How to select the world’s most reliable off-the-shelf capacitors
Process and screening options deliver industry’s most dependable COTS tantalums for high-rel applications
James C Lewis, KEMET’s Senior Technical Expert for Applications and Technology, looks at two new technology breakthroughs that allow designers of high-reliability applications to select high-grade tantalum capacitors that deliver the highest possible reliability levels in the field.
Companies developing high-tech equipment for defense, aerospace, medical and other high-reliability applications seek Commercial Off The Shelf (COTS) – rather than custom - solutions to deliver high-performance products, quickly and at the lowest possible cost. As governments continue to cut their defense budgets, for example, COTS products are expected to be the strongest category in the military and aerospace sector with CAGR of around 6% according to a 2012 report by Databeans .
As a result there are detailed guidelines for contractors covering the use of diverse types of COTS components, including system-on-chip and discrete semiconductors, optoelectronic devices and connectors, and passives such as resistors and capacitors. While the various strengths of the commercial sector are recognized, the guidelines also acknowledge challenges such as the wide variety of suppliers with differing approaches to product quality, screening and testing.
It is up to component manufacturers to ensure COTS devices deliver the cost savings and technical advances that procurement executives expect, while also continuously improving quality, performance and long-term durability in the field. That’s why KEMET recently announced new technologies that enable solid tantalum capacitors to deliver even better performance through greater ruggedness, longer lifetimes and lower derating.
To enhance the reliability of tantalum capacitors for COTS applications, two possible approaches are valid. The first is to address the root cause of device failure such as imperfections in the dielectric and weaknesses in the connection between the anode and the lead wire. The second is to create new screening techniques that enhance the ability of manufacturers to isolate those capacitors that are likely to deliver the best possible reliability in the field. Both of these approaches have been taken by KEMET with the announcement of F-Tech manufacturing technology and a new testing process known as Simulated Breakdown Screening.
F-Tech technology introduces two innovative techniques that control the carbon and oxygen content of the tantalum powder during sintering and associated processes that form the capacitor anode and dielectric. These carbon and oxygen control processes prevent the formation of tantalum carbide and tantalum oxide, which are the main contaminants that encourage crystallization.
For carbon control, KEMET has developed a liquid de-lube process whereby the anodes are washed repeatedly in solution and analysis performed to insure the carbon content is at or below the level of supplied tantalum powder (Figure 1). Regarding oxygen, KEMET has developed an exclusive de-oxidation process using magnesium vapour as well as a unique passivation process for the tantalum anodes. Magnesium has a higher affinity for oxygen than tantalum and when deposited on the anode, actively removes the oxygen from the tantalum. After the deposition of magnesium a leaching process removes the magnesium oxide. Again, oxygen levels are monitored to ensure the level of oxygen in the anode is at or below the levels of the tantalum powder as delivered.
F-Tech technology has been shown to significantly increase reliability; after 2000 hours accelerated life testing at 85°C and 1.32 x rated voltage, KEMET F-Tech capacitors have demonstrated zero degradation while standard tantalums have 1.5 orders of magnitude degradation in leakage current. F-Tech capacitors also benefit from an innovative wire-attachment technology that produces a stronger mechanical connection between the anode and the lead wire, hence preventing the major cause of open-circuit failures.
Simulated Breakdown Screening or SBDS is a patented (Patent Number US 7,671,603 B2) technique allowing non-destructive testing to ascertain true breakdown voltage (BDV) and removal of parts where BDV is below a pre-determined minimum.
By simulating capacitor BDV without damaging the dielectric, SBDS allows KEMET to determine the breakdown voltage of each individual capacitor and hence select only the strongest capacitors from each lot (figure 2). The benefit of SBDS is illustrated by a minimum breakdown voltage greater than 2 x rated voltage, as compared to Weibull graded parts with breakdown voltages as low as 1.25 x rated voltage. Also, this is done without inducing wear-out, which is a phenomenon sometimes associated with Weibull grading.
By providing an effective means of isolating capacitors with low breakdown voltage, SBDS allows engineers to apply lower derating at design-in and hence benefit from greater efficiency. Without such screening it is quite common for conventional tantalum capacitors to be derated by up to 50%, which can result in a 90% reduction of efficiency.
Effective in the field
F-Tech and SBDS technologies can be applied independently, and hence give users the flexibility to specify conventionally screened F-Tech devices, or SBDS screening to guarantee minimum breakdown voltage of conventional capacitors. A combination of SBDS screening applied to F-Tech devices provides the ultimate assurance of reliability in the field.
Indeed, from third party feedback it can be estimated that when using 1,000,000 SBDS-screened F-Tech tantalums the first failure would occur in 10,000 years, as compared to other solutions that are likely to see the first failure between six months and one year.
Tantalum capacitors – degradation and breakdown voltage
Tantalum capacitors are polarized, electrolytic capacitors built with a pure tantalum sintered anode that allows a very thin dielectric layer. Compared to other types of capacitor they offer higher capacitance per unit volume, improved high-frequency characteristics and good long-term stability. They are widely used in applications such as power supply filtering and microprocessor decoupling. In addition, their low leakage current supports excellent performance in sample-and-hold circuits, where a long hold time is required. Tantalum capacitors are used in military, aerospace and medical equipment such as power supplies, DC/DC converters, guidance and targeting systems and communication equipment.
The major degradation mechanism in tantalum capacitors is the crystallization of anodic oxide films. This crystallization disrupts or breaks the amorphous structure of the anodic oxide film causing increases in leakage current including breakdown and short circuits. The crystallization rate in anodic oxide films depends on several factors but paramount are chemical purity and mechanical robustness. From a chemical purity perspective, the presence of carbon and oxygen in the basic anode have found to be the major cause for the formation of crystalline structure and the growth rate of these structures.