How do I select conformal coating for an RF PCB assembly in a harsh environment?

Conformal coating is a thin protective layer applied over assembled PCBs to protect against moisture, salt spray, contamination, and fungal growth. For RF circuits, the coating must provide environmental protection without significantly affecting electrical performance: (1) Coating types: acrylic (AR): thickness: 25-75 μm. Dk: 2.5-3.5. Df: 0.02-0.05. Advantages: easy to apply and rework (soluble in solvents). Good moisture resistance. Low cost. Disadvantages: limited temperature range (-65 to +125°C). Moderate chemical resistance. Silicone (SR): thickness: 50-200 μm. Dk: 2.5-3.0. Df: 0.01-0.03. Advantages: widest temperature range (-65 to +200°C). Excellent flexibility (accommodates thermal cycling). Low stress on components. Disadvantages: difficult to rework (must be physically peeled or abraded). Soft (can be easily damaged by handling). Polyurethane (UR): thickness: 25-75 μm. Dk: 3.0-4.0. Df: 0.02-0.04. Advantages: excellent chemical and solvent resistance. Good abrasion resistance (hard coating). Disadvantages: high Dk relative to other coatings (may affect impedance at mmWave). Difficult to rework. Parylene (XY): thickness: 5-25 μm. Dk: 2.65 (Parylene C). Df: 0.001-0.01. Advantages: ultra-thin and uniform (vapor-deposited; conformal to all surfaces including under components). Excellent moisture barrier (pinhole-free). Lowest RF impact (thin + low Dk + very low Df). Disadvantages: expensive (vacuum deposition process, $5-50 per board). Cannot be reworked (must be mechanically removed for repair). Masking is difficult (areas that must not be coated, such as connector mating surfaces, must be masked before deposition). Epoxy (ER): thickness: 50-100 μm. Dk: 3.5-5.0. Advantages: hardest coating, best chemical resistance. Disadvantages: rigid (can crack under thermal cycling), high Dk (significant impact on RF, especially at high frequencies), and not reworkable. Not recommended for RF circuits. (2) Selection criteria for RF: frequency of operation: at frequencies < 6 GHz: any coating type is acceptable (the thin coating has negligible impact on impedance and loss). At 6-40 GHz: use Parylene (thinnest, lowest Dk/Df) or silicone (low Dk, flexible). At > 40 GHz: Parylene is strongly preferred (even thin acrylic or polyurethane can measurably affect transmission line impedance and loss). Environmental severity: mild (indoor, controlled environment): acrylic is sufficient. Moderate (outdoor, non-marine): silicone or polyurethane. Severe (marine, military field use): Parylene or silicone. Temperature range: wide temperature range (military -55 to +125°C): silicone or Parylene.