Characterization of Tantalum Polymer Capacitors

Solid-electrolyte tantalum capacitors were first developed and commercially produced in the 1950s. They represented a quantum leap forward in miniaturization and reliability over existing wound-foil wet electrolytic capacitors. While the solid tantalum capacitor has dramatically improved electrical performance versus wet-electrolyte capacitors, especially at low temperatures, today’s electronic circuits require even better performance. In response to this need, steady improvements in the equivalent series resistance (ESR) of tantalum capacitors have been made. Low ESR is the most important attribute of capacitors used to filter and decouple power for high-speed digital electronics. While the first patented tantalum capacitor was claimed to have ESR of roughly 2.0 ohms, a similar capacitor today has ESR of about 0.1 ohms. Even so, today’s digital electronics frequently require capacitors to have ESR in the low milliohms, a level that can be achieved with conventional tantalum capacitors only by connecting many in parallel. A substantial fraction of the ESR of a tantalum capacitor comes from its solid electrolyte material, manganese dioxide (MnO2). While MnO2 is substantially more conductive than almost all wet electrolytes, especially at low temperatures, capacitor manufacturers search for higher conductivity materials to replace MnO2. Today’s solid electrolyte material of choice is the conductive polymer PEDT (polyethylenedioxythiophene) which has up to 100 times MnO2’s conductivity and has generally acceptable compatibility with tantalum pentoxide, the tantalum capacitor’s dielectric. With the introduction of conductive polymer electrolyte, remarkable improvements in capacitor ESR are possible. But ESR isn’t the only capacitor performance characteristic to benefit. For lower-voltage capacitors, improved dielectric strength and long-term reliability are also observed. Also, substantially more capacitance stability with frequency is observed. But there are also limitations to the technology including marginal material stability at elevated temperatures, muted self-healing capability, and reduced dielectric reliability at high rated voltages. This document briefly describes the origin of MnO2-based solid tantalum capacitors, their methods of processing and construction, and their defining electrical characteristics versus the wet electrolytic capacitors they replace. The case for improved electrolyte conductivity is briefly presented and conductive polymer is identified as the candidate material of choice to replace MnO2 in the solid tantalum capacitor. The defining electrical characteristics of capacitors made with conductive-polymer solid electrolyte are described. Typical processing options and material issues related to conductive polymer electrolyte are identified. Details of the step-by-step processing of typical tantalum polymer capacitors from tantalum powder to assembled and encapsulated devices are photographically presented. The electrical performance, dielectric robustness, reliability, and environmental stability of tantalum polymer capacitors are discussed in some detail. Competitive testing of these capacitors will occur during the FY06 task. Results of this competitive evaluation will be published in FY06 as the phase two deliverable of this project.