ENGINEERING ANSWER
A common two-layer concept uses an RF signal layer, a RO4350B core and a ground plane. The core thickness is the signal-to-ground spacing only when that ground plane is directly adjacent and uninterrupted.
What to decide before release
Select the core for the RF geometry and mechanical interface. Keep power and digital routing from cutting the ground return path. Include finished copper and surface treatments when checking final thickness and connector fit.
Checks to put in your design brief
- Reserve the back layer for a continuous reference where possible
- Confirm core and finished-board thickness
- Add a coupon to the panel
Important distinction
A nominal 20 mil core does not produce a finished board of exactly 20 mil. Copper and coatings add thickness.
Sources & basis
Material facts follow the cited source. Design checks are preliminary guidance, not supplier process instructions.