Thick film resistors are often required to operate in harsh environments. These include downhole applications, military applications, low-temperature environments and many industrial plant applications.
In this post, we cover the impact harsh environments can have on thick film resistor performance. We also discuss some potential solutions.
Environmental Damage To Resistor Components
The main challenges found in extreme environments are high humidity, high (or low) temperatures and chemical elements. These factors can act alone or in combination. The worst-case result can be a component failure. Other issues include component interface failure, subtle performance changes or performance drift.
A high moisture level environment can cause several problems. It can cause component failures and/or compromise the component/system board interface. Water ingress into the resistor package material can damage the resistor material, cause tracking or a short circuit.
In extreme cases, water ingress can ‘pop’ a package if the component is subjected to a thermal cycling event. At the component/system board interface a high moisture environment can cause mould, metal migration and lead corrosion.
Thermal issues and their impact on resistor performance are discussed elsewhere on this blog. High temperature can cause overheating of the thick film resistor element. This can affect resistor performance and long term resistance stability. Sometimes it can lead to component failure. Low temperature can damage the resistor packaging and cause lead and contact embrittlement.
Sulphur in the environment is a common cause of chemical environment failures. It can damage the package and any coatings and facilitate secondary damage via moisture ingress. When packages or coatings are damaged sulphur can damage resistive elements, lead coatings and the lead/system board interface.
Protecting Resistors From Harsh Environmental Conditions
Protecting thick film resistors from chemical elements is generally straightforward. A variety of coatings are available to protect the resistor package. In extreme applications, barrier layers can be incorporated into the resistor design. Specialist materials are available for the resistor package. The main problem is protecting the resistor lead and the system board interface.
Addressing high (or low temperatures) is primarily a resistor component design issue. If the environment is known in advance the resistor design, cooling and system board layout can be considered. Action taken in advance will reduce the risk of component failure.
High moisture environments can be more difficult to address. Choosing appropriate resistor package materials or coatings can resolve the problem at the component level. However, issues at the system board interface remain an issue. System board design and layout, cleaning processes and the correct manufacturing systems can all help.
Utilising thick film resistors in harsh environments is a challenge. There are issues to address at both the component level and at the system board level. Threats can be interrelated and the impact of one can make the component more susceptible to damage by another. Secondary protection can protect standard resistor devices. Alternatively, a specialist thick film resistor manufacturer can design an application specific device.