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Metal exposed to moisture generally results in undesirable corrosion. Just as it can occur on the terminals of a car battery, it also can occur on a precision thin-film resistor in a critical portion of a circuit. For a battery, the simple solution is regular maintenance and a wire brush. However, until recently, the only proven solution for nichrome resistors was to replace them with thin-film resistors having tantalum nitride elements.
The self-passivating property of tantalum nitride elements made them necessary for applications where prolonged exposure to moisture is likely, such as automotive or industrial electronics. However, the cost of these resistors limited their use to only critical circuits where they were absolutely necessary. Recently, new materials and processing techniques have changed the thin-film resistor landscape and allowed certain nichrome-resistor types to serve as more cost-effective alternatives to tantalum nitride resistors.
It is possible for any nichrome resistor element to experience corrosion in a humid atmosphere. However, there are certain conditions that act as catalysts and greatly increase the chance for excessive resistance shift or even open failure. The presence of electrical potential is definitely a catalyst, but it is not the only factor contributing to corrosion.
High resistance values appear to be more susceptible to corrosion, which is logical because they are more likely to be operated at or near their maximum voltage and not at maximum power. A resistor running at rated power usually generates enough heat to prevent the accumulation of surface water. Conversely, this same logic implies that a part of any resistance value running from 10% to 20% of its rated power will have a higher susceptibility to corrosion.
But even with these factors present, the only failures due to nichrome resistor corrosion at the customer or manufacturer levels also involved a flaw, misalignment or pinhole in the outer protective coating. The manufacture of these precision-resistor types, while not at the extreme speeds of commercial thick-film chips, are still done in such a way that an imperfection as mentioned previously is rare but still possible. If the previously mentioned conditions were present for a resistor with this type of passivation flaw, it could result in severe electrical damage.
One might then suggest that a solution would be to slow down the passivation processes to the point where no misalignments or pinholes are possible. To a certain degree, some manufacturers do this. But in the real world of today's manufacturing, to guarantee no passivation miscues would be impractical because the manufacturing costs for this nichrome part would then increase to essentially the same cost as the tantalum nitride equivalents that the design engineer is trying to replace.
A better solution is to introduce materials that are more glasslike in nature, both as an addition to the nichrome resistive element itself and as a secondary line of defense against moisture under the final passivation coating. The addition of small amounts of this silicon-based material to the nichrome element itself reduces its susceptibility to corrosion. As long as the amounts are kept to a minimum, the excellent electrical and stability properties that a nichrome resistive element provides are unchanged. The physical structure of a nichrome resistor with these features is shown in Fig. 1.
Additionally, the use of a silicon-based material as an initial protective layer against moisture under the final passivation layer makes the chance of a pinhole through both layers so remote that it is essentially irrelevant. Both of these additions increase cost only in small increments. Another property that can help minimize the effects of moisture on this type of part is substrate smoothness. A smoother surface will have fewer valleys where the condensation can collect or pool up. Again, if one of these valleys were to line up with a passivation flaw or hole, chances for corrosion are increased.
All of these techniques are easily incorporated into the manufacturing of nichrome resistors and lead to products that show resistance shifts over humidity testing that are equal to or even less than those for a tantalum nitride element. Examples of these new devices are shown in Fig. 2. The typical humidity test parameters used in the industry are done at 85°C, 85% or greater relative humidity, 10% rated power, and at least 1000 hours in length.
Tables 1 and 2 show the results of a moisture performance test comparing standard nichrome and tantalum nitride resistors to SEI's new special-passivation NiCrSi devices. The procedure for both tests were as follows:
- Place a drop of deionized water on each resistive element
- Apply 9 Vdc to each resistor for 4 minutes
- Visually inspect each resistor
- Electrically test each resistor.
As evident from the results of these tests, the processing applied to SEI passivated nichrome resistors results in a significant improvement over standard nichrome elements, as well as superior performance over tantalum nitride technology.
All market segments have price pressure, and power supplies depending on the end application are no different. Power supplies use precision resistors at or near rated power more often than many other applications. There are still many occurrences of precision resistors used in voltage division, or to set gain or other types of feedback where the need for reliable, cost-effective precision resistors are critical.
One example would be automotive power circuits. Automotive power design engineers must take into account all reasonable weather conditions and environmental effects. Given the challenges these engineers face, resistor cost and moisture sensitivity would likely not be the first issues considered. This new technology offers a significant advantage by reducing potential concern over resistor cost and moisture sensitivity.
Given their controlled operating environment, power circuitry for the medical industry is presumably less vulnerable to moisture than automotive circuitry. However, medical devices typically have a much smaller margin for error. Furthermore, there is a growing market trend to make medical instruments smaller and portable so they can be included onboard ambulances. The reliability of precision resistors in these applications, with respect to all environmental conditions, then becomes paramount.
Instrumentation is another obvious application where there is very little room for environmental variance, especially in aerospace applications. Instrumentation aboard boats and other marine vehicles also would fall under this category. This would include both the circuitry used for the control systems and also gauges that measure and report vehicle dynamics. For aerospace applications, those dynamics could include speed, altitude, pitch, banking, fuel, landing-gear position and radar data. For nautical vessels, the instrumentation applications may include speed, weather conditions, depth-finder data, and fuel and trim settings, to name a few. Both of these application segments may also use instrumentation for direction or GPS positioning.
These applications represent the most extreme examples within the range of mobile electronic equipment having to cope with uncertain operating environments. For any device fitting these criteria, whether portable such as a PDA or embedded in a mobile platform such as those previously mentioned, the concerns are largely similar. While the space constraints for an automobile are obviously quite different than those for a PDA, both will be required to provide predictable, reliable power for extended periods of time, regardless of geographic location. In that sense, designers for both may be forced into a requirement for an environmentally robust precision resistor.
The communications industry of today is being pushed to faster and more efficient levels than previously thought possible. If the equipment for this voice and data communication is housed within an office building, then there will obviously be fewer concerns about environmental conditions. However, if this equipment is at the base of a remote communications tower, the potential for weather-related issues certainly exists. This communications hub could be a microwave or fiber-optic link, but in either case this high-speed communications equipment would definitely need the reliability that a precision moisture-tolerant resistor would provide. And given increasing equipment speeds and corporate downsizing, cost is also becoming increasingly important to this market segment.
Industrial power and controls are also another application where operating conditions can be very unpredictable. Elevator and escalator controls are typically inside a controlled atmosphere, but they also may be exposed to the elements at places such as parking lots. Other types of industrial power or controls such as boiler controls, machinery such as ovens, fans, and ventilation, and motors of many different types must consider both the allowable price point as well as the long-term reliability of their design.
Finally, this new precision nichrome resistor is a good solution for companies with insufficient technical manpower to determine the long-term reliability of their design. It provides assurance that moisture corrosion will not be an issue in the design. And it accomplishes this for only a minimal increase to the cost of conventional nichrome resistors, as opposed to a tantalum nitride solution, which being typically 40% to 70% higher in cost would require the design engineer to make absolutely certain that the application needed such a part before designing it in.
|SEI passivated (400 Ω)||Standard nichrome (1 kΩ)|
|SEI passivated (400 Ω)||Tantalum nitride (1 kΩ)|