Phosphorus-containing liquid cycloaliphatic epoxy resins for reworkable environment-friendly electronic packaging materials

Novel thermally reworkable, phosphorus-containing, di- and tri-functional liquid cycloaliphatic epoxy resins were designed and synthesized. Their chemical structures were characterized by means of MS, FTIR, 1H NMR and 31P NMR methods. After curing, the products were transparent and stable up to 220 °C, while exhibited quick thermal decomposition at the temperature range of 255–280 °C. The removal test showed that, after heat-treatment at 260 °C in air atmosphere for only 4 min, the residual char on the glass substrate could be easily wiped off. This unique degradation behavior was attributed to the synergistic effect of two factors: the thermally-labile phosphate groups evenly distributed within the three-dimensional network and the in-situ catalyzing of phosphoric acid generated from the cleavage of phosphate bond on the pyrolysis of adjacent other phosphate and ester groups, as evidenced by the results of molecular modeling, isothermal TGA and FTIR spectra. In addition, compared to the commercial cycloaliphatic epoxy resin ERL-4221, the newly synthesized epoxy resins had increased limiting oxygen index (LOI) by 31%. The combination of excellent reworkability, non-halogen flame retardancy, high glass transition temperature of 227 °C and high mechanical modulus endows them the potential for environment-friendly microelectronic and optoelectronic packaging applications.