What Are Thick Film Devices?

Thick film devices are passive and active electronic components fabricated by depositing functional pastes onto a substrate through a screen-printing or stencil-printing process, then firing the deposited layers at elevated temperatures to sinter the material and bond it to the substrate. Film thicknesses typically range from 10 to 50 micrometers, distinguishing thick film technology from thin film processes that deposit layers of nanometer to submicrometer thickness using vacuum techniques. Thick film technology has been a production-proven platform for hybrid microelectronics since the 1960s, offering a versatile, low-cost route to integrating resistors, conductors, dielectrics, and inductors onto ceramic or other substrates in compact assemblies.

The primary substrate material is alumina ceramic, valued for its high thermal conductivity relative to organic circuit boards, good dimensional stability at firing temperatures, and chemical compatibility with the paste materials. Other substrates including beryllia, aluminum nitride, and low-temperature co-fired ceramics (LTCC) are chosen for applications requiring higher thermal performance or three-dimensional multilayer interconnect.

Screen-Printed Circuits and Thick Film Conductors

Screen printing transfers a paste through a fine-mesh screen onto the substrate surface. The screen is patterned by blocking mesh openings in areas that should not receive paste, so that a squeegee forces paste through only the open regions. Conductor pastes based on silver, gold, silver-palladium, or copper are printed to form interconnect traces, bonding pads, and via fills. After printing, the substrate is dried to remove solvents, then fired at temperatures between 500°C and 900°C for standard thick film systems, or below 900°C for LTCC. Research on multilayer LTCC interconnect technology published through IEEE Transactions on Advanced Packaging documents advances in conductor paste formulations and co-firing process control.

Silver-palladium conductors provide a good balance of conductivity, adhesion, and compatibility with air firing atmospheres. Copper conductors offer lower resistivity but require controlled-atmosphere firing to prevent oxidation, adding process complexity. Conductor sheet resistance, typically 2 to 50 milliohms per square depending on paste and fired thickness, determines trace resistance for a given geometry.

Thick Film Resistors

Thick film resistors are formed from pastes containing ruthenium dioxide or other conductive oxide particles dispersed in a glass matrix. After firing, the glass encapsulates the conductive particles and bonds the resistor to the substrate. Resistance values are set by the paste formulation (sheet resistance), the aspect ratio of the printed pattern, and post-firing laser trimming. Trimming cuts grooves in the resistor body to increase resistance, enabling final values to be set to tolerances of 0.1 percent or better. Thick film resistors cover a wide range from a few ohms to many megaohms and are a standard building block in hybrid circuits for automotive electronics, telecommunications, and instrumentation. NIST Standard Reference Materials for resistor calibration support the measurement traceability that production testing requires.

Thick Film Inductors

Thick film inductors are planar spiral coils printed on the substrate surface, sometimes with multiple layers separated by printed dielectric to increase inductance. Their self-resonant frequencies and quality factors depend on the conductor geometry, substrate permittivity, and interlayer capacitance. They are most practical at frequencies above roughly 100 MHz where small physical inductance values are needed and where the relatively high series resistance of printed conductors is acceptable for filter and matching network applications. Multilayer LTCC processes allow embedded inductors within a sintered ceramic block, enabling compact RF modules. Proceedings of the International Symposium on Microelectronics document many advances in thick film passive integration for RF and power applications.

Applications

• Hybrid microelectronic circuits for automotive engine control units and transmission controllers
• Resistor networks and voltage dividers in precision instrumentation and medical devices
• RF modules using LTCC multilayer technology for wireless communication front ends
• Heater elements for sensor hot plates and fluid-heating assemblies
• Substrates for high-power LED modules requiring thermal management capability
• Sensor elements including piezoresistive pressure sensors and thermistor arrays on ceramic