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.
Power Resistor Substrate Options
There are four main substrate technologies used in thick film power resistor manufacture, they are:
1. Alumina (Al₂O₃)
2. Aluminium Nitride (AlN)
3. Beryllium Oxide (BeO)
4. Silicon Nitride (Si₃N₄)
Glass/ceramic substrates and steel substrates are not included above. Glass/ceramic substrates suffer from poor heat dissipation, hot spots and thermal ageing. Although Steel substrates are robust with high-temperature capability, their electrical performance is poor and they are generally only used in heater applications.
Choosing A Power Resistor Substrate Technology
The type of power resistor substrate should be driven by the demands of the application. In consultation with the thick film power resistor manufacturer, the key issues to consider are:
- Thermal conductivity.
- Electrical insulation properties.
- Mechanical stability and strength.
- Environmental stability.
- Useability.
- Availability & Cost.
Thermal Conductivity – Using a substrate with poor heat dissipation leads to performance degradation, reduced reliability, and system-level issues.
Inadequate heat transfer from the resistor can result in hot spots or thermal runaway. Elevated temperatures can cause changes in resistive material properties and resistance drift over time. Poor thermal performance can force resistor derating and a corresponding increase in resistor dimensions.
When selecting a substrate, resistor manufacturers will consider the Coefficient of Thermal Expansion (CTE). They will match substrate CTE with resistor paste and other materials to avoid delamination or cracking during temperature cycling.
Electrical Insulation Properties – Poor insulation can cause performance degradation. In high-voltage applications, it can lead to leakage, short circuits, or catastrophic failure. To maximise electrical insulation, substrate material must have high dielectric strength (KV/mm).
Poor insulation means leakage currents can flow to the heatsink or nearby circuit board tracks. Leakage paths can change the effective resistance value and introduce noise into sensitive circuits.
Mechanical Strength and Stability – In many industrial applications, thick film power resistor substrates must withstand vibration, mechanical shock and short-term overload conditions caused by electrical surges or pulses.
Mechanical stresses can cause sudden resistor failure, but more often they cause micro-cracking. Over time, these small cracks can expose the conductive layer to moisture or contaminants, causing resistance drift or shorting, and accelerating failure under vibration or shock.
Resistor manufacturers find that poor substrate strength makes it difficult to maintain tight tolerances because substrates can warp or fracture under processing stress.
Environmental Stability – Defines thick film resistor behaviour when exposed to high humidity, temperature cycling, or corrosive atmospheres. Poor substrate environmental stability can cause resistance drift, leakage, corrosion and substrate cracking,
Water ingress can cause leakage currents across the substrate surface or through microcracks, reducing insulation resistance and also promoting corrosion. A chemical attack weakens the substrate’s mechanical strength and reduces dielectric performance. For power resistors in industrial or automotive environments, this is a major long-term reliability issue.
Useability – Covers a range of factors from health and safety to ease of handling, including dicing and mounting. Resistance to chemicals used in cleaning, processing, or operating environments is a key consideration. There are RoHS/REACH compliance issues to consider
Availability and cost – In any design process, there are cost vs performance trade-offs to consider. Sourcing some substrate materials can be difficult, significantly increasing lead times.
Alumina is by far the most common power resistor substrate technology. The material delivers a level of performance suitable for most applications at the lowest cost.
Different grades and formulations of Alumina substrates are available to match the demands of various applications. The material has excellent electrical insulation, excellent long-term stability, acceptable thermal conductivity, good mechanical strength and resistance to wear and corrosion
Aluminum Nitride (AlN) offers a significant thermal advantage over alumina, with thermal conductivity that is 4 to 6 times higher. However, this performance comes at a cost as AlN typically costs eight times more than alumina and availability can be an issue. In other key areas, including substrate strength, electrical properties, and long-term stability, aluminum nitride performs similarly to 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 onto the aluminium heatsink itself.
Beryllium Oxide (BeO) substrates offer high performance but are rarely used as the material is toxic if mishandled. BeO costs are high and availability is poor.
Silicon Nitride (Si₃N₄) substrates are usually selected when mechanical robustness and environmental performance is critical, but costs are high compared to Alumina and Aluminum Nitride
When considering the best substrate technology for complex 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.