Synergistic Regulation of Thermal Expansion and Conduction in Epoxy Composites via (Ca,Sr)SnF₆ Ceramic Filler

Low thermal expansion and high thermal conductivity are essential for advanced integrated circuit packaging materials. In this study, an isotropic negative thermal expansion fluoride, Ca_0.85Sr_0.15SnF₆ (CSSF), exhibiting a linear coefficient of thermal expansion, α_l of −7.3 × 10⁻⁶/°C, is synthesized via a hydrothermal method. TEM results capture the diverse lattice distortion in CSSF, including attractive Moiré fringe, dislocations, and obvious lattice strain. Subsequently, CSSF is introduced into epoxy resin (EP) to design the unconventional CSSF/EP composites. Compared with the pure epoxy, the composite with 60 wt.% CSSF filler exhibits a significant reduction in α_l, from 69.5 × 10⁻⁶/°C (glassy region) and 172.0 × 10⁻⁶/°C (rubbery region), down to 35.8 × 10⁻⁶/°C and 87.4 × 10⁻⁶/°C, respectively. Meanwhile, the thermal conductivity increases from 0.19 to 0.48 W·m⁻¹·°C⁻¹. Moreover, a higher Shore hardness (93.3 HD) is achieved for the CSSF60/Epoxy composite. Furthermore, the composite demonstrates desirable dielectric properties, with a dielectric constant of 9.48 and a dielectric loss of 0.166 at 1 MHz. In particular, fracture surface analysis reveals that CSSF filler achieves strong interfacial adhesion with the epoxy matrix without surface treatment, which can be attributed to its isotropic nature. This unconventional strategy of incorporating NTE fluorides into EP presents a viable approach for advanced electronic packaging applications.