By Ron Demko; AVX Corporation
The evolution of automobiles from their birth as simple horseless carriages to the current wide range of highly complex models has been nothing short of breathtaking. Today’s automobiles exhibit significantly greater performance, luxury, and safety compared to the automobiles offered just a few years ago.
The goal of the next generation cars will be to offer the consumer a blend of even higher levels of driving performance, more luxurious amenities, increased levels of safety, and better fuel efficiency. Without a doubt, we can safely predict that performance levels unheard of a few years ago will be possible through the use of advanced engine control units, various electronic stabilization packages, a broad array of sensors, and high speed communications bus structures.
Initial Electronic Content:
Many of us can recall the days of control cables, linkage rods, and belts on automobiles. Electronic content first emerged in the form of non-critical entertainment and comfort offerings. AM radios are the best example of this event. AM radios quickly evolved into the various forms of early audio options. Among them were FM radio, 8 track players, and cassette tape players. As these entertainment options were accepted by consumers, the auto manufacturers became efficient in large volume electronic manufacture. Automobile accessory manufacturers began to improve the electronics design, efficiency, reliability, and size through process improvements and the well-known transition from tube-based circuitry to semiconductors. Ceramic capacitors, as well as most other passive components, met that challenge by evolving from typically higher voltage leaded devices to intermediate voltage devices in axial and miniature radial packages. Ultimately SMT packages were offered when reliability performance levels could meet and exceed the requirements of the Big Three (Ford, DaimlerChrysler, and General Motors).
Somewhere along that semiconductor and system evolution process, common mode chokes and capacitive based
filters were used in OEM designs to improve the performance of these options through better voltage quality. More advanced noise filters began to emerge as aftermarket options for the home mechanic. In fact, a case can be made that the home mechanic audio market was so great and appeared so lucrative that the Big Three realized that major design efforts should be placed there. Advances in aftermarket audio choices reinforced automotive manufacturers’ commitment to audio and entertainment product development efforts. The audio and entertainment trend led to expectations for the cost reduction, electrical performance, and reliability demanded by the electronic divisions of the Big Three. In most cases, evolution of passive components centered upon improving the reliability of ever increasing electrical quantities in smaller SMT case sizes—e.g. more capacitance in smaller case sizes. Other efforts were placed upon improved electrical stability over temperature and increased voltage ratings for new breeds of power semiconductors.
Secondary Electronic Content Phase:
The next area of electronic content occurred in the offering of comfort options on automobiles. Developments like power windows, power locks, speed alarms, intermittent wipers, and air conditioning all began to involve some form of passive components for power stabilization, filtering, and sensor functions. Though the function of these systems still was not considered as critical to automobile operation, the manufacturers
placed increased reliability and performance demands on suppliers.
As time progressed, performance enhancement systems were developed and introduced at a later phase in this trend. Typically, these systems involved using redundant circuitry and systems to maintain overall automobile performance and accuracy. Further, designs were structured in such a way that an electronic system failure would not inhibit total vehicle operation.
Third Electronic Content Phase:
Eventually, semiconductor trends, costs, and process capability evolved so much that cost effective reliable VLSI ICs became available for designers. Additionally, low cost, reliable power semiconductors came onto the market.
These broad developments would forever change automotive electronics. Miniature, reliable VLSI chips are crucial
to the control of engine, power train, and vehicle stability functions. Next generation VLSI ICs will be called upon to improve automobile fuel economy through the control of advanced items such as variable displacement engines, variable CAM timing, variable compression ratios, direct injection, and variable advanced multi-speed transmissions. Most importantly, from this article’s perspective, the VLSI chips cause an exponential need for the implementation of efficient ceramic capacitors in designs. Efficient capacitors are needed for voltage stabilization and decoupling, as well as EMC control associated with the high operating speed and the small trench size associated with VLSI ICs. In fact, EMC control and compliance is a major goal driving many automotive electronic designs. Passive components and advanced passive components can be utilized to simplify and solve EMC design issues. As additional specialty ICs are created (stability, occupant sensor, IR control/ sensor, etc), there will be an additional need for high operating temperature range capacitors and high quality bulk decoupling capacitors. Power ICs allow designers the option to control loads and modules in a fast, direct method. These power ICs are driving the need for higher voltage capacitors exhibiting low inductance, as well the need for miniature higher power circuit protection devices.
One way to group the advances in passive components is by end application family. A simplified grouping of application types yields three general end sectors: 1. generalpurpose electronics, 2. high temperature and harsh environment solutions, and 3. EMI/Transient suppression solutions.
