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Capacitors survive extreme down-hole drilling conditions

Posted: 20 May 2015 ?? ?Print Version ?Bookmark and Share

Keywords:electronic controllers? capacitors? MLCCs? X7R? Highly Accelerated Life Testing?

As well as offering the outstanding temperature stability of a C0G dielectric, the calcium zirconate high-temperature formulation has significantly higher starting Insulation Resistance (IR) at room temperature than other C0G or X7R high-temperature dielectrics, leading to greater system reliability. Although IR of both C0G and X7R types tends to reduce as operating temperature increases, an X7R capacitor's IR can fall from a few Giga-ohms at 25C to just a few hundred kilo ohms as temperature increases.

Enhanced mechanical properties
The mechanical properties of MLCCs can also be enhanced through a combination of material selection and capacitor construction. Mechanical shocks and vibration can be particularly severe in modern drilling applications, and especially where electronics are to be installed in the drilling tool itself. MLCCs in larger case sizes are known to be particularly vulnerable to cracking when exposed to high levels of shock and vibration.

Reducing the MLCC case size to an outline such as 0603 has been shown to reduce vulnerability to damage caused by mechanical shock. In addition, new high-break-strength C0G materials have been developed, which are capable of withstanding shocks up to very high G-forces. KEMET's high-temperature C0G dielectric has high break strength, with a modulus of rupture more than twice that of the industry's best X7R material.

Drop tests have been performed, applying a force of 500G to standard X7R and high-break-strength high-temperature C0G capacitors in two case sizes, to illustrate the improved resistance to cracking afforded by the new C0G dielectric. Capacitors were soldered to a PCB test card and subjected a total of 120 drop tests in x, y and z directions. The crack failure rate for the X7R dielectric was four in 40 drops, whereas zero failures were recorded with the advanced C0G dielectric.

Molded C0G MLCCs with radial leads and a molded temperature-resistant housing have demonstrated even greater shock resistance. Attaching leads to the capacitor provides strain relief, which improves the mechanical durability. However, care must be taken to ensure that the solder joint is capable of withstanding high operating temperatures. KEMET's C052H and C062H molded radial high-temperature C0G MLCCs are assembled using high melting-point (HMP) solder to ensure the integrity of all lead attachments. Adhesion tests performed at 200C using a force tester have demonstrated breaking strength greater than 3kg. This significantly exceeds the minimum force specified at room temperature, which is 1.8kg. Finally, gold plating of the leads ensures optimum performance up to high temperatures.

Reliability testing
High-temperature C0G MLCCs that incorporate these advanced features are available in surface-mount packages, or as molded radial-lead devices for extra strain relief. The stability of the special high-temperature C0G dielectric ensures zero change in capacitance with respect to time and voltage, and capacitance change with temperature limited to 30ppm/C from -55C to 200C. In addition to high insulation resistance, these capacitors also have low dissipation factor at temperatures up to 200C, as well as low ESR at high frequencies.

Highly Accelerated Life Testing (HALT) performed on the molded radial capacitors at different voltages and 200C, with Weibull analysis and prediction using the Prokopowicz and Vaskas (P-V) equation, has predicted Mean Time to Failure (MTTF) of 8.64 x 107 years.

In addition, 1000 hours of life testing at 200C and rated voltage has been completed. Measuring insulation resistance at zero hours and then 250, 500 and 1000 hours, to assess reliability, recorded no failures and no degradation in the measured values. Other long-term tests including humidity tests, storage-life tests, -55C/+220C thermal shock, and mechanical shock and vibration tests have shown zero failures indicating very high reliability for these devices.

Conclusion
The increased demands placed on electronic control systems operating in today's deepest oil wells call for improvements in numerous aspects of component design and construction. New and advanced dielectrics, combined with high-performance solders and surface finishes, have improved both the electrical and mechanical characteristics of capacitors to withstand the harshest ultra (uHPHT) and extreme (xHPHT) environmental conditions defined by the oil industry.

About the author
Reggie Phillips is with KEMET Corp.


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