Multilayer capacitors can take the heat

28th October 2009

Commercial grade multilayer capacitors can be used for extended temperature operation. Matt Ellis, Senior Engineer at Syfer Technology, outlines some basic guidelines.

In contrast to the high profile portable wireless products sector, with its emphasis on cool-running, ultra-low power operation, burgeoning high-reliability markets, including defence, aerospace, automotive and energy exploration are increasingly demanding components that operate beyond the standard temperature range. Indeed, high temperature operation is becoming a critical challenge for electronics operating in a host of promising new applications in seismology, oil exploration, geothermal analysis, detonators and vehicle emission control. Already, OEMs are looking for devices that operate beyond the standard 125°C limit, typically up to 200oC, often to eliminate the need for forced cooling. Within the next 3 to 5 years this is likely to increase to 250 ºC to 300ºC, and longer term, up to 350ºC.

Multilayer ceramic capacitors are typically manufactured from Class 1 and Class 2 dielectrics, the most common of which are C0G (NP0) and X7R. The properties of these materials are well defined over the industry standard temperature range of -55°C to +125°C. Capacitors based on the X8R dielectric are an option, but this is defined only up to 150°C. There are some capacitors available rated to higher temperatures, but the higher temperature rating inevitably means higher cost.

To demonstrate that commercial grade MLCCs rated to 125°C can often provide suitable solutions for applications at significantly higher temperatures, European capacitor manufacturer, Syfer Technology, has undertaken an extensive program of testing and characterisation. The aim is to provide information and guidance to designers on the behaviour of standard, commercial grade products under extreme operating conditions, including high temperature environments. The Norwich, UK-based team has generated a valuable set of empirical data on the functionality and reliability of multilayer ceramic capacitors under a range of conditions. Customers can take advantage of this work to specify components with a known predictability of operational reliability beyond the standard qualified temperature range.

Thermal stress
The reliability of ceramic capacitors is affected by a function of the voltage and temperature conditions applied to them in operation. The rate of increase in temperature is highly significant, more so in fact, than voltage (see Fig.1).

Thermal stress alone is sufficient to cause electrical failure. Thermal breakdown takes place when heat is generated in the dielectric at a higher rate than it can be conducted away. This leads to increased conductivity in the dielectric layers, more heat generation, and eventually to instability in the form of an uncontrolled, often very rapid temperature rise. The temperatures attained when a capacitor discharges through a region of localised thermal runaway can be high enough to melt the dielectric material. Reliability is not the only factor to consider when using capacitors beyond standard operating conditions. It is well understood that there is a capacitance shift with temperature. A low temperature coefficient of capacitance (TCC) is important to ensure circuit stability and function, not only at elevated operational temperature conditions, but to tolerate temperature swings. TCC is defined as ±15% for EIA dielectric classification X7R (-55°C to +125°C) and X8R (-55°C to +150oC) and ±30ppm/°C for C0G/NP0 designation (-55°C to +125°C).

Syfer’s comprehensive accelerated life testing programme has always included 1000hours endurance testing of its standard parts at the maximum specified temperature of 125oC. The extended programme includes testing at intervals at higher temperatures, up to 215oC. In addition, Syfer has characterised the effect on capacitance under the same conditions. Subsequent failure analysis has enabled the company to build a valuable knowledge base for its ranges of MLCCs.

As can be seen in figures 2 and 3,

once beyond the standard specification temperature range, capacitance shift can be quite extreme. Note the case of X7R dielectrics with almost 60% capacitance drop at 215°C. Insulation resistance is also adversely affected by the rise in operational temperature. The value is reduced by almost a factor of 10 between room temperature and 200°C (see Fig. 4).

Reliable predictability
At first glance it would seem that the specification of COTS (Commercial Off The Shelf) MLCCs for use at elevated temperatures is far from desirable. But in fact, the data does enable recommendations, even guidelines, to be made which allow these components to be used reliably and predictably. The first option is to consider the X8R dielectric range of MLCCs which will operate from -55°C to +150°C, with a maximum capacitance change ±15% (without applied voltage). The devices are available in sizes 0805 to 2225, with voltage ranges from 25V to 200V and capacitance values from 1nF to 1.8µF.

If design constraints demand the X7R or C0G ranges, Syfer recommends that for use at temperatures up to 160°C, engineers should specify a minimum 30% increase in the voltage rating of the selected capacitor. For example if the user requires an 0805 50V 10nF as standard, then a 100V version with a capacitance of 15nF would provide a reliable solution with comparable remaining capacitance at a higher operational temperature.
Between 160°C and 200°C there are no generic sets of rules or guidelines, and much will depend on the specifics of an individual design. However, armed with years of experience, knowledge and test data, Syfer is willing and able to make recommendations on a case by case basis.

For further information on Syfer products and test methods, see: www.syfer.com
Matt Ellis is a Senior Engineer at Syfer Technology Ltd.