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

Task description:

The aim of this task is to learn the basic fabrication processes of thick film hybrid circuits, going through the technology step by step.

 

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. When two subsequent printed layers are not overlapping, their firing can be done in the same step.

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

 

Further important processing steps of the production of thick-film hybrid IC-s are as follows: trimming, mounting, encapsulation and packaging to an electronic circuit module or system.

 

Contents:

1. Thick film circuit fabrication step by step:

  • Screen printing and firing a conductor layer onto a ceramic substrate;
  • Screen printing and drying various resistor layers and firing them in a single step;
  • Laser trimming of resistors;
  • Solder mask printing and firing;
  • Solder paste printing;
  • Mounting components;
  • Reflow soldering.

2. Laser trimming of resistors:

  • Trimming technologies, laser trimming of thick film resistors;
  • Schematic structure of a laser trimming system;
  • Laser system for trimming and marking.

3. Main parameters of cermet and polymer thick-film resistors.

 

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. For special high power applications glazed metallic substrates are also in use.

 

Screen printing is followed by leveling and drying prior to firing. When two subsequent printed and dried layers are not overlapping, their firing can be done in the same step.

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. When no dielectric layer is used, 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 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 to prepare connections of the module to the other parts of the system, encapsulation, testing and marking.

 
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