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The importance of high-temperaturecapacitors

Developing SMD tantalum capacitors forcontinuous operation at 200C

BY RADOVAN FALTUS, TOMAS ZEDNCEK,and M. KALOUSAVX, Czech Republichttp://www.avx.com

Industries such as oil-drilling, aerospace, andautomotive have applications that requirecomponents to work continuously attemperatures up to 200C. Solid tantalumcapacitors are well-known for their excellentreliability, robustness, and stable parameters.They have also been available for continuousoperation at 125C since commercialcapacitors were first developed. Surface-mountdevices with 150C capabilities wereintroduced in 1998 followed by 175C devicesin 2003.

Capacitors are one of the key components in any electronic device and system. Their main functionsinclude power supply voltage smoothing, supporting the energy source, and filtering.

Application requirements

Reliability is key requirement in typical aerospace and defense applications where engines, turbo fans,

and control and sensing electronics are placed near outer shells of rockets and space shuttles. Automotiveapplications, such as small gearboxes or embedded alternators/starters, also require reliability and long lifeat elevated temperatures.

In the oil and gas industries, high temperature capacitors are needed for the Dc/dc converters used indrilling heads that experience rising ambient temperatures the deeper they drill. Oil drilling applicationsrequire capacitors for use in control circuits at 3.3 V (digital) and 5 /15 V (analog). The oil industry also hasspecific service interval requirements: the minimum-requested continuous operating time is 1,000 hours at200C. Recent industry demands have shifted conditi ons to 2,000 hours operating time at 215C.

Automotive applications require 12 and 24 V for power lines that, allowing for derating, meaning voltagesfrom 25 V and up must be provided. In combination with high temperature, this is a real challenge.

Standard tantalum capacitor technologies have an operating temperature range of 55to 125C, which

covers the needs of consumer electronics, computers, mobile phones and in-cabin automotive electronics.There are a variety of automotive applications, specifically situated near the engine, which requirecomponents to work at higher operating temperatures. Some manufacturers have introduced automotiveproduct families to satisfy the needs of the automotive industry, which expand the use of tantalumcapacitors to engine compartment systems with operating temperatures of up to 175C.

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Materials development

Higher operating temperatures affect both the chemical and mechanical stability of all materials used in theproduction of capacitors. The tantalum anode, encapsulant and terminations are the three key areas thatrequire special attention when extending operating temperatures to 200C, and it is necessary to consi derthe effects of both operational stress and storage at high temperature.

The real challenge when moving from 175to 200C i s dealing with the epoxy-based compounds that are

used in capacitor manufacture, such as the moulding resin and silver adhesives. These materials have aglass transition temperature of around 175C. The v alue of the coefficient of thermal expansion issignificantly higher above the glass transient temperature, so cracks and delaminations in the capacitorlayers are much more likely to occur at elevated temperatures. Thus the selection of suitable materials isvery important.

Tantalum anode

Reliability testing coupled with the long-term production experience of many capacitor manufacturers hasproven that tantalum capacitors with MnO2 cathodes are capable of being stored at high temperatures,provided specific procedures are followed.

The type of tantalum powder that is used is crucial for creating a robust anode. Powders from various

producers differ marginally in granularity, porosity, and purity, factors that all influence capacitor reliability.

The forming of homogenous dielectrics with maximum safety formation parameters, together with a robustanode wall structure results in a tantalum anode that can absorb thermomechanical stresses and tosurvive adverse environmental conditions including high temperatures and high humidity.

Encapsulation

The encapsulation material must be able to absorb stress from thermal shocks as well as create aneffective 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 thermalexpansion and high glass transition temperature. High strength at high temperatures and good adhesion tothe lead-frame reduces the chance of cracks forming in the encapsulant, and allowing humidity to

penetrate. IPC level 2 a minimum required level of cracking resistance requires zero cracks after 168hours at 85C in 85% relative humidity and three ti mes 90 s solder immersion at 220C. Humidityresistance is further improved if the encapsulant has a low water absorption tendency.




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Today, lead-free solder is the most common assembly process. Components that are lead-free process-compatible must be able to withstand reflow at peak temperatures up to 260C. They must also meetenvironmental standards that ban using Sb/Br compound as fire retardants. Encapsulants should also becolor-stable and markings must remain legible after storage at 200C.

Terminations

Conventional tin/tin-lead terminations have melting points between 183and 210C, which is too low fo rreliable operation at 200C. Among the applications that require 200C operational capacitors are handsoldered or mounted hybrid circuits, so the most suitable terminations are gold plated ones that areenvironmentally friendly, highly reliable, and temperature stable.

Developmental results

Experience in producing capacitors that are capable of continuous operation at 175C has enabledmanufacturers to specify materials (powder, silver, moulding resin, etc.), designs and procedures thatresult in high-reliable devices even after storage and cycling at 200C.

Tantalum capacitors are 100% screened during production at accelerated conditions at 200C to eliminat epotential failures. Devices are purposely over-stressed by combinations of high voltage and temperature,cyclic thermal shocks and current surges. This guarantees reliability of screened parts.

Leakage current (DCL) stability is an important measure of production process quality. Using a standardaging process (capacitors are exposed to high temperature at the defined voltage for several hours, thenfailed capacitors are removed) certain leakage current flyers were noted after final life testing. To improvestability, additional long-term aging and further leakage current testing at capability limit with screening wasput into the production process.

Leakage current flyers of E100 F/16 V after 200C, 1,000 hours life test. (Standard aging)




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Leakage current stable, E100 F/16 V after 200C, 1 ,000 hours life test, (Long term aging performed)

Stability of leakage current, E100 F/16 V at 200C , 0.3 Ur, M1 after 1,000 hours, M2 after 2,000 hours

200C tantalum caps

Capacitors require voltage derating at high temperature. Maximum operating voltage considering actualoperating temperature is termed category voltage, which at 200C is equal to 30% of rated voltage atroom temperature (0.3 x Ur). A 16-V device thus suits 5-V analog circuits at 200C and a 10-V part can beused for 2.5- and 3.3-V digital circuits in similarly high temperatures, such as drilling heads. Leakagecurrent (DCL) doesnt exceed 1 mA after 1,000 hours of continuous operation at 200C for both p/n 100F/16 V and 220 F/10 V.

The introduced 200C tantalum capacitors provide co ntinuous and reliable operation in many harsh

environment applications, opening new possibilities for use in oil and gas industry drilling systems.


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