Feature Power Electronics


Increasing the capacity C


T. Zednicek, M. Biler, J. Petrzilek & I. Pinwill, all enginners at AVX Corporation explore the design characteristics of a new hermetically sealed SMD tantalum capacitor structure to solve limited stability in high temperature environments


ertain electronic applications such as oil drilling demand ever higher operating temperatures. Recently these have increased from 200


0


C with an increased operating life of one thousand hours and beyond, due to continuous development of oil drilling heads and sensors for deeper drilling or drilling in more difficult geological con- ditions. Capacitors with high capacitance value are a common part of the electronic boards used in these applications, but over 125


230 0 0 C/175C, capacitor choice is very 0


limited. Tantalum SMD capacitor technology has proven reliability up to 175


0 0 0 C and


there is a limited choice of special designs up to 200


C. Long life wet tantalum capaci-


tors up to 200C are available from multiple sources, or up to 230C with a very short


0


operational life from a single source. The traditional cathode material - MnO2 - provides good mechanical robustness and relative temperature and humidity stability that suit many applications.


The two major disadvantages of MnO2 tantalum capacitors are the thermo runaway failure mode and relatively high equivalent series resistance (ESR) capability.Both can be solved by using cathodes based on con- ductive polymers, which provide lower ESR and safer failure mode, but strongly limit possible operating temperature. The solution to the problem of limited stability in erratic high temperature environments is to use an MnO2 cath- ode, but even these are subject to potential degradation if exposed to extreme conditions.


Keeping the capacitor’s internal struc- ture away from humidity and oxygen can significantly increase its ability to withstand extreme temperatures. Both of these conditions can be met by using capacitors which are hermetically sealed with inert atmos- phere. Additionally, the relatively high ESR given by the MnO2 cathode can be reduced by use of parallel connection of more capacitors inside one package. AVX has designed and tested a new hermetically sealed SMD tantalum capacitor structure with the capacitor elements hermetically sealed in a ceramic housing and attached by a common termination.


Nitrogen used as an inert gas inner atmosphere inhibits thermal failure mode of the manganese dioxide solid electrolyte. The housing includes a lid (typically metal) placed on an upper surface of the ceramic side walls after


Figure 1: MnO2 cathode layer


of 100µF/35V capacitors with cathode


hermetically sealed in ceramic housing before, After Performance Life Test


C to


the capacitor element is positioned. The ceramic housing and the capacitor element have a similar shape to provide maximum volumetric efficiency. A resistance seam welding process was used and 100 percent hermeticity testing performed on all manufactured parts. The company tested and analysed


the new design of the 100µF/35V MnO2 hermetically sealed surface mounted tantalum capacitor.


Life tests performed on the new capacitor design show good stability at 200°C and voltage rating up to 60 percent of Vr. Tests at 230°C show perfect stability in the case of no voltage applied. Very good stability is obtained up to 33 percent of applied voltage; further increases in voltage has an impact, increasing ESR and producing a slight capacitance decrease after 500 hours. DCL starts to grow after 500 hours as well. A potential wear out mechanism was identified based on FESEM analysis as chemical and morphological changes of the MnO2 cathode induced by bias under elevated temperatures. The phenomenon of capacitance decrease is directly connected to conductivity changes in the cathode material – MnO2. The FESEM image in Figure 1 shows high contrast with sharp edges, illustrating high conductivity of the MnO2 material.


As shown, 100µF/35V capacitors operating at 17.5V can provide good stability up to 1000 hours at 230°C with final ESR below 100 mOhms. This demonstrates the high level of performance of such high capacitance, small and stable SMD capacitor devices that may open up a new gener- ation of designs for 230°C applications. AVX Corporation www.avx.com Enter 207


Enter 15 22 OCTOBER 2013 Electronics


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