Choose the correct resistor for thick film power

This article begins by discussing the basic construction of a thick film power resistor product. We then cover the potential performance limitations this construction can cause and discuss issues to consider when selecting a power resistor for a specific application.

Building a thick film resistant product is a sequential process. The substrate (usually alumina) is imprinted with a resistive paste consisting of metal oxides and glass-based fillers, which together with the solvent form a paste. The substrate and paste are then fired to burn off irregularities, solvents and harden the substrate. Then the resistor element is sealed to prevent moisture and gas ingress.

Although thick film resistors are often the cheapest components on a PCB, failure can be just as catastrophic as a more expensive active component. Choosing the right quality thick film resistor begins with a description of its use as follows:

Description of use and application.

Description of the usage environment. The main issues to consider are temperature, humidity, aggressive gases, mechanical stresses, and electromagnetic compatibility requirements.

long-term reliability requirements.

Determine the required quality standards.

A power resistance device may only be required for a consumer application. In this case, the long-term effect on performance due to electrical stresses or environmental factors such as temperature may be irrelevant.

Quality standards may not be an issue if the power resistor is for a low cost product with a short expected life. However, if the device is intended for military application, environmental factors and quality levels will be a major concern.

In high reliability applications, component storage, traceability and production methods will be important. Fine cracking in the substrate due to improper handling or trimming can cause failure. Failure to properly seal the device to prevent entry of moisture and gases (particularly sulfur and chlorine) can also occur.

Thick film power resistors are relatively small, low cost, and highly reliable in the majority of applications. But some knowledge of the manufacturing process and possible failure modes is useful to ensure the correct selection of components. Being clear on the exact requirements of the device will increase the chances of component reliability in the long term.

When an application-specific resistor is the only solution, close cooperation with a specialized resistor manufacturer can yield the best results. Leverage the expertise of your suppliers’ engineering teams to ensure your power resistor is fit for purpose.

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