:: IPCI ::

The aim of this task is to learn the screen printing process, which is the principal step of thick film technology for layer deposition and patterning.

The screen printing process is followed by settling, drying and firing. While screen printing determines the pattern of the thick film layer, firing creates the structure of the layer and - as a consequence - determines the properties of the thick film components.

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

Contents:

- Screen printing: the characteristic layer deposition and patterning process of thick film circuits

- Screen mask: the patterning tool tool for screen printing

- Types of screen masks:

1.) emulsion masks and

2.) metal masks

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 temperature between 500 and 1000 deg C.

Screen printing is the basic layer deposition and patterning process in thick film technology, while the following firing process creates the layer structure and determines the properties of the thick film components.

The substrate is generally made of alumina, beryllia or aluminum-nitride (AlN). Glazed metallic substrates are also in use for special high power applications.

The circuits are produced using conductive, resistive and/or dielectric pastes (or inks), which characteristics are mainly determined by their composition. 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.