Capacitors for high temperature applications
29 May 2013
There are many uses for high temperature electronics, including the downhole oil and gas, avionics and automotive industries, with more markets emerging. Key requirements for high temperature components are reliability, shock and vibration capability as well as being able to operate under conditions that include high humidity and dust.
The majority of capacitor technologies currently available on the market are limited to a maximum temperature of 125ºC, 150ºC or even lower, but the need for capacitors with electrical performance stable at high temperature has grown in recent years. Given the wide range of commercially available capacitor technologies, only few can reliable operate at elevated temperatures. In general, for temperatures of 175ºC and above, tantalum and ceramics capacitors are used the most.
Most MLCC high temperature ceramic offerings are designed to operate at maximum temperatures of 150ºC or 200ºC. There are practical size limitations which prohibit reliable direct mounting of chip capacitors larger than 2225 to a substrate. These large chips could be subject to thermal shock cracking and thermal cycling solder joint fatigue. In order to achieve the required capacitance stacked X7R or COG ceramics are common in high temperature applications (Fig 1). In order to limit the mismatch of thermal expansion (CTE) between the ceramics chip and the PCB, N-lead through-hole or J-lead/L-lead surface mountable configurations are commonly used.
Many designs use custom parts with soft, Teflon coated wires (Fig 1b). This option typically requires mechanical securing of the component, but allows flexibility of soldering with High Melting Point (HMP) solder without the use of PCB. The additional benefit of such design is the ability to solder at a safe distance from large ceramic capacitors. This effectively eliminates the need for preheat and eliminates any risk of thermal shock to the components.
X7R (Class II) type capacitors feature good volume efficiency and lower cost. The X7R type of dielectric is usually considered as a semi-stable. The capacitance over the temperature range from -55ºC to 125ºC is relatively flat; nevertheless the capacitance drop off at 200ºC can be very significant, depending on the CV (capacitance/voltage) of the part (Fig. 2). The other factor important to consider is also drop of capacitance with applied voltage. Small capacitance loss is also present due to ageing, but this is not significant in comparison to the temperature coefficient of capacitance (TCC) and voltage coefficient of capacitance (VCC).B. COG (Class I) type capacitors have very low TCC compare to X7R. Capacitance is very stable with reference to temperature and voltage, however, capacitance values are lower in comparison to Class II X7R, so much larger building block might be needed in order to achieve sufficient capacitance.