Failure Analysis of Dielectric Breakdowns in Base-Metal Electrode Multilayer Ceramic Capacitors

Leakage current measurements of BaTiO3-based X7R multilayer ceramic capacitors (MLCCs) with base-metal electrodes (BMEs) have revealed three distinct failure modes: avalanche breakdown (ABD), thermal runaway (TRA), and slow degradation. Failure analysis (FA) was performed for a number of BME capacitors that failed with the aforementioned three failure modes. The samples that failed with ABD had damage sites that were easily found and that were characterized by the existence of incompletely burned binder particles that were surrounded by transverse cracks that extended through several layers of electrodes from the damaged site, clearly a sequence caused by thermal damage. The samples that failed with TRA also had a particle-like processing flaw with high carbon content, but the flaw was smaller than that of the ABD failure samples. The failure site was also surrounded with extensive transverse cracks that extended through many dielectric layers. Degraded dielectric and conglomerates of nickel spheres were also revealed, indicating a severe thermal event that generated excessive heat and that resulted in the melting of the local dielectric and electrodes. There is no fundamental physical difference between ABD and TRA failures for BME capacitors. The failure mode is a combination of ABD and TRA and is referred to as “catastrophic.” The failure analysis on the samples that failed with a slow degradation indicates a failure process that can be described as follows: The electromigration of oxygen vacancies not only gives rise to a gradual increase of leakage current against stress time, but also changes the initial stoichiometry of BaTiO3 grains and causes the local hollowing and melting of dielectric grains and the formation of cracks. The molten dielectric dissolves the internal nickel electrodes and transports the nickel along the transverse cracks, which causes the resistive short. Some of the cracks with dielectric degradation will eventually result in a catastrophic failure. The failure process involves a localized high temperature that can melt both dielectric and nickel. There was no evidence of nickel migration in the BME capacitors, even under highly accelerated life stress conditions. The failure analysis indicates that failure mechanism in BME capacitors with BaTiO3 dielectrics can be more accurately described as a two-stage dielectric wearout that begins with a slow dielectric degradation, characterized by a gradual increase in leakage current with stress time, and followed by a thermally dominated catastrophic breakdown (either ABD or TRA).