1. General Purpose Electronics
Examples of general-purpose electronics are modules, which do not expose electronics to excessive environmental demands. These can range from dashboard electronics to passenger compartment mounted ECU systems, or from RKE units to wiper control circuits. Other examples of emerging general-purpose circuits are internal mapping/in car navigation, speech recognition, biometrics, and interior seating controls. Regardless of the exact circuit there is always an overall desire to make the circuit smaller, cheaper, and more reliable. Passive components are addressing the designers’ general improved cost, size, and reliability goals through the introduction of new types of bulk decoupling capacitors, low inductance–efficient decoupling capacitors and high device density capacitor arrays.
Niobium Oxide Capacitors (OxiCap™) are lead-and halogen- free capacitors that offer automotive designers high reliability and fail-safe operation ideal for use in high reliability applications. The niobium oxide capacitor was
developed as a higher performance alternative to aluminum and other SMT technologies. These capacitors are capable of lead free reflow process temperatures of 260°C through a 3-pass operation. The OxiCap is based
upon a niobium oxide ceramic anode material that is processed through the same steps as Tantalum capacitors. OxiCaps exhibit high reliability (0.5%/1000hours) as a result of the Nb2O5 dielectric in combination with the
self-healing MnO2 cathode. Niobium Oxide capacitors are packaged in the same EIA case sizes as Tantalum
capacitors and are available in both standard and low ESR series. For instance, standard and
low ESR Oxicaps are available in B, C, D, E, V, and Y case sizes, from values of 100μf to 680μf and ESR values from
50 to 150mΩ. The typical applications OxiCap capacitors address are low voltage bulk decoupling around ICs and
DC:DC converter output filtering (typically on 5 volt lines).
As previously stated, OxiCap devices offer high reliability and fail-safe operation. OxiCap devices fail in a high
resistance mode, which in the great majority of cases will not impact the functional operation of the circuit. A
comparison of typical failure resistance distribution between Niobium-Oxide capacitors and polymer types is shown in Figure 1. The data illustrates a high resistance failure mode relative to other capacitor types.
Further, a failed OxiCap has a 95% reduced ignition failure mode compared to conventional tantalum capacitors, due to an ignition energy requirement nearly three orders Niobium also has a favorable burn rate of approximately one tenth that of Tantalum. Data comparing the burning rate and ignition energy between
Niobium Oxide, Tantalum, and Niobium Polymer is shown in Figure 2.
Low Inductance Decoupling Capacitors help designers meet stringent EMC requirements associated with today’s vehicles by lowering overall PCB impedances and minimizing PCB geometry. It is possible that a designer may actually reduce the number of decoupling capacitors used on a design with the use of low inductance capacitors. Additionally, low inductance decoupling capacitors offer designers a capacitor with a dramatically higher self-resonant frequency of operation— thereby actually extending the useful frequency range of the capacitor. The progression of advanced, low inductance decoupling capacitors is shown in Figure 3. Generally speaking, in addition to aspect ratio rules, the larger the number of connections to the capacitor terminals, the lower the capacitor inductance. If alternate termination connections are placed in close proximity to one another, the inductance is further minimized due to magnetic field cancellation. Figure 3 illustrates those rules—SMT capacitors offer favorable geometries over axial leaded capacitors. Standard EIA case size capacitors’ inductance can be minimized by a geometric optimization, terminating SMT capacitors on the long side vs. the short sides.
As process capability developed, multiple connections were alternately made to the terminations, thus improving magnetic field cancellation and further reducing capacitor inductance. Finally, geometric rules and I/O optimization were combined to create capacitors with inductances minimized in the low tens of ph (or lower). Presently, most of the automobile interest in low inductance capacitors is limited to reverse geometry capacitors (those terminated on the long side—typically like 0612, 0508, and 0306 sizes).
High Device Density Capacitor Arrays are a space-saving alternative to discrete capacitors in parallel bus and redundant system circuitry. Capacitor arrays will be used extensively in engine control units, entertainment systems, and smart modules. Four element capacitor arrays are typically available in 0508 and 0612 packages, with voltage ratings up to 100 volts. Both NPO and X7R dielectrics are available in values up to 470pf and 0.1μf respectively. An emerging trend is to use dual value capacitor arrays on analog circuitry. Alternatively, dual value arrays can be used to broaden the frequency response (Figure 4) of a decoupling capacitor on ICs sensitive to Vcc quality and EMI. Two-element array usage is expected to increase because it saves space in filtering drivers of multiplex bus communications within automobiles.
2. High Temperature and Harsh Environment Solutions Sensors and modules are examples of high temperature and harsh environment electronics, which are typically under- hood mounted. These assemblies experience significant periods of high temperature operation, large numbers of temperature cycling, as well as random vibration/shock. Specific modules and assemblies can range from engine, clutch, and gearbox controllers to turbocharger sensors. Hybrid automobiles also have a variety of high temperature and harsh environment electronics modules. Typically these are inverter/converter circuits, motor control, and water pump applications.
