ENGINEERING ANSWER
A typical thermal conductivity of 0.69 W/(m·K) at 80°C is reported for RO4350B. This is a laminate property, not a complete board thermal resistance. Power devices usually need a deliberately designed heat path.
What to decide before release
Trace heat from the device pad through copper, vias, interface material and heat sink. Use package thermal guidance and a board-level model. Improving one layer may have little effect if another interface dominates the path.
Checks to put in your design brief
- Estimate dissipated power at worst case
- Model copper and thermal vias
- Measure temperature on the assembled prototype
Important distinction
Do not use a dielectric-conductivity value to promise junction temperature without package and boundary-condition data.
Sources & basis
Material facts follow the cited source. Design checks are preliminary guidance, not supplier process instructions.