ENGINEERING ANSWER
Surface finish selection balances RF loss, solderability, assembly, storage and reliability requirements. There is no universally best finish for every RO4350B circuit or frequency.
What to decide before release
Compare candidate finishes on the actual exposed RF geometry. Nickel-bearing finishes may influence loss, while other finishes introduce their own handling and reliability considerations. Ask the fabricator to state process thickness and storage guidance.
Checks to put in your design brief
- List RF and assembly priorities
- Model or measure relevant finish effects
- Confirm shelf-life and handling
Important distinction
Do not recommend ENIG for every RF board merely because it provides flat pads.
Sources & basis
Material facts follow the cited source. Design checks are preliminary guidance, not supplier process instructions.