Photoimageable dielectric epoxy resin system film

Photoimageable dielectrics in film form, rather than paste or liquid form, are desirable for a number of reasons. For example, liquid photoimageable dielectrics and paste photoimageable dielectrics run into and partially plug holes, such as via holes; upon development, they typically leave a residue in the hole. This leads to defective circuitized substrates and reduces yields. The film photoimageable dielectrics are also employed to cover "tent" holes, thereby eliminating hole filling operations which are expensive and inconvenient. However, for photoimageable dielectric films to remain flexible and not crack during fabrication of the film, application and processing, about a 12% solvent content is typically employed. The presence of the solvent, however, leads to additional problems. Often, such photoimageable dielectric films have a low Tg which in turn impairs dielectric properties of the photoimageable dielectric layer in the circuitized substrate. Also, such photoimageable dielectric films tend to stick to the photo-tool used in photoimaging. In an attempt to overcome these problems, such photoimageable dielectric films are baked prior to photoimaging, to drive off a portion of the solvent. However, the bake often causes the photoimageable dielectric film to become quite tacky while the photoimageable dielectric film is being heated. As a result, dust and debris from the bake oven and the ambient surroundings adheres to the surface of the photoimageable dielectric film. A method for fabricating circuitized substrates which reduces shorts, and does not require baking and resulting film. The method employs a photoimageable dielectric film, having a solvent content less than about 5%, and a glass transition temperature, when cured, which is greater than about 110° C. A photoimageable dielectric film is provided having from about 95% to about 100% solids, and comprising: from 0% to about 30% of the solids, of a particulate rheology modifier; from about 70% to about 100% of the solids of an epoxy resin system (liquid at 20° C.) comprising: from about 85% to about 99.9% epoxy resins; and from about 0.1 to 15 parts of the total resin weight, a cationic photoinitiator; from 0% to about 5% solvent; applying the photoimageable dielectric film to a circuitized substrate; and exposing the film to actinic radiation.