In some applications, the noise generated by current flowing through a thick film resistor can cause serious system errors. This post examines the sources of thick film resistor noise and explains how to minimise it through appropriate material selection and optimised manufacturing processes.
Electrical Noise Defined
Electrical noise distorts a desired electrical signal and introduces error into a system. The impact depends on the comparative amplitude of the noise and the desired electrical signal. This is why noise is a concern in sensitive equipment where the signal of interest can be overwhelmed by noise.
Thermal noise exists regardless of current flow, while current noise is related to the physical microstructure of the conduction path.
Current noise typically has a 1/f characteristic, which means its impact is concentrated at low frequencies and falls as frequency increases. This makes it most significant in precision, low-frequency, and DC measurements. In many applications, such as high voltage applications, other performance issues are of greater concern than noise.
Resistor Material Selection
The resistor paste, the substrate and the termination material can all impact thick-film resistor noise, but the paste tends to be the main contributor.
Noise performance is heavily influenced by the number of conductive particles in the resistor paste and how effectively the particles combine to form current paths.
Different resistor material compounds exhibit different noise characteristics. Higher Ohmic resistors are generally noisier as the resistor pastes have fewer conductive particles than lower Ohmic value resistors.
Closely related is the composition of the glass frit, which influences how effectively conductive particles contact after firing.
Beyond the composition of the resistor material, the substrate material is also a factor, as its properties directly affect film uniformity. Both the purity of the substrate material and its surface roughness influence how evenly the resistor material deposits and fires and this uniformity in turn impacts current flow and noise performance.
Finally, terminations and termination material can also affect noise performance. Their impact is influenced by the cleanliness and resistance of the resistor film to the termination junction.
Noise is generated at the termination solder junction. The noise level is influenced by the quality of the solder joint, the chemical stability of the termination plating alloy and the plating process.
Thick Film Resistor Process Optimisation
Process optimisation begins with tight control over the screen printing stage to deliver a consistent wet film thickness across the resistor substrate. Consistent film deposition helps create a more uniform resistor structure after firing.
The firing process is equally important as it impacts the glass phase and the formation of the conductive network within the resistor material. Poor control of printing or firing can produce a less uniform conductive network, disrupting current flow and creating noise.
Trimming is another important consideration as it changes the geometry of the resistor and the way current flows through the resistor film. Different trim patterns create different levels of current crowding and therefore influence noise performance. The impact can be minimised by designing the pre-trim resistance closer to the final value.
Once trimmed, the final thick film resistor coating protects the resistor from moisture and contaminants that could impact stability and noise performance.
Conclusion
Thick film resistor noise arises due to the granular nature of thick film construction and is shaped by both material and process choices.
Although thick film resistor noise cannot be eliminated, it can be reduced. Specialist thick film resistor manufacturers can work with customers to optimise thick resistor performance to match a specific application.
The final choice of a resistor is always a compromise depending on performance (including noise), cost, precision, power rating, size, robustness, stability and frequency response.