Armed with the resistor performance specification and full details of the application a specialist thick film power resistor manufacturer will select the most appropriate substrate technology to deliver a reliable resistor solution at an appropriate cost.
There are four main substrate technologies used in thick film resistor manufacture, they are:
2. Aluminium Nitride
A range of other materials may be used but they tend to be for more specialist, niche applications and their availability can be limited. There can also be availability issues with Aluminium Nitride material.
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.
Various different grades and formulations of Alumina substrates are available to match the demands of a wide range of applications. The technology 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 resistor technologies discussed in this post Alumina and Steel are the cheapest.
Aluminum Nitride Substrates
Aluminum nitride (AlN) thick-film substrates tend to be 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.
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 are relatively cheap (similar cost to Alumina), are robust and may 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 and therefore tends to be used extensively in heaters and other applications where performance is not a major issue.
There are two main types of silicon substrate – silicon Nitride and Silicon Carbide. Both deliver high levels of mechanical strength at high temperature and high thermal shock resistance. Silicon carbide is highly corrosion resistant. Silicon substrates tend to be used in specialist applications and are expensive when compared to Alumina.
When considering power resistor design for specialist applications it is often best to talk through the material and design options with a specialist manufacturer with long term experience of thick film resistor technology.