Thick Film Resistor Design Considerations
When designing any electronic system there are compromises to be made in component selection. This extends to what may be perceived as the simplest components including resistors and capacitors. In this post, we consider typical thick film resistor design and application issues the system designer needs to consider.
The Perfect Resistor
A perfect resistor will have a constant resistance value over its entire in service life and zero capacitance and inductance. The resistance value will be maintained regardless of any external stresses including over voltage and surge events, ESD, mechanical stresses, changes in temperature, moisture levels and environmental conditions.
In reality, the perfect resistor does not exist and there are trade offs to be made in the design and manufacturing process to deliver a resistor that meets the required specification at an acceptable cost. It is important the system designer understands these design compromises when selecting a resistor component. Given the demands of a particular application and the environmental conditions, the designer should consider
Load life stability
Depending on the voltage applied, the resistor material and manufacturing method a resistor will be subject to some self-heating and this must also be accounted for during the selection process.
A thick film resistor has many advantages including its relatively small size, excellent thermal properties, relatively low cost, low impedance and robust construction but it is not the best choice in all applications.
Thick Film Resistor Manufacture
The most common thick film manufacturing process is based on a resistor pattern deposited on a flat ceramic (usually Alumina) substrate. A resistive paste consisting of a mixture of metal oxides, a carrier and a binder is applied to the substrate.
The carrier consists of organic solvents and holds the resistor pattern in paste while the glassy frit is designed to hold the resistor material in place post firing. The resistor material is a granular film of metal oxides. Resistance is a function of the dimensions of the resistor track (LxWxH) and the film resistivity.
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 then 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.
Thick Film Resistor Design Compromises
Resistor performance issues including resistor tolerance, temperature coefficient of resistance (TCR) drift, voltage coefficient of resistance (VCR), power rating and surge withstand capability are all directly linked to the appropriate choice of materials. The resistor manufacturer may have a wide choice of materials available but there are always choices to be made based on the demands of the application, manufacturing capability and cost.
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 resulting in ‘hot spots’ and variance in performance between resistors of the same lot. Choice of material and manufacturing methods can correct both of these issues (to a point) if the final application and environment are understood.
VCR has a negative impact on the resistance value. This effect is difficult to measure in production but during the resistor design process, a series of tests and calculations may be made to compensate for the VCR effect that may apply in a particular application.
The final coating process of the resistor determines the insulation resistance of the device and its dielectric withstand capability. Both of these characteristics are important in the design of precision high voltage resistors.
A vast range of standard resistor devices is available. Given an understanding of the trade offs in resistor design, manufacture and cost the system designer may make an appropriate selection based on published data. For specialist applications or where no standard resistor is available to match the demands of the application, a partnership between the system designer and a specialist thick film resistor manufacturer can deliver the required resistor device.
Typically, the manufacturer will need to understand the required:
Dimensions (with tolerances)
Nominal resistance (with tolerances)
Nominal power load
Operating temperature range
Power /current rating
Length of pin
Armed with this information the thick film resistor manufacturer may design and manufacture a resistor fit for purpose in the required volume.