistors could be accelerated by kneading the resistor body of leads, or by short time overload to trig the tension off. 

Thunder Precision Resistor Co., Ltd. makes a lot of high power metal oxide film resistors with huge over loading capability that the resistors could stand 9 times rated power for more than 60 seconds without burn out even the body were heated to bright red!