Strategies for Manufacturing Ultra Low ESR Ta Capacitors

Leading edge designs for portable electronic devices require capacitors which deliver volumetric efficiency, high reliability, low cost, reduced propensity for ignition, and low Equivalent Series Resistance (ESR). Tantalum capacitors employing a conductive polymer cathode are uniquely capable of providing all of these critical characteristics. Tantalum capacitors have a long track record for unsurpassed volumetric efficiency and reliability. The introduction of conductive polymer cathodes enabled dramatic reductions in ESR and greatly increased resistance to ignition. The lower ESR and enhanced cap retention at high frequency of the conductive polymer construction enabled circuit designers to reduce the number of capacitors required to achieve design solutions, resulting in reduced cost. Although Multiple Anode Tantalum (MAT) designs are capable of achieving the lowest ESR possible with valve metal capacitors (< 6 milliohms), single anode Ta/polymer capacitors with ESR less than 9 milliohms are now. Single anode capacitors with ESR levels less than 7 milliohms are within reach. The ability to achieve ultra low ESR levels with single anode designs provides the lowest cost, high performance solution to many design requirements. This paper describes the strategy used by manufacturers to provide solid state capacitors with ever decreasing ESR levels. Introduction The market demand for low ESR, high capacitance capacitors is driven by industry trends toward reduced circuit voltages operated at increased frequency with higher current demands than in the past. These trends coupled with the need to improve energy efficiency, reduce heat buildup, and increase battery life for portable electronic devices fuel the need for ever lower ESR capacitors. Reed, Marshall, and Prymak provide the mathematical equations which demonstrate the relationship between total capacitance and ESR for an output filter capacitor of a switchmode power supply. In order to accurately calculate the number of capacitors required for a specific application consideration must be given to the loss of capacitance at high frequency. The loss of capacitance is directly related to the resistance of the solid electrolyte in the pores of the capacitor. Thus to minimize the number of capacitors required low ESR, high capacitance, and good capacitance retention at high frequency are needed. Component manufacturers have made continuous improvements in these areas to provide the electronics industry with components required for today’s design. Manufacturing Strategies In order to manufacture an ultra low ESR capacitor the manufacturer must examine all of the elements used in the construction of the capacitor and seek to reduce the resistance of each element. Tradeoffs between reduced resistance and cost must be understood and taken into consideration in order to provide the lowest cost solution. The general equation for resistance is shown below: R=ρl/A Where: R=resistance ρ=resistivity l=path length