Composition of Photo-Imageable Solder Resist Mask and Demand Properties

A recent trend of increasing circuit density and functions of PCB diversifies demands for a solder resist mask. When the solder resist mask was introduced, its demanded property was simply to prevent solder bridge on PCB. However, a photo-imageable solder resist mask, which correspond to high circuit density PCB design was developed to overcome the limitation of the solder resist mask's resolution made by silkscreen printing method. Now days, the photo-imageable solder resist mask that is contact UV exposure and alkaline develop type is a most common solder resist mask for PCB fabrication. The photo-imageable solder resist mask is used in various PCBs e. g. motherboards on PC or TV, IC package (BGA and CSP) substrates, flexible PCBs, and etc. Since these PCBs are made for specialized purposes and functions, demands for solder resist mask for these PCBs become more specific in features and performances.In this paper, basic composition of the photo-imageable solder resist mask and its curing system, features and performance to cope with environmental issues that are halogen free and lead free, UV exposure types, and difference of liquid type and dry film type solder masks are discussed.