Field failure mechanism and reproduction due to moisture for low-voltage ZnO varistors

Abstract

This paper presents an investigation of the insulation resistance/leakage current failure of low-voltage ZnO varistors in the field. Failed varistors resumed normal operation after surface polishing, and the fault site was located in the Si coating layer and the outermost side of the bulk ZnO. To reproduce the field failure phenomenon, an effective stress that causes failure was suggested. Accelerated reproduction tests were conducted to determine if the chosen stress caused the field failure. To accelerate the formation of the defect, a water precipitation/reflow precondition was applied. A biased HAST after preconditioning could reproduce the same field failure phenomenon. The failure mechanism and root cause were determined by comparison with a sample that used an improved coating method. A failure mechanism was proposed in which the insulation resistance degradation was caused by Zn(OH)2, which is generated by moisture absorbed into the surface of the varistor. Further, the ZnO and leakage current degradation was caused by H+ ions generated by dissociation of the diffused H2O to the ZnO grain boundaries on the surface of the bulk ceramic. Finally, the effective factors for field failure of the ZnO varistor were defined as the weakness of the Si coating, moisture ingress, and bias.