ENGINEERING ANSWER
A four-layer hybrid can place the RF path over a RO4350B dielectric while using other materials for less sensitive functions. The RF field must remain within the intended dielectric region for that concept to work.
What to decide before release
Start with the signal and its adjacent reference plane. Then add bonding and digital/power layers with a fabricator-approved construction. Review symmetry and copper balance rather than assembling arbitrary thicknesses in a drawing.
Checks to put in your design brief
- Identify the RF reference plane
- Name the pressed bonding layers
- Review bow and twist requirements
Important distinction
“Rogers plus FR-4” is an incomplete stackup. The exact FR-4 and bonding system must be agreed.
Sources & basis
Material facts follow the cited source. Design checks are preliminary guidance, not supplier process instructions.