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Advanced decoupling capacitors for aerospace applications

Mon, 09/09/2013 - 4:46pm
Ron Demcko, AVX Fellow, AVX

To keep pace with the ongoing technological advancements in the aerospace industry, electronics destined for use in aerospace applications must not only meet a stringent set of safety and reliability qualification requirements, but must also satisfy continual market pressures to provide maximum functionality and improved reliability in a physical form that is both smaller and lighter than the components that are currently available.

Advancements in multilayer ceramic capacitor (MLCC) technology have provided several new configuration options for these critical passive components in aerospace applications.

Aerospace systems are achieving higher levels of performance via the adoption of increasingly high speed ICs and high frequency RF links, both of which require the intended signal to be completely decipherable against background noise. One of the most effective methods for optimizing signal-to-noise ratios is controlling, transforming, or eliminating the internal parasitics of advanced ceramic capacitors.

Two examples – DC blocking capacitors and advanced decoupling capacitors – are provided to demonstrate the importance of controlling MLCC parasitics. In both cases, the resulting component types offer true advantages, particularly for aerospace circuits. Advantages include better electrical performance, smaller size, lower weight, and higher reliability of the overall system.. At the individual component level, the aerospace industry and its user community have reached the point where component reliability is a given, so the stringent quality and reliability standards must be met or exceeded in every case.

DC blocking capacitors
The purpose of a DC blocking capacitor is to eliminate the DC offset voltage so that it cannot enter later stages of an electronic circuit. Regardless of the exact circuit type – RF, optic, etc. – the capacitor serves to isolate this DC voltage from subsequent stages. The advantage of an ultra broadband DC blocking capacitor is that it isolates stages across a wide frequency spectrum through the use of a single component. Utilizing patented material and manufacturing methods that control the parasitic elements, a typical ultra broadband DC blocking capacitor can effectively pass frequencies from 16KHz to 40GHz.

A component photo, together with cross section of its internal design, is pictured in Figure 1 below. These ultra broadband DC blocking capacitors are available in 0201, 0402, and 0603 case sizes.

The advantage of using discrete ultra broadband DC blocking capacitors is that these devices save size, weight, and the inherent reliability risks that necessarily follow from having several solder joints within a multi-component circuit.  Additionally, ultra broadband filter characteristics become more important as operating frequencies increase.  Frequency responses associated with ultra broadband DC blocking capacitors are shown in Figure 2.

Decoupling capacitors
IC manufacturers recommend using low inductance decoupling capacitors to achieve adequate levels of high frequency noise filtering required for stable operation in aerospace electronics applications.

While an ideal capacitor can instantly transfer all of its stored energy to a load, a real capacitor has parasitics that prevent the instantaneous transfer of a capacitor’s stored energy. The true nature of a capacitor can be modeled as an RLC equivalent circuit. For most simulation purposes, it is possible to model the characteristics of a real capacitor with one capacitor, one resistor, and one inductor.

The RLC values in this model are commonly referred to as equivalent series capacitance (ESC), equivalent series resistance (ESR), and equivalent series inductance (ESL). The ESL of a capacitor determines the speed of energy transfer to a load; so the lower the ESL of a capacitor is, the faster its energy can be transferred to a load.  Consequently, reduction of the MLCC’s parasitic inductance (ESL) plays a major role in creating a high efficiency, low inductance decoupling capacitor and it is the geometry of the capacitor that affects the capacitor’s inductance.

Trends driving low-inductance capacitor evolution

Trend 1: Reverse geometry MLCCs

The first trend is explained by the following three illustrations of two terminal devices. The far left illustration is that of a traditional MLCC in a standard EIA case size, which is terminated on its ends. This traditional device is used as a baseline to compare reduced inductance devices against.

The middle illustration shows the same case size capacitor now terminated on the long side of its body. This is the most simple of reverse geometry MLCCs and has the effect of reducing the ESL nearly in proportion to the reduction in spacing/loop area of the opposing electrodes; or, in this case, a reduction of approximately 60% compared to a standard MLCC device.

The third illustration shown under the two-terminal devices grouping shows the effect that vertical electrodes have in reducing inductance through a reduction in the current loop area. The two-terminal inductance loop reduction is limited by the practical limits of MLCC dimensions, though. Once that limit is reached, multi-terminal MLCCs must be used.

Trend 2: Multi-terminal MLCCs

Multi-terminal MLCCs enable loop inductance between the electrodes to be reduced well beyond that of most two-terminal devices.

The first of the multi-terminal devices to be introduced was the Inter-Digitated Capacitor (IDC).  Terminals of alternate polarity are brought out of the capacitor’s body in such a way that the loop inductance is approximately 80% lower than a standard MLCC.

The next devices in the multi-terminal family were the low inductance capacitor array (LICA) and its successor, the multi-terminal land grid array (LGA), both of which utilize vertical electrodes to further reduce loop area.

Material trends – base metal electrode MLCCs
To address the extreme cost pressures on low inductance ceramic capacitors, component manufacturers have turned to the use of a base metal electrode (BME) material system, which has a much lower material cost than traditional palladium silver electrode (PME) devices. Despite their notably higher pricing, PME-type capacitors will not be discontinued any time soon, as their exceptional reliability is necessary for the most critical of missions. However, the new BME-type MLCCs offer comparable performance with the highest capacitance ratings for extremely competitive prices, which, due to the prevalence of associated applications that require high capacitance components, has led to their increasingly broad use within the aerospace industry.

Base Metal Electrode capacitors were initiated into the consumer electronics sector more than twenty years ago and, shortly thereafter, began being designed into demanding industrial and automotive applications. Both of these market sectors, which also have high reliability requirements for electronic components, have long accepted the fact that the low insulation resistance failures caused by the oxygen vacancy issues that were found in some of the initial BME devices decades ago were satisfactorily solved shortly afterwards through firing and formulation means. As such, BME devices have since earned a reputation for exhibiting high reliability performance, In fact, today BME capacitors, both in RoHS and tin-lead termination options, are gaining ever-greater acceptance in high reliability aerospace applications – a trend that will accelerate as BME component manufacturers expand their test data bases and customers learn about the success that materials and statistical process control (SPC) processes have had on BME component performance.

Summary
The aforementioned examples – the DC blocking capacitor and the advanced decoupling capacitor – demonstrate the importance of controlling MLCC equivalent model parasitics in high reliability applications. In both cases, the resulting component types offer true advantages to the aerospace electronics community, including better electrical performance, smaller size, higher reliability and lower weight, while still meeting or exceeding the stringent quality and reliability standards for aerospace components, which the industry now accepts as a given. Now, the optimization of parameters such as ESL is one of the major goals to be mastered by component manufacturers and optimally implemented by end users.

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