Underfills are materials made of epoxy that are used in different electronic assemblies to handle the gaps that exist between components and on the PCBs. Applying an underfill protects the components from vibration, thermal cycling, drop, and shock.
Website: https://www.epoxyadhesiveglue.com/best-top-10-bga-underfill-epoxy-adhesives-and-underfill-encapsulants-manufacturers-in-china/
https://www.youtube.com/@bgaunderfillepoxyadhesive/about
https://www.pinterest.com/bgaunderfillepoxyadhesive/