Synthesis of Chemically Modified Lignin-Epoxy Composites with Enhanced Interfacial Adhesion for Underfill

The inherent high cross-linking network of epoxy underfill (UF) makes it inherently brittle, and the density of surface active functional groups is insufficient. To address this, we report a high-performance lignin-epoxy composite UF using chemically demethylated lignin (DL) synthesized via a LiBr/HBr system. Demethylation reduced methoxy content by 63.7% while increasing phenolic hydroxyl groups by 54.1%, significantly enhancing lignin-epoxy compatibility. The optimized composite with 4.14 wt% DL (LEP3) achieved an interfacial adhesion strength of 27.20 MPa, representing a 104.2% improvement over the unmodified system (LEP0, 13.32 MPa), with 60.08% retention post-aging. Fracture toughness (KIc) increased by 11.00% due to hydrogen bonding and covalent crosslinking between DL and the epoxy matrix. The strategy replaces part of petroleum-derived epoxy with renewable lignin while meeting the needs of the 5G chip packaging industry.