Composite epoxy glass-microsphere dielectric for hermetic SMA type RF connectors

The development of a composite epoxy glass-microsphere dielectric for hermetic SMA type R.F. connectors is described. The emphasis is on the selection of a moisture resistant combination of epoxy resin, curing agent, glass microspheres, and silane coupling agent that meets the electrical and physical requirements for an SMA connector up to 26.5 GHz. The material selected for evaluation in prototype connectors has a dielectric constant of 2.06, insertion loss of 1.06 dB/inch @ 26.5 GHz, and a coefficient of thermal expansion of 25 ± 2 × 10⁻⁶cm/cm/°C. Prototype connectors were fabricated by injecting the uncured dielectric into the connector barrel, then curing the material to form a hermetic seal. The connectors maintained a leak rate ≤1 ×10⁻⁷ cc He/sec. before and after exposure to MIL-STD 883, Class B environmental tests. The physical strength of the epoxy dielectric compares favorably with glass-to-metal seals.