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.
Heat in the high power resistor must be dissipated without impacting on nearby temperature sensitive electronic components. Thick Film Resistors can reach temperatures which exceed safe operation within seconds. It is therefore important to keep the operating temperature range within maximum specified limits – typically 50 – 60C.
Heat is generated in the high power resistor as current passes through the resistor element. This heat must be dissipated to prevent damage to the resistor materials. At the power resistor component level, the classic solution is to manufacture a resistor with sufficient mass to dissipate the heat. However, this solution can compromise component packaging density at the system level.
The solution is to use a appropriate combination of materials with superior thermal properties. But high performance materials all come at a cost. At the system level, cooling using heat sinks, forced air cooling, water cooling and heat pipes, either individually or in combination, to keep the operating temperature range within specified limits.
Heatsinks are sometimes used in isolation but require excellent thermal bonds between the resistor element and heatsink to avoid thermal fractures of the resistor. Choosing an appropriate resistor material may also improve thermal performance.
While small package size is desirable this must not compromise heat dissipation of the resistive element. Choosing a specialist power resistor substrate material may maximise heat dissipation and minimise component size but this must be weighed against material cost and potential complications in the manufacturing process.
There are four main substrate technologies used in thick film power 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.