Long term reliability and performance are critical regardless of the exact circuit under discussion. Size and cost are important considerations as well. Miniature and highly reliable versions of 150°C rated ceramics and 175°C rated Tantalum capacitors have been developed to address high temperature applications. In addition, new termination material systems have been developed that greatly improve ceramic capacitor temperature cycling performance.
High Temperature X8R Ceramic Capacitors exhibit a temperature stability of +/-15% across their rated temperature range of –55 to +150°C. X8R capacitors have been available for some time in standard and automotive AECQ-200 qualified parts. Typically X8R capacitors were available in tin and conductive epoxy terminations for hybrid applications.
A significant new development is that AVX now offers RoHS-compliant X8R capacitors in a termination technology called FLEXITERM™ (available on other standard dielectric discrete and array capacitors as well). FLEXITERM™ is a flexible termination layer that helps maintain electrical integrity under external forces such as vibration, bending, and temperature cycling. This flexible termination is achieved by coating the copper termination of a ceramic capacitor with a conductive polymer, then plating with nickel and tin. The flexible, high conductivity termination material allows nearly three times the board flexure of standard ceramic capacitors, thus enhancing the components’ mechanical and temperature flexure performance to prevent system failures due to cracking.
At high temperatures, the mismatched coefficient of thermal expansion (CTE) between capacitors and PCBs can cause excessive forces at the connection interface of the dissimilar materials. This force can be significant in automotive applications where varying load conditions cause large temperature fluctuations and vibration. FLEXITERM’s flexible properties lessen the stress on the component, reducing the system failure risk. It is important to note that some manufacturers offer flexure protection by using an adapted internal electrode design, where an increased gap is made between the end of the electrode and the side of the chip. This design helps to reduce low resistance and short circuit failures due to the higher chance of the crack passing through one polarity group of electrodes and not the worst case scenario of shorting though both polarities. However, depending on how high the crack spreads through the component, an increasing loss of capacitance occurs due to the disconnection of electrodes. Another limitation of adapted internal designs is that higher capacitance values may not be offered for these components.
The X8R capacitors reduce the transfer of mechanical stress on the component; preventing damage during module manufacture, board flexing, vibration, and temperature expansion. It has been designed so that failure
due to board flexing/temperature cycling would be contained in the termination area and in open mode. This stops the current supply, removing the risk of damage or fire from a short circuit failure. With FLEXITERM™ the open circuit occurs in a small area of the termination with little or no degradation to the capacitor’s performance.
These capacitors can withstand over 4mm board flexure and AECQ 200 temperature cycling requirements for 3000 cycles without failures.
High Ttemperature Tantalum Capacitors are available in AVX’s new THJ series. These devices combine high temperature operation and higher basic reliability for optimal performance in automotive applications. Continuous operation at +175°C is possible with a 50% derating of voltage ratings. The level of reliability of THJ capacitors is 0.5%/100 hours at rated voltage, rated temperature, and 0.1Ω circuit impedance. THJ capacitors are available in A, B, C, and D case sizes in values up to 150μf and 35V ratings.
3. EMI and Transient Suppression Solutions:
EMI and transient suppression solutions are needed on virtually all automotive electronics. Examples range from air bag igniters to motor driver circuits. The most common circuits in need of both EMI and transient protection are multiplex bus communication networks within vehicles. There is a variety of SAE recommended practices for vehicle multiplexing, with one of the most common being CAN bus. Within a CAN bus, each node has an interface circuit that needs to be protected from transient events as well as filtered for EMI emissions and interference. Multilayer Varistors (MLVs) can provide both transient suppression and EMI filtering in a single component, based upon a predictable bi-directional zener clamping capability and a definable off state capacitance. Figure 5a shows the equivalent model for a MLV. Figure 5b shows how a dual element MLV (each element represented by a bi-directional zener contained within the blue box – which represents a single 0508 package) can be utilized as a single component solution to CAN bus protection. It is very significant to note that new versions of MLVs now offer lower leakage current than zener diodes, thereby allowing sensors to be more accurate and systems to consume less power. Additionally, new choices for CAN bus protection are now offered, based on CAN bus speed (Figure 6).
There is strong evidence that automobile electronic content will continue to increase due to consumer demands and government regulatory requirements. At the same time, passive component usage will also increase due to increased general electronic content within vehicles. Given these trends, high performance passive components will need to continue to evolve in order to meet the demands of next generation semiconductors, sensors, and modules.
Companies Who Reviewed This Article Also Reviewed: (1) Value-Added & Application Specific Ceramic Capacitors (High Voltage, High Frequency and High Temperature): World Markets, Technologies & Opportunities: 2005-2010