Study on the curing process and the gelation of epoxy/anhydride system for no-flow underfill for flip-chip applications

No-flow underfill is used in the assembly of microelectronics to increase the productivity and to decrease the cost of the flip chip manufacturing. The curing process, especially the gelation of the no-flow underfill, is essential for the yield and reliability of the flip-chip assembly. A different scanning calorimeter (DSC) and a stress rheometer are used to study the curing process of epoxy/anhydride system at different curing rates and different isothermal temperatures. The gel point is found when the storage modulus and the loss modulus of the resin measured by the rheometer equals to each other. The degree of cure (DOC) at gelation is calculated according to the results from DSC. The results indicate a strong dependence of the DOC at gelation on the heating rates and the curing temperatures. In order to further investigate the difference in the curing process at various heating rates. FTIR spectra of the resin are taken during curing. The change of different functional groups is recorded and compared. The results do not show a significant difference in the chemical structure at different heating rates. The early gelation at high heating rate/high temperature can be caused by the structure difference in the epoxy network at the early stage curing due to the chain initiation and propagation of the molecules in the curing process.