Designers often say they need a “rugged” component, but don’t adequately define what kind of “rugged” they need. In fact, it’s important to clarify the situation so the appropriate component can be selected from the many possibilities available.
These special circumstances don’t apply solely to active components such as transistors and ICs; they also apply to passive ones – resistors, capacitors, and inductors – as well. It’s also critical to consider what level of rugged performance is acceptable in the situation. While an outright failure is clearly not desirable, a modest change in a component’s key parameter beyond its initial tolerance limits due to operating conditions may be acceptable.
What are the environmental and operating situations which these passives may have to tolerate? Extended temperature is the first one that usually comes to mind. Among the commonly available ranges are −40°C to +100°C for commercial operation, −40°C to +125°C for industrial and automotive applications, and −55°C to +125°C or +155°C for military designs. However, there are many other ranges defined by various industry standards, as well as the extreme cold and heat of space.
Further, it’s not only the absolute high/low temperature span that degrades a component. The repeated thermal shock of going from nominal to one extreme (or the other) repeatedly is also detrimental, and can induce cracks as a result to the stresses in materials induced by thermal cycling.
Another factor in ruggedness is continuous vibration, where it causes material fatigue and subsequent faults. The solution here may involve using a standard component rather than a mechanically more rugged one, but used in an enhanced mechanical mount or support. How and where the component is placed on the circuit board or in the product plays a large role in minimising the effects of such vibration.
Special consideration is often required in cases of high humidity. This can cause a change in the impedance between a component’s leads, and in extreme cases even foster mould growth which has serious consequence. In such cases, adding a conformal board coating or even epoxy-based potting may the logical solution, but those may also be “large” solutions where a smaller one will be sufficient.
For example, the WIN series of thin-film chip resistors from TT Electronics uses a water-insoluble nitride film to avoid the moisture vulnerability problems of nichrome and passivated nichrome technologies; this film self-passivates which forms a very thin, highly stable oxide layer. The result is very low and predictable drift and high reliability even under conditions of high humidity, even in the event of damage to the component coating layer itself.
These resistors are available in 0603, 0805 and 1206 sizes, Figure 1 with 0.1 W, 0.25 W and 0.33 W ratings respectively, and resistance values from 5 Ω to 1 MΩ with a tolerance of ±0.05% and TCR of ±15ppm/°C.
Figure 1: Although it is indistinguishable in appearance from a standard resistor, due to its water-insoluble nitride film, the WIN series of thin-film chip resistors from TT Electronics is highly resistant to moisture.
Other factors which can cause resistor failure are surges and pulses, ranging from lightning all the way down to far-smaller but still significant spikes which are common in industrial and automotive systems, or energy-metering and power-condition monitoring. Even with these modest pulses, where the average power which the resistor is handling is within its rating, a short but high-power pulse can cause failure due to overheating and other modes, and a series of repetitive pulses is even more damaging. Note that neither forced air-cooling nor heat sinks will have an effect on overload resistance for durations below a few seconds.
For these situations, designers can consider various pulse-withstanding resistors which meet various standards for such tolerance, as defined by the appropriate telecom, industrial, and automotive standards bodies. For example, the WRM-HP range of precision metal-film resistors, Figure 2, are qualified to the automotive AEC-Q200 requirement, but of course can be used in other applications as well. Note that this series also has a very wide operating temperature range of −55°C to +125°C.
Figure 2: Surges can be a “hidden” killer of components, so resistors such as TT Electronics WRM-HP precision metal-film resistors are carefully designed and tested to meet defined requirements for protection against these transients.
Radiation resistance is another type of ruggedness, and is needed for spacecraft designs at tolerance levels which are a function of distance from Earth. Low Earth orbit (LEO) satellites undergo less radiation exposure than those in geostationary orbits, while spacecraft which go beyond Earth’s protective layer must endure much higher levels. (For a brief, interesting perspective on space radiation and some solutions, see the recent blog “Multi-Chip Arrays (MCAs) pave the way for ‘NewSpace’ projects.”)
Finally, consider the situation where the possibility of resistor failure for any reasons must be assessed, in the context of an intrinsically safe (IS) design. This sort of fail-safe mode is one for which a short-circuit failure mode is considered impossible, and therefore need not be simulated during analysis or testing. The failure inherently limits the current available to pass through the barriers which restrict energy transfer between parts of an IS system.
The solution may be to use a resistor such as the ULW series, which uses wirewound construction, similar to a conventional wire-link fuse but with the wire instead wrapped around an insulated rod. The only failure modes here are parametric (a shift of a few percent in ohmic value) or open circuit, both of which meet the IS mandate, while the component is both a resistor and a fusible link.
Working on your next design? Find out more about our full range of resistors here.