High resolution liquid photopolymer coating patterns over irregular

Abstract

High resolution photoimaging with liquid photopolymers of pastelike consistency overlying irregular surface topography such as provided by conductors extending from the surface of a printed wiring board substrate is obtained without air entrapment or starvation of the superimposed liquid polymer. Thus, for example, solder mask coatings of high resolution may be prepared. This is achieved in two photosteps, the first of which is a partial polymerization obtained by radiating with the polymer surface exposed to air, thereby polymerizing the layer portion unexposed to air adjacent the substrate surface and leaving the air exposed surface in the liquid state. This step uses an image with a greater dimension of opaque pattern area than that used in the subsequent step, which obtains high resolution by surface contact of the image with the polymer. The radiation source in each case is non-collimated. The resulting product because of the partial polymerization covering the surface produces no starvation over the rough surface because of pressing the second image into surface contact. The liquid polymer layer covers rough surfaces without entrapment of air. By radiating when the surface is exposed to air, noise causing poor resolution is eliminated, such as from surface interface reflections and pinholes in opaque surfaces, etc. A layer of liquid photopolymer is disposed upon its permanent substrate cured through only part of its thickness adjacent the substrate to leave a liquid tacky outer surface.