IR microscopic observation of plastic encapsulated TAB inner lead bonds degrading during thermal cycling

Infrared (IR) microscopy investigations of TAP (tape automated bonding) contacted chips are described. The ability of the method to detect mechanical stress fields in silicon as induced by a bump or a load is demonstrated. In the case of plastic coated TAB devices the degradation of the chip after thermal cycling is observed. IR microscopy revealed that the periodic thermomechanical stress induces cracks within the metallization under the bumps, which may grow over a long period of time or spontaneously change into shell-shaped breaks in the silicon substrate. This form finally results in an electrical interruption of the lead.<>