Resistor performance issues including resistor value, tolerance, temperature coefficient of resistance, drift and power rating are all directly linked to the thick film resistor manufacturing process and the appropriate choice of substrate and resistor materials.
Thick film resistors are manufactured by depositing resistance film patterns onto a flat substrate (usually Alumina). After firing the patterned substrate at high temperature the resistor pattern may be modified (abraded) to deliver the required resistor value. Finally, a coating is often added for environmental protection.
The resistive paste consisting of a mixture of metal oxides, a carrier and a binder is applied to the substrate usually by a screen printing type process. The carrier consists of organic solvents and holds the resistor pattern (metal oxides). The binder consists of a glassy frit. Its purpose is to hold the resistor material in place post firing.
The resistive pattern is printed onto a substrate at high temperature (typically 850°C). The carrier material burns off, the metal oxides combine to form the resistor film and the glassy frit melts to hold the resistor material in place. Resistive layers may be added sequentially to create the required resistance pattern and value.
The final resistor pattern is often modified (abraded) to deliver the required resistor value. A final coating process determines the insulation resistance of the device and its dielectric withstand capability. Resistance is a function of track length divided by track width multiplied by ink resistivity.
Temperature effects and electrical and mechanical stresses can all impact on the performance of a resistor over time. They may impact on the resistor value or cause complete failure. Thick film resistor manufacturers select materials and modify the manufacturing method to address the particular demands of the application.
Larger manufacturers tend to produce devices that are suitable for a wide range of applications. They therefore tend to ignore applications that have more extreme requirements. Specialist manufacturers operate in this space offering application specific thick film resistor devices that tend to be manufactured in low to medium volume.