Photo‐imageable Screen Printed Soldermasks

Over the last few years, the increase in size of printed circuit boards, together with the increase in density of components, has made successful application of soldermask by conventional screen printing more and more difficult. This is despite improvements in both the screen printing resists and the equipment used. The accuracy produced by the photo‐imaging technique has been firmly established with the now almost universal use of dry‐film resist for the plating and etching of printed circuit boards. This led to the introduction of the dry‐film soldermask, but, unlike the dry‐film resists used for plating and etching, this type of soldermask has failed to gain universal acceptance, mainly because of technical shortcomings and high costs. The application of liquid soldermasks overcomes many of these technical problems, although the first attempts to achieve acceptable results required special equipment and huge capital investment. Photo‐imageable soldermasks which can be applied using the conventional printing and exposure equipment, available at printed circuit manufacturers, are now available. Some are processed in aqueous solutions, whilst others are processed in halogenated solvents of the types used in the processing of dry‐film resists. The introduction of such soldermasks makes available the combined advantage of liquid application and photo‐imaging, which will increase the overall quality of printed circuit boards produced, while utilising existing screen printed, oven, photo‐exposure unit and conveyorised spray developer.