In thick film power resistor design material selection is essential so that the resistor can both dissipate the right amount of heat and retain a reliable resistance after a while.
Important issues to think about in thick film resistor design include:
- Resistor tolerance
- Temperature coefficient of resistance
- Resistance drift
- Power rating
Frequently the ability rating and resistance drift would be the key but all be a consequence of the best option of materials. Such as the power rating and temperature coefficient of resistance are largely with different selection of substrate material. Whereas resistance drift is much more connected while using resistor paste and processing.
Not merely one substrate material is a perfect choice which satisfies all of the important characteristics of thick film substrates. A range of substrate materials therefore are frequently connected while using electrical, thermal and mechanical demands within the application. A perfect thick film substrate must have:
- Low dielectric constant
- Good dimensional stability during processing
- Good adhesion between substrate and printed material
- High thermal conductivity
- A thermal coefficient of expansion matching many other materials within the circuit
- High electrical resistivity
Alumina is a kind of choice which satisfies most of the above needs in several applications nevertheless it’s brittle and may snap (particularly with large substrates), shaping may also frequently matter. To find the best power dissipation Aluminium Nitride is frequently utilized because it is >5 occasions the thermal conductivity of Alumina but it’s relatively pricey and needs using special printing pastes and processing.
While using the substrate material established the resistor material must be defined. Resistor materials are employed in paste form by screen printing and they are then fired by heating having a warm. Resistor pastes have three primary ingredients:
- The key elements (metal alloy or oxide particles)
- Binder (glass particles)
- Organic solvents and temporary binder
The temporary binders give control of the printing process. They evaporate at the outset of the firing process combined with solvents. The glass particles melt within the firing process, bind the active particles together, keep to the substrate and provide stability for that circuit.