The Era of Technology

In thick film power resistor design material selection is critical to ensure the resistor can both dissipate the required amount of heat and retain a stable resistance over time. Important issues to consider in thick film resistor design include: • Resistor tolerance • Temperature coefficient of resistance • Resistance drift • Power rating Often the power rating and resistance drift are the most important but all are related to the appropriate choice of materials. For example the power rating and temperature coefficient of resistance are largely dependent on the choice of substrate material. Whereas resistance drift is more related to the resistor paste and processing. No single substrate material is an ideal choice which satisfies all the important properties of thick film substrates. The choice of substrate material is therefore often related to the electrical, thermal and mechanical demands of the application. An ideal thick film substrate should have: • Low dielectric constant • Good dimensional stability during processing • Good adhesion between substrate and printed materials • High thermal conductivity • A thermal coefficient of expansion matching other materials in the circuit • High electrical resistivity Alumina is a common choice which satisfies the majority of the above requirements in most applications however it is brittle and can snap (especially with large substrates), shaping can also often be an issue. For very high power dissipation Aluminium Nitride is often used as it has >5 times the thermal conductivity of Alumina but it is relatively expensive and requires the use of special printing pastes and processing. With the substrate material established the resistor material should be defined. Resistor materials are applied in paste form by screen printing and are then fired by heating to a high temperature. Resistor pastes have three main ingredients: • The functional elements (metal alloy or oxide particles) • Binder (glass particles) • Organic solvents and temporary binder The temporary binders give control over the printing process. They evaporate early in the firing process together with the solvents. The glass particles melt in the firing process, bind the active particles together, adhere to the substrate and provide stability to the circuit. The design of a thick film power resistor therefore requires a thorough understanding of the requirements of the application, available materials and processing technology. There is no perfect substrate or resistor material so appropriate design often requires a number of compromises. Cost is also a key consideration as materials with the best performance can come at a considerable cost.