Enhancement of Fine Line Print Resolution due to Coating of Screen Fabrics
In the LTCC technology, printing is the most reliable and cost effective process for line deposition on ceramic layers. An enhancement of resolution is required to realize modules in the highest frequency range which requires small lines and distances. The fine line print resolution is limited by two factors of processing. One is paste rheology itself including viscosity particle size and thixotropic behaviour. The other factor is the screen with limitations in mesh size, wire thickness, calendring, and angle of the screen fabric in the frame. From the viewpoint of the screen the idea was to improve the fabric by coating. It can be shown that especially in concentric circles even with fine line screens, arrays occur, where no printing happens. These paste free areas exhibit a moiré effect and follow exactly the string angle of the screen. This is caused by the crossing of the strings of the fabric which does not allow a passing of the paste due to very narrow gaps. To f acilitate the paste passing, the idea is to coat the fabric with a "hydrophobic" surface, so that the adhesion of the paste on the fabric wires becomes drastically reduced and therefore enhance the detachment. This coating is only appropriate for the bottom side of the screen and the spaces between the meshes. On the contrary the thixotropic effect of the paste requires a good adhesion to achieve the necessary shear effect. Therefore, the topside of the screen should be coated "hydrophilic" to provide a significant paste adhesion to the screen which is necessary for this shear stress. This development is supported by numeric simulation to predict the appropriate direction for these coated screens to improve the paste rheology.