01207 230 728 sales@tsec.ltd.uk

Precision thick film resistor networks are critical components in many high-performance electronic systems, from instrumentation to advanced test equipment. In this post, we cover the complexities of achieving tight resistor tolerances in these networks.

Resistor Tolerance Fundamentals

Resistor tolerance refers to the permissible deviation from the nominal or stated resistance value of a resistor. This tolerance is expressed as a percentage and indicates how much the actual resistance value of the resistor can vary from its stated value.

A precision thick film resistor is suitable for applications requiring high accuracy, such as instrumentation, test equipment, and precision voltage dividers.

Thick Film Resistor Network Manufacturing Limitations

Controlling resistor tolerance is a significant manufacturing challenge. The composition of materials can have a major impact on the outcome.

Thick film resistors are manufactured by depositing a resistive paste onto a substrate. The exact composition of the paste affects the tolerance. Impurities or variations in the resistive material can lead to inconsistencies in resistance.

The flatness of the substrate and the screen printing process used to apply the resistive paste can impact resistor tolerance. Variations in the thickness and uniformity of the resistor film can result in resistance variations.

After printing, the resistors are fired at high temperatures. Fluctuations in temperature, or differences in the firing profile (time and temperature), can cause variations in the physical and electrical properties of the material, thus affecting the resistor’s tolerance.

Post-firing resistors often undergo a laser or abrasive trimming process to achieve precise resistance values. The precision and control of this process are crucial; any inconsistency can result in variations that impact the final tolerance.

During manufacturing, factors such as humidity and temperature can influence the behaviour of materials and impact processes like printing and firing. Controlled environments are necessary to minimise these effects.

Ageing and Drift

Ageing refers to the changes in the properties of thick film resistors over time. Ageing is inevitable and resistor manufacturers typically specify an ageing rate for resistors, over a defined time, under specified environmental conditions.

Resistance drifts due to changes in the microstructure of the resistive paste Environmental factors such as temperature, humidity, and exposure to chemicals can accelerate these changes.

Frequent temperature changes can cause mechanical stresses that alter the resistor structure. Under normal operating conditions, the resistor will experience stress from the regular current load. This can lead to thermal effects and gradual degradation of the materials.

The absorption of moisture can change the conductivity and dielectric properties of the materials used in thick film resistors. Exposure to corrosive environments
can lead to chemical changes in the resistor materials

Design Considerations for High-Precision Networks

In precision thick film resistor network layout, interconnect routing, and resistor placement are critical in maintaining tolerance specifications. The design process should aim to mitigate the effects of individual resistor tolerances and other factors that can introduce errors. Potential design approaches include:

Resistor Matching: Ensure resistors have similar temperature coefficients and ageing characteristics. This technique is useful in differential amplifiers or bridge circuits.

Thermal Matching: One advantage of a resistor network is the close proximity of resistors on the substrate. This ensures they are subjected to the same temperature changes, reducing variability.

Temperature Compensation: Thick film resistor networks can be designed with temperature-compensating features, such as using resistors with opposite temperature coefficients that cancel each other’s temperature effects.

Averaging: Multiple resistors can be arranged in parallel or series to average out errors. This can help to smooth out variations caused by individual resistors.
Achieving high precision in thick film resistor networks requires a detailed understanding of material science and network design and manufacturing techniques. There are limitations and trade-offs are often required to deliver a resistor network device optimised for a particular application.

Understanding what can (and cannot) be achieved is crucial for senior engineers working on applications where resistor accuracy directly influences system reliability and performance. Where reliability and/or safety are critical it is often best to work with specialist resistor network manufacturers.