Armed with the resistor performance specification and full details of the application a specialist manufacturer will select the most appropriate thick film power resistor substrate technology to deliver a reliable solution at an appropriate cost.
Correct resistor substrate material selection can maximise heat dissipation and minimise component size. This must be weighed against the material cost and potential complications in the manufacturing process.
Substrate Options
There are four main substrate technologies used in thick film power resistor manufacture, they are:
1. Alumina
2. Aluminium Nitride
3. Steel
4. Silicon
A range of other materials are available, but they tend to be for more specialist, niche applications. Often their availability and cost can be an issue.
Selecting A Substrate Technology
Alumina Substrates
Alumina is by far the most common thick film power resistor substrate technology. The material delivers a level of performance suitable for most applications at an acceptable cost.
Different grades and formulations of Alumina substrates are available to match the demands of various applications. The material has good thermal conductivity, mechanical strength and resistance to wear and corrosion.
Alumina substrates also have good high-frequency performance, above-average electrical insulation properties and excellent long term stability. Of the four thick film power resistor substrate technologies discussed in this post, Alumina and Steel are the cheapest.
Aluminum Nitride Substrates
Aluminium nitride (AlN) thick-film substrates are generally used where excellent thermal management is required. The thermal conductivity of Alumina Nitride tends to be 4 to 6 times higher than Alumina.
Substrate strength, electrical properties and long term stability tend to be similar to Alumina. However, the improved thermal conductivity comes at a cost as Aluminium Nitride is typically eight times the cost of Alumina. There can also be availability issues with Aluminium Nitride material.
Aluminium substrates are an alternative to Aluminium Nitride. There are thick film inks for use on aluminium (often used for LED applications) where the thick film materials are printed and fired on the aluminium heatsink itself.
Steel Substrates
Steel substrates are relatively cheap (similar cost to Alumina), are robust and can be cut into various shapes. The technology is capable of withstanding high levels of temperature, vibration and shock.
The electrical performance of steel is poor when compared to the other technologies listed in this post. Therefore steel is often used in heaters and other applications where performance is not a major issue.
Silicon Substrates
There are two main types of silicon substrate – Silicon Nitride and Silicon Carbide. Both deliver high levels of mechanical strength at high temperatures and high thermal shock resistance. Silicon carbide is highly corrosion resistant. Silicon substrates are used in specialist applications as they are expensive compared to Alumina.
When considering the best substrate technology for specialist applications, it is often best to discuss the material and design options with a specialist resistor manufacturer. They can use their long term experience of resistor substrate technology to guide the selection process.