What are the advantages of thick film resistors over their thin film cousins? In this post, we consider the ideal resistor and discuss how thick film and thin film resistors compare with that ideal.
The Ideal Resistor
The perfect resistor has zero capacitance and inductance. Its resistance value is precisely known and does not vary over its entire in service life regardless of any external environmental factors such as moisture or temperature.
The ideal resistor is robust and its resistance value will not vary if it is subjected to external stresses including over voltage and surge events, ESD and mechanical stresses. It has a high resistance per unit area.
Thick Film Resistor construction
A resistive paste consisting of a mixture of metal oxides, a carrier and a binder is deposited on a flat (usually Alumina) substrate. Resistive layers are added sequentially to create the required resistance pattern and value. Abrading of the resistance pattern delivers the final resistance value.
The carrier is based on organic solvents and holds the resistor pattern in paste during processing. The glassy frit binds the resistor material in place post firing and provides protection from external contaminants. The thick film resistor material is a granular film of metal oxides.
The resistive pattern is fired onto a substrate at high temperature (typically 850°C). The carrier material burns off, the metal oxides combine to form the resistor film and the glassy frit melts to hold the resistor material in place.
Thin Film Resistor construction
Thin film resistor technology uses a deposition rather than a printing process onto an Alumina, Silicon or GaAs substrate. A multi step sputtering process is used to add mechanical and electrical foundation layers followed by a bulk conductor layer of copper. The resistor layer is typical 1000x thinner than used in thick film. A photo-lithographic process is used to form both a conductor and resistor pattern.
Key Performance Elements
Although both thin film and thick film resistors use a resistive layer on a ceramic their performance is completely different. Using the ideal resistor definition above as a basis for comparison
Capacitance and inductance. Thin film features lower parasitic inductance and capacitance and is therefore generally more suited to very high frequency applications than thick film resistors.
Resistor value: Thin film resistors tend to be available with lower resistor values than thick film.
Resistor tolerance: Thick film resistor tolerances tend to be 10x higher than those available from Thin Film.
Stability: Thin film resistors have higher long term stability and lower noise than thick film.
Noise: The structure of the resistive material generally means thin film resistors are preferred in low noise applications. However, the use of specific materials and processing means thick film resistors can also be used in low noise applications, including audio.
External stresses: Thick film resistors are significantly more robust than thin film. Surge and ESD event survivability of thin film technology is poor.
Power handling: Due to their construction Thin film resistors are generally not used in power applications.
Cost: Thin film resistors are more expensive than thick film devices.
Thick Film vs Thin Film Conclusion
Primarily due to their power handling capabilities, resistance to external stresses and lower cost thick film resistors are used in a wide variety of applications. They tend to be used in power applications which are beyond the capabilities of thin film resistors. In contrast thin film resistors tend to be used in precision and high frequency applications.