By optimising the industrial power resistor manufacturing process, manufacturers can improve resistor performance and reliability, tolerance, voltage rating, TCR, stability and power rating.
In this article, we cover the manufacturing process of industrial power resistors. We discuss material selection and production techniques and their potential impact on performance.
The Perfect Resistor
Throughout its in-service life, a perfect resistor will have a constant resistance value 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. To deliver a resistor that meets the required specification at an acceptable cost, requires trade-offs. The system designer must understand these compromises when selecting a resistor. They should consider
- Power dissipation
- Voltage rating
- Tolerance
- Load life stability
Power resistor thermal management and the impact of self-heating is also a consideration. This depends on the applied voltage, the resistor material and the power resistor manufacturing process.
Thick Film Power Resistor Manufacture
The first step is to deposit a resistance film pattern onto a flat substrate (usually Alumina). Resistance is a function of track length divided by track width multiplied by ink resistivity.
In screen printing, a metal oxide mixture, combined with a carrier material, and a binder is applied to the substrate. The carrier is a mixture of organic solvents and the binder, a glassy frit. The binder holds the resistor material in place post-firing.
After firing the patterned substrate at high temperatures (typically 850°C) the resistor pattern is often abraded to deliver the required resistor value. Finally, the resistive element is coated to protect it from the environment. This determines the insulation resistance of the device and its dielectric withstand capability.
During the high-temperature firing process, 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. Manufacturers add resistive layers sequentially to create the required resistance pattern and value.
Improving Power Resistor Performance
Optimising thick-film industrial power resistor manufacturing processes can maximise performance and reliability. However, It is worth noting that the pursuit of higher performance must be balanced against cost. More precise manufacturing processes may lead to increased production costs.
Power Dissipation
The power rating of an industrial power resistor is the maximum amount of power the resistor can safely dissipate (in a given environment) without exceeding its specified temperature limits.
Given the required power handling capability, a definition of the operating environment, cooling (if any) and the space available on the system board, the resistor manufacturer can make appropriate design choices.
The power dissipation capacity of thick film resistors is influenced by material selection. Different materials have varying thermal conductivity and temperature coefficients. High thermal conductivity materials can handle more power without significant temperature rise.
The substrate material and dimensions are crucial. High substrate thermal conductivity improves heat transfer and increases the power handling capacity of resistors. Larger resistors have more surface area for heat dissipation, allowing them to handle higher power levels. The shape of the resistor can also have an impact on power dissipation.
Material selection and design are crucial, but manufacturing controls can be just as important. As an example, the final abrading of the resistor track can cause track thinning. This can result in ‘hot spots,’ which can cause a power resistor to fail.
Voltage
In high-power applications, the resistor’s power handling capability is usually the first consideration, but it is also vital to consider the maximum voltage rating. Different thick film resistive materials have varying dielectric strengths. This determines the resistor’s ability to withstand high voltages without electrical breakdown.
The substrate material is also a factor to consider. Substrates with high dielectric strength increase electrical insulation and the resistor voltage rating.
As with power ratings, the physical dimensions and shape of the resistor play a role. Larger resistors have more space between conductive elements, reducing the risk of voltage breakdown.
Tolerance
Tolerance refers to the range within which the resistance value can deviate from its specified value. Variations in resistor material resistivity cause variations in the resistors nominal value.
Tolerance is also impacted by variations in the power resistor manufacturing process. Although manufacturers aim to control these variations, this must be balanced against the cost implications.
Stability
Temperature effects and electrical and mechanical stresses can all impact the performance of a resistor over time. They may affect the resistor value (stability), or cause complete failure.
The granular structure of the resistor film makes it susceptible to thermal, electrical or mechanical stresses. Resistor material selection can mitigate this issue (to an extent), but there are performance and cost trade-offs to consider.
A vast range of standard resistor devices is available. Larger manufacturers tend to produce devices suitable for a wide range of power resistor applications. They tend to ignore applications that have more extreme requirements. Specialist industrial power resistor manufacturers operate in this space. They offer power resistor devices manufactured in low to medium volume.