Variables affecting uniformity in the screen process printing of printed and fired-on films and the development of a squeegee design for improving uniformity
INTRODUCTION THE general use of screen process printing in the formation of "Printed-and-Fired" or "Thick" films in microcircuitry is described as a new application of the old graphic arts method of printing. As such, this new application requires more understanding of the process. Discussion is limited to the area of off-contact printing of both resistive and pure glazes on ceramic substrates to form hybrid, resistor or resistor-capacitor networks and conductive paths. The assumption is stated that microcircuit applications differ from other forms of screen process printing mainly because resulting components or circuits are extremely sensitive to the thickness of the resultant printed film. Basic areas in process and materials used are pointed out as of primary importance in controlling the printed film thickness. Those areas highlighted in the printing process are (1) the squeegee, (2) the screen or mask and (3) the machine or equipment used. Areas added by the basic materials are (4) the ink or composition to be printed and (5) the substrate. I tem (6), post treatment, is considered the final area of major concern. These six general areas are broken down into forty-seven specific variables which might affect uniformity of results. Since the "squeegee" design and its controlled application can compensate for some of the other variables, this area is assumed of major importance. A theory is stated and discussed in detail as to why a squeegee causes printing to occur. This theory is that: "The resultant force from the speed of squeegee travel and pressure on the squeegee, combined and directed by the angle of attack on the squeegee blade, creates a hydraulic pressure in the composition being printed which causes openings in a screen mesh to be filled with composition under pressure, and as contact occurs, this fill is transferred to the substrate." I t is then concluded that any factor which would affect or vary the hydraulic pressure which causes printing must affect the deposited film thickness. Sketches of various basic designs of squeegees used in screen process printing are shown and discussed and then a new squeegee design is shown and arguments are made as to why it will have superior performance in laying down uniform films. The design basically is a broad 45 ° included angle blade formed by the edge of a ~ in. square cross-section, and mounted so as to hold it as rigid as possible during use.