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Thick Film Technology
Screen Printing
Firing of Thick Films
    Tutorials (1)
Fabrication Steps of Thick Fim Circuits
Thick Film Circuit Design
Felületi szereléstechnológia (in Hungarian)
Task Firing of Thick Films

Task description:

The aim of this task is to learn the basic properties of thick film materials and the principle of the firing process, which fabrication step creates the layer structure and - as a consequence, together with the materials characteristics - defines the properties of the thick film components.

 

The firing process is preceded by the screen printing of the layer. Screen printing determines the pattern of the thick film layer, and firing creates its final materials structure.

 

Since a thick film circuit is usually a multilayer structure, its fabrication needs several printing and firing processes, one after the other.

 

Contents:

  • Thick film materials: substrates; conductive, resistive and dielectric layers
  • The firing furnace
  • The firing process: the temperature profile, and its drying, firing and annealing phases

Introduction:

The principal steps of thick film technology are screen printing of pastes onto ceramic substrates and - after leveling and drying - firing them at a typical temperature of 850 deg C, or between 500 and 1000 deg C, in a conveyor type furnace. The firing process creates the layer structure and - together with the materials characteristics - determines the properties of the thick film components.

 

Prior to the firing process, paste is screen printed to the substrate, which is generally made of alumina, beryllia or aluminum-nitride (AlN). Glazed metallic substrates are also in use for special high power applications.

 

The circuit layers are produced using conductive, resistive and/or dielectric pastes (or inks), which characteristics are mainly determined by their composition. A wide variety of pastes is available to suit different application and requirements. They are classified in accordance with their functions into three groups: conductor, resistor and dielectric pastes. The pastes are colloidal systems that become ceramic like composite layers as the result of high temperature firing.

 

At the beginning the first conductive layer, generally using a Ag/Pd paste, is printed, dried at 150 deg C and fired in a belt furnace. Then follows the printing and drying of the crossover and other dielectric layers. Two printings are required for dielectrics in order to ensure good insulation from the conductive layers. After firing the insulating layer, the second conductive layer is printed and fired. The different resistor pastes are then printed and dried. All the resistor pastes are fired together. All firing is done at 850 deg C in a belt furnace, using a very accurate temperature vs. time characteristic. The last step of the print-and-fire process is the printing of the overglaze and its firing at 500 deg C.

 

Further important processing steps of thick film technology include laser trimming of the integrated resistors, soldering leads to the interconnection patterns to prepare connections of the module to the other parts of the system, encapsulation, testing and marking.
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