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Thick Film Technology
Screen Printing
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Thick Film Circuit Design
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Felületi szereléstechnológia (in Hungarian)
Task Thick Film Circuit Design

Task description:

The aim of this task is to learn the basic design rules of thick film hybrid circuits, especially of thick film integrated resistors.

 

The technology of thick-film substrates for passive networks and hybrid circuits is based on an additive process: the various layers and their patterns are formed by screen printing in the same step. Screen printing is followed by leveling, drying and firing/burning. The structure of the layer and all of its essential properties are formed during firing. Both the pattern and the properties of the thick film integrated components are influenced by the laser trimming of resistors.

 

When designing a thick film integrated circuit, in particular its integrated resistors, in addition to determine the layout to meet the electrical requirements of the circuit, it should be taken into consideration that the layout will be realized by using the screen printing and laser trimming processes.

 

Subsequent processing steps, which should be considered as well, are as follows: assembling, soldering, encapsulation and the final mounting of the circuit module to an electronic system.

 

Contents:

  • Design rules and shapes of integrated thick film resistors;
  • Design of the resistor elements;
  • The most common shapes of integrated film resistors and the recommended l/w aspect ratio values;
  • Typical laser cutting shapes for thick film resistor trimming

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. While the various layers and their patterns are formed by screen printing, the firing process creates the layer structure and - together with the materials characteristics - determines the properties of the thick film components.

 

The substrate is generally made of alumina, or other ceramics. These materials have quite good heat conduction, which mainly determines the admissible specific power dissipation of the resistors and the whole circuit.

 

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 main goal of the design process is to select the suitable pastes for the elements of the circuit and to determine the layout of the integrated elements and the interconnection layers to meet the electrical requirements of the circuit.

 

The technology of the integrated thick film substrate is completed by laser trimming of the integrated resistors, and by the print-and-fire process of an overglaze for solder resist. Further important processing steps of thick film technology include solder paste printing, soldering leads to the interconnection patterns and encapsulation. Therefore the laser trimming routes and strategy, as well as, the layout of the solder resist, the solder paste layers and the whole circuit package should also be designed.

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