Printed thick film resistors allow resistance elements to be integrated directly onto ceramic or other stable substrates. Good resistor design requires coordination between target value, paste sheet resistance, geometry, trimming strategy, power density, and reliability conditions.
Sheet resistance and geometry
The starting design uses the paste sheet resistance and the number of squares defined by resistor length and width. Long narrow shapes increase resistance but may increase sensitivity to process variation and trimming damage. Short wide shapes can carry more power but need enough adjustment range.
Terminations
Resistor-to-conductor overlap and termination design influence contact resistance, stability, hot spots, and trimming behavior. Material compatibility between conductor and resistor paste is important.
TCR and tolerance
Temperature coefficient of resistance matters when the circuit works across a wide temperature range. Tolerance is affected by paste, print thickness, firing profile, resistor geometry, and laser trimming accuracy.
Laser trimming allowance
The layout must leave enough area for the trim path. The trim should raise resistance to the target without creating excessive current crowding, heat concentration, or drift. For high stability applications, the trim design should be verified after thermal cycling and aging.
Power density
Power creates local temperature rise. Thermal path, resistor area, substrate material, conductor heat spreading, and package design all affect safe operating limits. For sensor circuits, power may also affect signal linearity.
Design checklist
- Target resistance and tolerance.
- Operating temperature range and TCR requirement.
- Maximum voltage, current, and power.
- Trim method and allowed drift.
- Substrate material and heat path.
- Environmental exposure: humidity, vibration, fuel, oil, or cleaning agents.