Publication date: 01 December 2009
There are applications in industries such as drilling for oil, aerospace and automotive, that require components to work continuously at temperatures above 200degC. Solid tantalum capacitors are well-known for their excellent reliability, robustness and stable parameters. Also they have been available for 125degC continuous operaton since commercial capacitors were first developed, with 150degC surface mount devices introduced in 1998 followed by 175degC products in 2003. Therefore they are an obvious choice for even higher temperature use. This article discusses the move to 200degC operation.
Capacitors are one of the key components in any electronic device and system. Their main functions include: power supply voltage smoothing; supporting the energy source; filtering, etc. In oil and gas industry, high temperature capacitors are needed for the DC/DC converters used in drilling heads that experience rising ambient temperatures the deeper you drill. Simply put, electronic components with a higher operating temperature will allow you to drill deeper, accessing harder-to-reach resources. As mining companies are forced to drill deeper to find new these reservoirs, the sensing and control electronic circuits that are mounted inside the drilling head - including tantalum capacitors - are required to withstand temperatures above 175degC.
Typical aerospace and defence applications include engines and turbo fans as well as control and sensing electronics placed near outer shells of rockets and space shuttles. A key requirement here is reliability under harsh conditions. Automotive applications such as small gearboxes or embedded alternators/starters also require reliability and long life at elevated temperatures.
Other considerations include voltage and reliability. Oil drilling applications require capacitors for use in control circuits at 3.3V (digital) and 5V/15V (analogue). The oil industry also has specific service interval requirements: the minimum-requested continuous operating time is 1000 hours at 200degC. Latest demands shift conditions even further to 2000 hours operating time at 215degC. Automotive applications require 12V and 24V for power lines – which, allowing for derating, means providing voltages from 15V upwards. In combination with high temperature, this is a real challenge.
Standard tantalum capacitor technologies have an operating temperature range of -55 to +125degC which covers the needs of consumer electronics, computers, mobile phones and also in-cabin automotive electronics. There are a variety of automotive applications, specifically situated near the engine, which require components to work at higher operating temperatures. Mainly to satisfy the needs of the automotive industry, some producers have introduced Automotive product families, which expand the use of tantalum capacitors to engine compartment systems with operating temperatures of up to 175degC.
Higher operating temperatures affect both the chemical and mechanical stability of all materials used in the production of capacitors. The tantalum anode, encapsulant and terminations are the three key areas that require special attention when extending operating temperatures to 200degC, and it is necessary to consider the effects of both operational stress and storage at high temperature.
The real challenge when moving from 175 to 200degC is dealing with the epoxy-based compounds used in capacitor manufacture – such as the moulding resin and silver adhesives. These materials have a glass transient temperature of around 175degC. The value of the coefficient of thermal expansion is significantly higher above the glass transient temperature – so cracks and delaminations in the capacitor layers are much more likely at elevated temperatures. Thus the selection of suitable materials is very important.
Reliability testing and five years of production experience at AVX has proved that tantalum capacitors with MnO2 counter (auxiliary) electrodes are capable of being stored at high temperatures provided specific procedures are followed.
The type of tantalum powder is crucial for creating a robust anode. Powders from various producers differ marginally in terms of granularity, porosity and purity, factors which all influence capacitor reliability.
The forming of homogenous dielectrics with maximum safety formation parameters, together with a robust anode wall structure results in a tantalum anode which is able to absorb thermo-mechanical stresses and to survive adverse environmental conditions including high temperatures and high humidity.
The encapsulation material must be able to absorb stresses from thermal shocks as well as creating an effective barrier to prevent penetration by moisture due to humidity.
The effects of thermal shocks can be reduced by selecting a moulding compound with a low thermal expansion and high glass transition temperature. High strength at high temperature and good adhesion to the lead-frame reduces the chances of cracks in the encapsulant which would open the way for humidity to penetrate. IPC level 2 - a minimum required level of cracking resistance - requires zero cracks after 168 hours at 85degC in 85% relative humidity and three times 90 seconds solder immersion at 220degC. Humidity resistance is further improved if the encapsulant has a low water absorption tendency.
Today, lead-free solder is the most common assembly process. Lead-free process-compatible components must be able to withstand reflow at peak temperatures up to 260degC. They must also meet environmental standards which ban using Sb/Br compound as fire retardants.
Finally, encapsulants should be colour-stable and markings must remain legible after storage at 200degC.
Conventional tin/tin-lead terminations have melting points between 183 and 210degC - too low for reliable operation at 200degC. Among the many applications that require 200degC operational capacitors are hybrid circuits where they are hand soldered or mounted, so AVX determined that the most suitable terminations are gold plated ones which are environmently-friendly, highly reliable and temperature-stable.
Five years experience producing capacitors that are capable of continuous operation at 175degC has enabled AVX to specify materials (powder, silver, moulding resin, etc.), designs and procedures which result in highly-reliable devices even after storage and cycling at 200deC.
Tantalum capacitors are 100% screened during the production at accelerated conditions at 200degC to eliminate potential failures. Devices are over-stressed by combinations of high voltage and temperature, cyclic thermal shocks and current surges. This guarantees reliability for the end user.
Leakage current (DCL) stability is an important measurement for production process quality. Using a standard aging process (capacitors are exposed to high temperature at the defined voltage for several hours, then failed capacitors are removed) certain leakage current “flyers” were noted after final life testing. To improve DCL stability, additional long term aging and further leakage current testing at capability limit with screening was incorporated into the production process.
Initially, AVX has made available two E-case (7.3 x 4.3 x 4.1 mm) size parts: a 220µF device rated at 10V and a 100µF part rated at 16V. Both parts are specified for continuous operation at 200degC and are 3-times 260degC lead-free reflow compatible. Construction uses gold-plated terminations and black encapsulation which assures legibility of marking even after storage at 200degC. THJ200 capacitors have a DCL of less than 1mA even after 1000 hours of operation at 200degC.
Capacitors require voltage derating when operating at high temperature. Maximum operating voltage considering actual operating temperature is termed ‘category voltage’, which at 200degC is equal to 30% of rated voltage at room temperature (0.3xUr). Considering this rule, the 16V device is suitable for 5V analogue circuits at 200degC and the 10V part can be used for 2.5V and 3.3V digital circuits in similarly high temperatures - such as drilling heads. Reliability is very good: 0.5%/1000 hours measured by failure rate defined at ambient temperature 85°C at full rated voltage.
The introduction of these new parts by AVX proves that continuous operation at 200degC, opening new possibilities for use in oil and gas industry drilling systems. More, the experience that AVX has gained concerning new materials’ behaviour and new aging and screening processes promises to result in 200degC capacitors with even longer operational times and lower leakage currents in the future – which may be suitable to meet the demands of even the automotive and aerospace industries.
