Resistors Answer Design Challenges for Harsh Environments
Many resistor applications have operating conditions that don’t require anything beyond a standard thick film chip resistor or general-purpose carbon film leaded resistor. However, when the application environment requires something more robust, choosing the correct resistor technology is an important factor to keep design costs down and time-to-market short. It is important for design engineers to understand the various resistor technologies and how they perform with respect to harsh environmental conditions. Two of the most important environmental considerations for resistors are high moisture and exposure to high amount of sulfur. Many engineers are surprised to learn that commodity thick film chips, the most commonly specified resistor today, perform well in high moisture environments, but have some long-term reliability concerns in high-sulfur environments. In addition, certain SMD and thru-hole metal film resistors are susceptible to corrosion due to moisture. Fortunately, there are answers to both of these design challenges and they don’t have to be expensive.
Thick film chip resistors use printed thick film conductor inks for their inner terminations, which are typically composed of a palladium silver alloy. Over the past few years, in an effort to reduce costs, the composition of inner conductor inks has trended toward a higher percentage of silver and a lower percentage of palladium. During the same time period, sulfur began to be more prevalent in the environments in which many types of electronic devices are found. Sulfur can be present in many types of rubber gaskets, hoses, and grommets. Some types of plastic used for connectors can also have a high concentration of sulfur, as can the atmosphere in highly industrialized areas. The combination of these two factors has led to a potential latent failure mode for thick film chip resistors, which has become more prevalent in the past five years. Silver is highly susceptible to reaction with sulfur; and high levels of sulfur can migrate through the outer plated terminations and barrier layer to the inner palladium/silver termination, leading to the formation of silver sulfide. Silver sulfide is non-conductive and its continued migration may eventually lead to open circuit failure. Fortunately, there are several solutions for sulfur contamination.
The most common solution to sulfur contamination for thick film chips is to reduce the amount of silver in the inner termination material and to slow down the overcoat print operation so that it can be more precise. These two remedies are extremely effective in slowing the growth of silver sulfide and are relatively low cost. However, since some silver is still present in the inner termination, the potential for sulfur contamination still exists. Another thick film solution, which provides a part that is impervious to sulfur corrosion, uses a gold sub-layer to protect the inner terminations. Since gold is impervious to sulfur, this is a reliable solution, but it is an expensive one and not practical for most commercial applications.
A more recent solution to this problem uses thin film technology for the inner terminations. Nichrome is the typical material for these solutions and is also impervious to sulfur contamination. Thin film technology can be scaled up for mass production when the TCR and tolerance requirements for the element are comparable to the above solutions. This significantly reduces the manufacturing costs. The same thin film elements can also achieve higher power ratings than the above solutions, and are completely lead-free. The previously mentioned thick film solutions are typically produced using a lead-containing dielectric glass, which is exempt from RoHS at this time. As a result, the thin film, high power anti-sulfur solutions are currently the most intriguing of the available solutions for sulfur rich applications.
Applications that have moisture-rich environments can also present challenges when using metal-film-based SMD and leaded resistors. Moisture, together with ionized impurities containing sodium, calcium, chlorine, or fluorine, combine with the carbon, nickel and chromium. For carbon, the ionized moisture combines with the carbon in the film resulting in CO2 gas; hence the resistive element evaporates away. For resistive elements containing nickel and chromium, they will combine with the oxygen to form nickel oxide or chromium oxide, which are non-conductive. Both of these chemical reactions lead to an increase in resistance and eventually will lead to the resistor failing open. The figures above show resistors that have experienced resistive element vaporization.
For film-based axial resistors, the solution to this performance issue requires special processes, improved process control and the use of special materials. First, the film process is optimized and the temperature adjusted to ensure the film is as robust as possible with respect to moisture resistance. Then all processes are closely monitored to ensure that ionized impurities are not introduced into the part at any point. Finally, a silicon-based intermediate layer is deposited on the film prior to the final coating process. This sub-layer is impervious to moisture and allows the final coating process to remain as it would for any axial leaded resistor, rather than try to optimize this coating to be the moisture barrier itself. Each of these product improvements individually has been shown to improve the moisture resistance of the product. The moisture withstanding performance of product with all of these improvements combined is exceptional, as can be seen in the graphs above. These product enhancements can now be implemented without a significant increase in manufacturing cost.
Metal film based chip resistors will have the same problems as the axial leaded metal film resistors because their nichrome resistive elements are essentially the same. For chip resistors, however, pinholes in the passivation print layer are more common than they are for through-hole resistors. The figures above show a standard thin film chip resistor structure on the left. Even tiny pinholes in the protective layer may allow enough moisture into a surface mount chip to cause a failure. The inspection systems and methods for detecting pinholes in mass production are not adequate to completely eliminate these from occurring. Like the coatings used for axial leaded resistors, most standard passivation materials for SMD chip resistors currently exhibit this potential for developing pinholes. For most applications using thick film resistive elements, pinholes are only considered a cosmetic defect. Passivation materials therefore have been optimized for high-speed manufacturing and low cost.
The solution is to use a silicon-based layer between the resistive element and the passivation to serve as the primary moisture barrier. Silicon oxide and silicon nitride can be sputtered onto the resistive element and thus will not have the same problem with pinholes that a printed epoxy passivation would. An additional anti-moisture passivation layer is also added prior to the final normal epoxy passivation. The figure on the right above shows a chip resistor with this structure. Thin film nichrome-based resistors with this type of moisture protection exhibits outstanding long term reliability under the harshest of biased humidity testing. The chart below shows 3000-hour life test data.
Film based resistors are the most widely used technology today. In the past, sulfur contamination has been a problem that was only a concern in a few specific applications. With the increased industrialization of many countries worldwide, it has become an issue for many more electronic applications than just automotive or industrial equipment. Fortunately there are low cost options for the design engineer that might be faced with this kind of environmental condition.
Thin film and metal film resistors have long been susceptible to moisture corrosion or evaporation under certain conditions. Through process improvements, unique designs and materials improvements, these two failure modes no longer need to be a barrier for the design engineer.