ENGINEERING ANSWER
A hybrid and an all-RF-material stackup should be compared by field placement, manufacturability, reliability and total cost. The lowest raw-material cost is not always the lowest finished-board cost.
What to decide before release
Identify which dielectric regions actually control critical signals. Compare two supplier-supported stackups with the same electrical requirements. Include bonding cycles, yield and qualification effort when assessing savings.
Checks to put in your design brief
- Map critical fields by layer
- Compare approved constructions
- Include process and test costs
Important distinction
Adding FR-4 to a drawing is not enough to demonstrate a cost reduction.
Sources & basis
Material facts follow the cited source. Design checks are preliminary guidance, not supplier process instructions.