Time-Dependent Short Mode Failures in MLCCs Due to Cracks in the Device Bodies

Abstract

Insulation resistance degradation is one of the most common failure modes in multilayer ceramic chip capacitors (MLCC). Degradation in insulation resistance facilitates short mode failures in long run due to certain failure mechanism. Therefore, it is important to understand, how these mechanisms resulted in short mode failures of MLCC. From many factors, those govern short mode failure in MLCC bias voltage is a major factor. There are two MLCC failure regimes defined by the bias voltage: low-voltage failure ( 5 V). The normal voltage failures in multilayer ceramic chip capacitors initiated due to crack developed in them during assembly process are described in this paper. MLCCs are susceptible to cracking, if subjected to sudden change in temperature (temperature gradient of ~250 °C) during soldering process. These cracks facilitate moisture ingression which resulted in short mode failures in ceramic chip capacitors over a period of time due to metal migration. In this paper, controlled experiments were conducted to demonstrate short mode failure in MLCCX7RU capacitors initiated due to cracks developed in them during assembly fabrication. Effect of cracks on insulation resistance of the capacitors is also demonstrated by an innovative method wherein capacitors having cracks were soaked by solder flux mixed isopropyl alcohol. This method effectively simulates the cleaning process of the capacitors after mounting them on to the PCB. Subsequently, these capacitors were tested at normal voltage (50 and 70 V) under ambient conditions (25 °C, 50%RH). Both single and stacked type MLCCX7RU capacitors were considered for this study. Some capacitors were conformally coated to see its effect against further propagation of failure. From this test, it is inferred that mechanism of short mode failure in ceramic chip capacitors are due to (i) crack in the capacitor body resulted during soldering, (ii) moisture/contaminants penetration during cleaning process, and (iii) potential difference across the capacitor during usage. With all these conditions, silver migration takes place between two adjacent electrodes resulting in short mode failure.