Moisture Induced Failure Mode in a Plastic Encapsulated Dynamic Timing Circuit

A specific failure mechanism in a dynamic storage node has been discussed and characterized. The failure mode, which occurs in a plastic encapsulated prototype timing circuit, was determined to be caused by the moisture induced conduction between two metal lines one of which is connected to the storage node due to the oxide surface conduction outweighs the charge restored by the junction leakage. This model explains quantitatively the failure of the flip flop cell, and also is consistent with other characteristics of the failure mode. Moisture induced surface conduction was determined to exist on top of the thermal oxide underneath the passivation layer. The activation energy of the oxide surface conductivity ranged between 0.42 and 0.72 electron volts and was found to decrease with the increased amount of moisture penetration. This result is consistent with the protonic conduction model proposed by Anderson. The activation energy of the junction leakage along with that of the oxide surface conductivity explains the higher failure rate of the timing circuit observed at a higher temperature such as 700C.