Comparison of Dry Film and Liquid Photo‐imageable Solder Masks for Surface‐mount Assemblies

The need for high packing density has led to a requirement for photo‐imageable solder masks, and the increase in surface mount technology is rapidly expanding the use of these systems into consumer electronics in addition to the traditional high‐technology outlets. This paper compares the properties and performance of the two broad types of photo‐imageable systems—dry film solder masks and liquid systems. The manufacturing and coating requirements are outlined. Dry film solder masks are supplied by major chemical companies as high‐quality pre‐formed films while producing coatings from liquid systems is the responsibility of the printed circuit board producer. The process for producing solder masked boards from the two systems is outlined. The relative performance of the systems is compared in detail. Resolution, encapsulation, topography and reliability are considered and these are discussed as well as the requirements for fluxing and defluxing and formation of tented via holes, a highly desirable feature for SMT. Finally, the attractiveness of the more established dry film technology for surface mount boards is summarised, with a request that designers consider the features available and build these into their designs for new circuit boards. Provided the appropriate application system is used and the thickness of dry film solder mask is matched to the requirement, cost advantages can be derived from their use.