Accelerated testing for failures of tantalum capacitors J. Virkki * , T. Seppälä, L. Frisk, P. Heino Tampere University of Technology, Department of Electronics, P.O. Box 692, FI-33101 Tampere, Finland article info Article history: Received 1 September 2009 Received in revised form 9 November 2009 Available online 2 December 2009 abstract This study focused on the use of accelerated testing to find out why tantalum capacitors fail. Stress effects of humidity, temperature, and ripple voltage were examined in different combinations. Results show that a standard 85/85 test with combined enhanced moisture and temperature does not result in failure of tantalum capacitors in 2500 h. However, with added ripple voltage, failures may occur in a relatively short time. High relative humidity and high temperature both affect water diffusion, but apparently increased ripple voltage in 85/85 testing causes tantalum capacitor characteristics to weaken and capac- itors to fail. The paper elaborates on the possible reasons. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction In today’s competitive market, it is important for a company to know the reliability of its products and to be able to control it for continued production at optimum reliability. That is why reliability testing of components has become a concern for electronics manu- facturers. Some common reasons for electronic failure are environ- mental contaminants and conditions, such as temperature and humidity, other failures deriving, e.g., from vibration, ripple volt- age, and overvoltage. Accelerated life testing techniques help investigate the reliability of electronic devices as regards certain dominant failure mecha- nisms under normal operating conditions. On the other hand, stan- dard accelerated tests can be designed to induce failure mechanisms other than those occurring under normal operating conditions. Failures must be identified in terms of the failure mechanisms and predominant stresses that induce failures. Operational stresses critical to the reliability of a certain device are determined by the operational environment and the dominant failure mechanisms. A common accelerated test is one in which temperature is held at a constant 85 °C and at a relative humidity of 85% (85/85) [1]. However, a problem with this test is to identify the individual ef- fects of humidity and temperature and those of other stresses on the failure mechanisms of the tested products. Nowadays, electronics design makes wide use of tantalum capacitors because of their high capacity density, size, and achiev- able levels of capacitance. Surface mount solid tantalum capacitors are typically manufactured in non-hermetic plastic cases (struc- ture shown in Fig. 1). In this study, we used accelerated testing to uncover reasons for tantalum capacitor failures. We examined the stress effects of temperature, moisture, and ripple voltage. 2. Failures in tantalum capacitors The reliability of the solid tantalum capacitor is heavily influ- enced by environmental conditions such as humidity. Moisture can penetrate the polymer encapsulating material, degrade the characteristics of the solid electrolyte, cathode attachment materi- als, and tantalum pentoxide dielectric, and cause the capacitor to fail [2,3]. The effects of moisture on tantalum capacitors have been studied before [2], and numerous humidity-related failure mecha- nisms have been reported for solid tantalum capacitors [4,5]. One failure model for tantalum capacitors in humid environments is based on the sleeping cells model and the oxidation of manganese oxide at self-healing sites. Variations in capacitance in dry and hu- mid environments have also been explained by the passive cells remaining inactive until activated by absorbed moisture. In humid environments, moisture diffuses into the passive cells, creating a conductive layer on the surface of the dielectric material. This water layer then works as a cathode electrode, which connects the passive area to the surrounding manganese dioxide, increasing the capacitance of the part. In humid environments, sleeping cells may become active in various ways. Furthermore, they differ in their kinetics and effectiveness in developing the conductive cathode electrode. These processes include formation of a mois- ture-adsorbed layer, capillary condensation, and moisturizing of the hygroscopic remnants in the cell [4,2]. The failure mechanism may also be related to the moisture- generated silver dendrite growing on cathode contacts. After short- ing, these dendrites ‘‘arc”, ignite the manganese oxide, and create a chemical reaction [5]. However, the clear reason for failure mechanisms of tantalum capacitors in humid environments has remained unexplained. Temperature is known to vary the rates of many physical and chemical reactions. Since failure mechanisms in a device are basi- cally physical and/or chemical processes, temperature is often used 0026-2714/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2009.11.006 * Corresponding author. E-mail address: johanna.virkki@tut.fi (J. Virkki). Microelectronics Reliability 50 (2010) 217–219 Contents lists available at ScienceDirect Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel