How do I select an encapsulant material for an RF wire bonded assembly that minimizes RF loss?

Selecting an encapsulant material for an RF wire-bonded assembly that minimizes RF loss requires choosing a material with low dielectric constant (Dk less than 3.0), low loss tangent (Df less than 0.005 at the operating frequency), low moisture absorption, and compatibility with the wire bond process. The encapsulant (glob top, dam-and-fill, or potting compound) protects the wire bonds from mechanical damage, moisture, and contamination, but: it surrounds the wire bonds and covers the MMIC die surface, directly affecting the electromagnetic environment. Encapsulant options: silicone gel (Dk: 2.6-2.8; Df: 0.001-0.003; very soft, low stress on wire bonds; the lowest RF impact of any encapsulant; examples: Dow Corning Sylgard 527; preferred for RF applications where minimum loss is critical), silicone RTV (room-temperature vulcanizing silicone) (Dk: 2.8-3.2; Df: 0.002-0.005; slightly higher Dk/Df than gel; provides more mechanical protection; good for moderate-frequency applications), epoxy glob-top (Dk: 3.5-4.5; Df: 0.01-0.03; provides excellent mechanical protection; but: the high Dk and Df significantly increase RF loss and change wire bond impedance; not recommended for RF applications above approximately 6 GHz), and no encapsulant (hermetic package) (the ideal for RF: the wire bonds operate in a dry nitrogen or vacuum environment inside a hermetically sealed package; no dielectric loading, no additional loss; but: requires a hermetic package (expensive); used for: military, satellite, and high-performance applications). The wire bond as a transmission line: at high frequencies, the wire bond acts as a short section of transmission line. Its impedance depends on: the wire diameter, height above the substrate, and the dielectric environment. Encapsulation changes this impedance, potentially creating a mismatch at the wire bond transitions.