The thick film resistor manufacturing process can directly impact resistor performance parameters. These include resistor value, tolerance, temperature coefficient of resistance, drift and power rating.
The Perfect Resistor
A perfect resistor will have a constant resistance value over its entire in-service life and zero capacitance and inductance. Its resistance value will remain constant regardless of any external stresses. These may include over-voltage and surge events, ESD, mechanical stresses, changes in temperature, moisture levels and environmental conditions.
In reality, the perfect resistor does not exist. Trade-offs are necessary for the design and manufacturing process to deliver a resistor that meets the required specification at an acceptable cost.
The system designer must understand these compromises when selecting a resistor. They should consider
- Power dissipation
- Voltage rating
- Load life stability
Self-heating is also a consideration. This depends on the applied voltage, the resistor material and the thick film resistor manufacturing method.
Thick Film Resistor Manufacturing
A resistance film pattern is deposited onto a flat substrate (usually Alumina). After firing the patterned substrate at high temperature the resistor pattern is 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 using a screen printing 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 paste is printed onto a substrate and then fired 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 are added sequentially to create the required resistance pattern and 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.
Manufacturing Process Impact On Performance
Temperature effects and electrical and mechanical stresses can all impact the performance of a resistor over time. They may affect the resistor value or cause complete failure.
The granular structure (see above) of the resistor film makes it susceptible to damage by thermal stress during operation. Final abrading of the resistor track can cause track thinning. This can result in ‘hot spots’ and variance in performance between resistors of the same lot.
A vast range of standard resistor devices is available. Given an understanding of the resistor manufacturing and design issues, the system designer can make an appropriate selection.
Larger manufacturers tend to produce devices that are suitable for a wide range of applications. They tend to ignore applications that have more extreme requirements. Specialist manufacturers operate in this space. They offer application-specific thick film resistor devices manufactured in low to medium volume.