The present invention relates to a pressure sensitive fluxing underfill composition that may be pre-applied to electronic components, such as CSP's, in order to increase the reliability of the component against mechanical stresses such as impact and bending. The composition contains an epoxy resin, a solid anhydride curing agent, and catalyst. Other materials, such as air release agents and fillers, may also be added as desired. The composition may be applied selectively to parts of the CSP, for example to the solder bumps. The composition provides sufficient tack in order to hold the electronic assembly together during the assembly process.