Fluorene‐based active ester significantly improved high‐frequency dielectric performance of <scp>SiO₂</scp>/epoxy composites

Abstract High‐performance epoxy composites are critical dielectrics used in advanced electronic packaging. A low dielectric constant ( D k ) and an ultralow dielectric loss tangent ( D f ) at high frequencies are highly desired for fast and lossless transition signals in devices. Active ester, a new type of curing agent for epoxy composites, can achieve low D k and D f . However, the thermal performance notably decreases compared to traditional amine and phenolic curing agents. Herein, to strengthen the thermal performance, a new type of active ester (BPFAE) constructed with a fluorene rigid group was successfully synthesized via a one‐step esterification reaction between bisphenol fluorene and p‐toluoyl chloride lasting for 5 hours. The cured SiO 2 ‐filled epoxy composites perform excellently in thermal performance ( T g ~ 149°C, T d5% ~ 384°C, and α 1 ~ 21 ppm·K −1 ) and ultralow D k (~2.4) and D f (~0.0037) at 20 GHz. After inundating in water for 24 hours and in boiled water for 1 hour, the composites still achieved an ultralow D k of about 2.45 and an ultralow D f of about 0.004. These results highlight the significant potential of BPFAE‐cured epoxy composites for high‐frequency, high‐performance electronic packaging applications. Highlights A Fluorene‐based active ester was synthesized via an esterification reaction. The BPFAE curing system performs high reactivity with a T p of 135°C. A high T g of 149°C and a low CTE of 21 ppm·K −1 were obtained. Excellent dielectric properties of D k ~ 2.4 and D f ~ 0.0037 were achieved at 20 GHz. Outstanding dielectric stability under humidity conditions was obtained.