This article starts 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 the issues to consider when choosing a power resistor for a particular application.
The construction of a thick film resistor product is a serial process. A substrate (usually Alumina) is printed with a resistor paste that comprises metal oxides and glass-based fillers, which together with a solvent forms a paste. The substrate and paste are then fired to burn off irregularities and solvents and to harden the substrate. The resistor element is then sealed to prevent moisture and gas ingress.
Although thick film resistors are often the cheapest components on a PCB a failure can be just as catastrophic as the most expensive active component. Choosing the right quality for a thick film resistor starts by describing its use as follows:
– Describe the use and application.
– Describe The environment for use. The key issues to consider are temperature, moisture, aggressive gases, mechanical stresses and EMC requirements.
– Long term reliability requirements.
– Specify the required quality standard.
It may be the power resistor device is only required for a consumer application. In this case, the long term impact 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 an expected short lifespan. However, if the device is for a military application environmental factors and quality levels will be a serious concern.
In high-reliability applications, the storage of components, traceability and production methods will be important. Microcracking in the substrate due to inappropriate handling or trimming can cause failure. As can failure to seal the device correctly to prevent the ingress of moisture and gases (particularly sulphur and chlorine).
Thick film power resistor devices are relatively small, low cost and high reliability in the majority of applications. But some knowledge of the manufacturing process and potential failure modes is useful to ensure correct component selection. The clarity in the precise requirements for the device will maximise the chances of long term component reliability.
When an application-specific resistor is the only solution, close cooperation with a specialist resistor manufacturer can yield the best results. Utilise the experience of the supplier engineering teams to ensure the power resistor device is fit for purpose.