The Basics of Screen Printing Thick Film Inks
Thick film inks are normally deposited on 96% Al2O3 ceramic substrates by a screen printing process. Understanding the basics of this method of deposition will ensure high yields and a resultant quality product.
The ink consists of four distinct groups of intermediates, which are thoroughly mixed and blended, yielding a homogeneous product:
- Functional Phase consists of metal powders (Pt,Pd,Ag,Au, etc.) in conductive inks, metals and/or metal oxides (RuO2, Bi2 Ru2O7, Pd, Ag) in resistors and ceramic/glass (BaTiO3, glass) in dielectric temperature firing.
- Binder Phase to hold the ink to the ceramic substrate, and merges with the ceramic during high temperature firing.
- Vehicle acts as the carrier for the powders and is composed of both volatile (solvents) and non-volatile (polymers) organics. These evaporate and burn off during the early stages of drying and firing, respectively.
- Modifiers are small amounts of proprietary additives which control behavior of the DuPont inks before and after processing.
Screen printing requires the ink viscosity to be controlled within limits determined by other rheological properties, such as the amount of inorganic powders in the ink. Ambient temperature will also influence the viscosity of the ink, altering a Spring® vehicle by as much as 4% per degree Celsius. Therefore, it is recommended the print room maintain a constant temperature of 72 +/-3° Fahrenheit. This will enable high quality prints to be made with squeegee pads as high as 8 inches per second.
The printing screen is prepared by stretching stainless steel wire mesh cloth across the screen frame and attaching it, maintaining high tension of the mesh. An organic emulsion is then spread over the entire mesh, filling all open areas. A common practice is to add 0.5 mil additional emulsion on the mesh. The area to be screen printed is then patterned on the screen. These screens can be purchased from your supplier already patterned, or you may choose to do it yourself.
As the squeegee moves the ink across the screen, a shearing action causes a decrease in viscosity, allowing the ink to pass through the patterned areas, onto the substrate. As the squeegee passes, the screen peels away and the ink viscosity recovers, leaving a well defined print.
|screen size (inches)||suggested snap off distance (mils)|
|5 x 5||20 - 30|
|8 x 10||30 - 40|
|12 x 12||40 - 60|
The lower limit should be used with a new screen, increasing the snap off distance slowly, as the screen mesh ages.
|screen mesh||wire diameter (mils)||typical dry print thickness (10-6)|
From these data, it can be seen the screen mesh is critical, when determining the desired thick film print thickness. In fact, when printing, it is the single largest change you can make in your parameters.
The downstop should be set, allowing the squeegee's travellimit to be 5-7 mils below the substrate surface. This will allow consistent print thickness across the substrate, while protecting the screen mesh from undue excessive pressure, which would cause stretching of the screen mesh.
Screen Size, Squeegee Length, Squeegee Travel
- Screen width should be 2-3 times the length of the squeegee, length should allow 5-8 cm distance both before and after the squeegee travel.
- Squeegee length set at 1-2 cm excess beyond the pattern area, on each end.
- Squeegee should drop down at least 3 cm prior to pattern area, and not lift until at least 3 cm past it.
- Special attention should be paid to these hints:
- Avoid excessive wiping of the screen, as this can wear the emulsion away
- Never apply solvent to the screen when ink is present
- Maintain the cleanest area possible
- Do not let ink dry on the screen, as this may cause voids in the prints
- Do not print with excessive ink on the screen, because it will dry out