Underfill composite kind of materials is created using epoxy polymer and filler. The other things added to the underfill formulation are dyes, adhesion promoters, and flow agents. Primarily, the underfills are used for flip-chip kinds of devices, but they can also be used in other areas. This includes ball grid arrays, CSPs and BGAs. https://www.deepmaterialcn.com/best-top-10-bga-underfill-epoxy-chip-adhesives-glue-manufacturers-in-china.html