A novel photosensitive low viscosity epoxy resin was synthesized by polyethylene glycol (PEG)-modified bisphenol-A epoxy resin (E51). The resin was modified by ethylene glycol, diethylene glycol, and different molecule weights (200,300,400) PEGs to optimize the minimum viscosity. FTIR was used to determine molecule structure. Cationic photoinitiator (UVI-6976) mixed with modified resin (10 wt %), was utilized to boost the resin curing under UV light. The curing degree was beyond 90% within 40 s and the whole process was monitored by photo-DSC. The modified resin diluted with ethylene glycol diglycidyl ether, was screen printed onto polyimide and polyethylene terephthalate substrate, and the properties of solder mask were up to China printed circuit association standard. The solder resist also meet all requirements under ink-jet printing technology as the viscosity is under 60 mPa center dot s and the curing duration is <1 min. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013