Composition for photoimaging

According to the present invention an improved photoimagable cationically polymerizable epoxy based solder mask is provided that contains a non-brominated epoxy resin system and from about 0.1 to about 15 parts by weight per 100 parts of resin system of a cationic photoinitiator. The non-brominated epoxy-resin system has solids that are comprised of from about 10% to about 80% by weight of a polyol resin having epoxy functionality; from about 0% to about 90% by weight of a polyepoxy resin; and from about 25% to about 85% by weight of an difunctional epoxy resin. Since the photosensitive cationically polymerizable epoxy based solder mask does not contain bromine it is particularly advantageous halogens in waste processing chemicals or in incinerated scrap circuit boards are regulated by environmental concerns. The photosensitive cationically polymerizable non-brominated epoxy based solder mask has a glass transition temperature greater than about 100 C. preferably greater than about 110 C. The solder mask dries to a tack-free film; thus artwork used in the photoimaging process will not stick to the dried soldermask film. The polyol resin which is a condensation product of epichlorohydrin and bisphenol A has a weight average molecular weight of between about 40000 and 130000. The polyepoxy resin is an epoxidized multi-functional bisphenol A formaldehyde novolak resin having a weight average molecular weight of 4000 to 10000. The epoxidized diglycidyl ether of bisphenol A has two epoxide groups per molecule a melting point of between about 80 C. and about 110 C. and a weight average molecular weight of between about 600 and 2500. The invention also relates to a cationically polymerized solder mask.