High resolution solder mask photopolymers for screen coating over

Abstract

High photo resolution of over 100 lines is obtained for solder masks of thicknesses greater than 0.002 in. (0.005 cm) with liquid photopolymers of special compositions deposited from mesh screens over printed wiring board traces and photoexposed with a phototool image in contact with the photopolymer. Special interrelated properties of the liquid photopolymer interacting in an explicit process of screen depositing two adjacent liquid polymer layers include freedom from volatiles, a paste-like viscosity permitting use with mesh screens of about 60 mesh to obtain a uniform thickness layer free of bubbles and mesh marks, solder mask insulator and heat resistance characteristics, an ability to photo flash through a phototool image in contact with one layer surface and the other surface exposed to air to polymerize through a significant thickness of about one-third of the layer thickness, photo response to polymerize to a depth of about 0.02 in. (0.05 cm) when exposed to about 1 joule per square cm, absence of ingredients refracting, blocking or reflecting the actinic radiation and good adherence to copper wiring. These properties react in the process of bubble-free lamination of two paste consistency layers, a first to cover uneven wiring traces and a second to provide a flat photo-image surface, thereby to achieve thicknesses of about 0.006 in. (0.015 cm) for covering wiring traces 0.004 in. (0.001 cm) thick with assurance of a covering of 0.002 in. (0.005 cm) thickness over all wiring traces. This is achieved by photo flashing the uppermost layer through a photo-image with one surface exposed to air to polymerize a surface layer that covers the traces permitting the unpolymerized liquid paste surface to meet in bubble-free surface contact with an initial layer deposited directly over the wiring traces.