FEATURE STORY
R
Predictable Print Quality Using Wim Zoomer
Technical Language
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F
iner lines and spaces in printed electronics demand correspondingly higher requirements in resolution and edge sharpness from printed line work. To get finer lines and spaces, the printer must apply direct emulsion to produce the highest resolution for the best possible edge sharpness. Consequently, stencil quality has to meet more stringent requirements. The screen maker is expected to guarantee the expected stencil quality. The printer, in turn, should evaluate stencils to predict print quality of the screen concerned before it goes to press. Using an eye glass and illumination, he is able to determine the presence of pinholes and edge defects. However, there is also a way to measure the stencil’s surface smoothness to ensure a perfect screen-printed edge sharpness. Besides capillary film, many companies use a direct emulsion to coat their screens. Direct emulsions are water-based systems with a solids content of approximately 50%. After drying, the coating should
be 50% thinner compared to the previously applied wet coat. During drying, the water evaporates from the emulsion. The emulsion shrinks in the mesh openings and creates a concave shape. This concave shape prevents the creation of a good seal between the screen and the substrate. A good seal ensures a perfectly printed edge sharpness. However, during printing, besides filling the screen’s image, the ink will also flow into the concave shape. Eventually, a printed image with bulges will be the result. But do not confuse bulges with another regular defect pattern called saw tooth (Figures 1 and 2). Bulges are outside image defects, whereas a saw tooth is an inside image defect. The actual image becomes smaller. The size of the bulges depends on several factors, such as: the quality of the seal, the angle of the image on the screen, the depth of the concave and the viscosity of the ink.
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SURFACE ROUGHNESS R Z MEASUREMENT The smoother the surface, the better the seal, and consequently, the better printed edge sharpness. The smoothness of the emulsion’s surface can be measured, which acts as a measure for the predicted edge sharpness. The lower the number, the smoother the surface, and the better gasket-like seal between substrate and screen. Result: a sharper edge. The stencil’s smoothness is measured with a surface roughness measurement device expressed in R z (in microns). Several devices are available. Using a probe with very fine needle with a 2µm diameter, pull it automatically over a specified distance to determine the stencil’s surface roughness. The path of the needle is divided into five equal zones. The device records the surface profile and calculates the average surface roughness per five zones (difference between max and minimum roughness value per zone). Here is the calculation: