Guidelines for Selection, Screening and Qualification of Advanced Wet Tantalum Capacitors Used for Space Programs

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  • Author: Alexander Teverovsky
I. Scope. These guidelines are developed for use by NASA space projects and are applicable to extended range, high volumetric efficiency wet tantalum capacitors similar to those manufactured per commercial specifications (e.g. Vishay SuperTan capacitors, ST-, STA-, and STE-style, AVX TWA-style, Evans THQ-, HC-style), DSCC drawings (e.g. #93026, #10004, #04003, #04005), and per military specifications (e.g. MIL-PRF-39006/33, CLR93). These extended range capacitors employ new cathode layer materials and design that are different from the one used in M39006 types capacitors (sintered tantalum powder with anodic oxide layer). Capacitors selected for space projects shall meet the requirements of MIL-PRF-39006 for “H” designated parts for all requirements that are not specified in Tables below. General comments for selection of wet tantalum capacitors per GSFC EEE-INST-002 should be used as applicable. In case of conflict between requirements this document shall prevails. II. Background. Wet tantalum capacitors are used typically in power supply lines, and their failure might have catastrophic consequences for the unit and space mission. Therefore all applications of wet tantalum capacitors should be considered as critical and they should be qualified for use in space systems after thorough screening and qualification for projects of all levels. Extended range, high volumetric efficiency wet tantalum capacitors feature a special design of cathode layers that are formed on the internal surface of tantalum case (see Figure 1). These layers might be made of different materials (e.g palladium, ruthenium oxide, or niobium oxide) that replace traditionally used cathodes made of sintered tantalum powder. This replacement allows for a substantial reduction of the thickness of the cathode layer and for using of the additional volume to increase the size of the anode slug. Considering that these parts are using smaller size tantalum powders (so-called high-CV powder), a substantial increase in capacitance and decrease of the equivalent series resistance (ESR), from 1.5 to more than 6 times, have been achieved without increasing the size of the parts. The range of rated voltages and capacitances for traditional wet tantalum capacitors (CLR79, 81, 90, and 91) and parts with high volumetric efficiency (DWG93026 and CLR93) for different case sizes is shown in Figure 2. A substantial improvement in the performance of the parts (increased capacitance and decreased ESR) did not come free. In addition to a general concern that capacitors with larger value of 3 capacitance have typically a greater risk of failure (because capacitors with a larger surface area are more likely to have a defect in the dielectric) there are two new design-related issues: increased sensitivity to mechanical stresses (shock and vibration) and to reverse voltage conditions. The first is due to the increased mass of the anode slug that raises pressure between the slug and Teflon insulator during mechanical testing, and to the presence of particles generated from the cathode layer. These floating in electrolyte particles can penetrate into pores of the slug and damage the dielectric when the slug is deforming as a result of stresses developed during mechanical vibration or shock. The second issue, sensitivity to reverse bias, is due to changes in the cathode materials. In a traditional design of wet tantalum capacitors, a specially formed oxide layer on the surface of tantalum powder sintered to the case prevents high leakage currents in reverse polarity. In the absence of this oxide barrier, reverse currents became large and result in electrolysis of the electrolyte, which is typically a 30% to 40% by weight solution of sulfuric acid in water. At sufficiently large currents a fast generation of gases (mostly hydrogen) increases internal pressure in the case and might cause its fracture, explosion, leak of the electrolyte onto the board thus shorting and damaging other components in the system. Reverse bias might also result in a substantial increase of forward currents due to electroplating of the cathode materials on the surface of anode slug, prohibiting self-healing, and resulting eventually in failures due to increased internal gas pressure. High reliability of wet tantalum capacitors, DSCC DWG93026 including, is due to a large degree to the self-healing process that results in oxide growing at the defective areas of the dielectric such as cracks, thin oxide areas, or microfissures. Experiments show that parts that failed after mechanical tests due to excessive leakage currents can recover with time of operation under bias. These excessive currents are typically due to mechanically-induced damage (e.g. cracking) to the tantalum pentoxide dielectric and recovery of these parts with time of operation under forward bias is a manifestation of the self-healing mechanism that results in a local electrochemical oxidation of tantalum at the damaged areas. The electrochemical oxidation results in the growth of the Ta2O5 dielectric similar to the process that forms dielectric initially during anodic oxidation of the tantalum pellet. This process goes along with the electrolysis of electrolyte and gas, mostly hydrogen, evolution. During the initial oxide growth the formed gases are dissolved in atmosphere, but when it happens in a hermetically sealed part, gas generation results in a buildup of the internal pressure. Hermetically sealed wet tantalum capacitors might sustain high pressures and some lots can pass life testing at high temperatures, up to 200 o C. However, diffusion of hydrogen into the case results in embrittlement of tantalum and increases the risk of its fracture. High internal gas pressure increases substantially the rate of electrolyte leaking to the glass seal area thus increasing leakage current on the surface of glass and the risk of corrosion of the weld between the tantalum tube and nickel lead. For these reasons, the capability of self-healing of wet tantalum capacitors should not be abused, and the risk of 4 damaging of tantalum pentoxide dielectric during mechanical stresses and the level of leakage currents under reverse bias should be limited by proper design, selection, and testing of the parts
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