Electronic components, including resistors, often have to operate in harsh, challenging environments. Given the fluctuating natures of climates all over the globe, the task that requires components to operate in high humidity, extreme dryness and incredibly moist environments comes as no surprise. Of course, moisture becomes a hazard for any type of carbon-based resistor, as water and carbon readily combine to form carbon dioxide and eat away at the component, resulting in an open circuit. Heat, on the other hand, tends to lower the resistance. What may come as a surprise is that thick-film SMDs, the resistor type most often used in today’s designs, are actually quite resilient to this common threat. It is their susceptibility to sulfur that takes many designers by surprise.
Sulfur is a Hazard
Sulfur is present in many common lubricants, and is also present in components made of rubber, including gaskets, seals and grommets. Thick-film chip resistors, in particular, use a combination of silver and palladium in their inner terminations, with silver comprising more and more of the mix because of its lower cost. Sulfur and silver are reactive to each other, forming silver sulfide. The intrusion of the sulfur into the resistor generally occurs in the tiny gap that is present on the border between the outer electrode, which is the electrical conduit between the device and the PCB, and the protective coating that protects the rest of the component. Once inside, the sulfur can readily react with the silver of the inner electrode, changing it into silver sulfide.
Over the course of an electronic product’s lifetime, this process of sulfuration causes considerable degradation. It is the source of added resistance, way beyond the resistor’s expressed tolerance. Another factor to take into account is that the silver sulfide formed in this process takes up more physical room than the silver that it replaces. The result is the occurrence of cracks, leaving a void through which more sulfur can seep in. The outcome can be that all the silver in the contact is engulfed, taking the resistor entirely out of the circuit, and leaving an open circuit.
Solutions to this problem are not inexpensive. They include going back to using more palladium and less silver, and even adding a tiny coating of gold, because gold does not react with sulfur. This is the solution effect by Panasonic’s ERJ-U030R00V thick-film SMDs.
Figure 1: Panasonic ERJ-U030R00V.
The inner electrode of Panasonic’s ERJ-U030R00V has gold, providing protection in a sulfurous environment. The datasheet reveals that this component is available in a variety of resistance value tolerances and power ratings.
A greater proportion of the global output of electronic devices of all sorts has been sold to businesses and consumers in Asia in recent years. In that part of the world, sulfur pollution is a major problem on city streets as well as in manufacturing environments—and it is an issue that is not likely to be ameliorated any time soon. For that reason, it’s not just specialized equipment that must be protected from sulfur, but consumer devices as well.
Fortunately, thin-film resistors, which are largely immune to problems involving sulfur, have become progressively cheaper to produce. The two main types employ resistive elements composed of tantalum nitride and nickel chromium, with the former also being able to resist problems with moisture. Their resistance to problems with sulfur is based on their inner electrode, which is comprised largely of nichrome, instead of silver. Thin-film resistors enjoy other advantages over their thick-film counterparts. These include lower stray induction and capacitance, better TCRs and they can be more efficaciously miniaturized.
Moisture and Humidity
It turns out that nichrome may resist sulfur, but not moisture, especially when combined with such common impurities as fluorine, chlorine sodium and calcium. The result is that the resistive element combines with the oxygen contained within the moisture of a humid environment, creating metal oxides that don’t conduct electricity, radically changing the value of the resistor.
For radial resistors, an extra layer of insulation from the outside world has proven effective in keeping moisture out. This, combined with extra care taken during the manufacturing process to keep impurities out, has proven effective and inexpensive. The extra coating layer option has also worked with SMDs, but the manufacturing process is more involved. Often, the extra layer comes in the form of a coating for the entire PC. This does add cost, and it can create problems if board repair or updates are necessary.
The resistive element of thin-film resistors can be nichrome or tantalum nitride. The latter, often described as TaNFilm® resistors, have the interesting property of forming an oxide layer in response to an electric field, and thus form a protective layer that can be quite effective against moisture.
TT Electronics produces a TaNFilm® resistor in the form of a voltage divider. Dubbed the PFC-D1206-03-1503-3091-BB, it can be specified in a number of different configurations. As the datasheet reveals, this device has been tested against elements of MIL-STD 202, a military standard for electronic components, and meets its requirements for resistance to moisture.
High Temperature, Pressure and Mechanical Shock
The oil industry is the largest consumer of electronic equipment designed for high temperatures. The deeper down oil explorers drill, the hotter the environment becomes, and temperatures of 150º C can be devastating to all types of film resistors. Wirewound resistors, on the other hand, are stable even at temperatures of 300º C and beyond. As might be expected, resistors that can stand up to high temperatures have excellent TCRs. Wirewound resistors conform to this generality, with TCRs in the range of 3 ppm/° C.
Metal-foil resistors are highly resistant to mechanical shock, which is a vital characteristic for avionics applications. This type of resistor is often confused with metal-film resistors, but, because the metal-foil resistor’s resistive element is far thicker, it is inherently more robust.
This characteristic is enhanced by a special construction technique, in which a flexible wire, whose only purpose is to absorb shock, is interposed between the resistive element and the lead that makes the external electrical connection. NASA mandates this method for metal-foil resistors in its S-311-P-813 specification. NASA requires that metal-foil resistors be able to absorb a 100G shock and withstand a 20G peak vibration. Their excellent TCR makes metal-foil resistors a great choice for high-temperature environments, as well.
Resistors, often thought of as the simplest of passive components, have different strengths and weaknesses in different threat environments. Designers who ignore these important factors do so at their own risk.