Underfill and preparing method thereof

The invention discloses an underfill, comprising, by weight, 30 to 70 parts of filler, 10 to 50 parts of epoxy resin, 2 to 20 parts of curing agent, 0.1 to 0.5 part of catalyst, 1 to 15 parts of flexibilizer, 1 to 25 parts of diluent, 0.1 to 0.3 part of dispersant, 0.05 to 1 part of defoamer, 0.1 to 1 part of coupler and 0.1 to 0.5 part of pigment. The underfill comprises, by weight, 30 to 70 parts of the filler; the filler is in a core-shell structure, a core is made of SiO2 having low expansion coefficient, and a shell is made of at least one of Al2O3, AlN, BN, SiC and Si3N4, having thermal conductivity; the underfill comprising the filler is high in thermal conductivity and helps transmit and dissipate heat in a packaging structure. Compared with the traditional underfill, the underfill is lower in thermal expansion coefficient and better in thermal conductivity.