TECHNICAL BRIEF Polymer Thick Film Resistors: Application and Design Criteria

Polymer thick film (PTF) resistors have been effectively used for many years in consumer goods like timers, motor controls, computers and a wide variety of high performance equipment. The potential for this technology has reached only a small portion of the market; its full potential is still to be determined. The cost savings relationship of printed resistors should be considered by factoring the design and the number of resistors printed. Printed resistor savings vs discrete component costs can be a factor of three. This is dependent on the number of printed resistors per board of the same sheet resistance (based on 400 printed resistors of the same sheet resistant values on a 18 × 24 panel vs discrete components). By minimising the printing operations, resistor costs will be effectively controlled. Resistive ranges of thick film material are from 1 ohm to 10 Mega Ohm.The printing quality of resistors may vary from 10‐20%; however, resistors may be trimmed to 1% in value. Many values of resistors may be achieved with the same decade of resistor pastes. The determining factor is the design and layout of the circuit. The power rating of the printed resistor is proportional to the geometric surface area and is highly dependent on the substrate selection. The ability for resistors to be printed and buried into the multilayer will give design engineers more flexibility in active circuit density. Up to a 30% increase in surface area may be realised if resistors are buried into the inner layer of the PCB or under other surface components.