Hierarchically packed liquid epoxy composites with ultralow CTE and viscosity for high-performance wafer-level packaging

Abstract

Highly filled liquid epoxy molding compounds (LMCs) are pivotal for advanced electronic packaging; however, simultaneously achieving high processability, mechanical reliability, and dimensional stability remains a formidable challenge. We report a synergistic design strategy that overcomes these inherent trade-offs by integrating matrix optimization, multiscale hierarchical packing, and interfacial engineering. By utilizing a ternary epoxy system, an ultra-high filler loading of 87 wt% (78.5 vol%) was achieved using bimodal fused silica, forming a dense and rigid framework. The strategic localization of core–shell rubber (CSR) nanoparticles within silica interstices creates a trimodal architecture that alleviates stress concentration without compromising the load-bearing network. This nanoscale confinement enhances toughness and T_g while suppressing the coefficient of thermal expansion (CTE), a combination rarely achieved simultaneously. To address the rheological jamming (>10⁴ Pa s) typically induced by such dense packing, epoxy-based surface functionalization was introduced to disrupt hydrogen-bond-mediated filler networks. This transformed the system into a highly flowable suspension with an exceptionally low viscosity (∼250 Pa s) at such high solid loadings, by establishing a lubricated filler–matrix interface. The resulting LMC exhibits an unprecedentedly low CTE of 8.20 ppm °C⁻¹, high T_g (∼170 °C), and superior flexural strength (∼150 MPa). This hierarchical–interfacial approach enables uniform, low-warpage wafer-level molding with robust hydrothermal reliability, providing a scalable blueprint for next-generation high-performance thermoset composites.