ENGINEERING ANSWER
Both materials belong to the RO4000 family, but their electrical values and flame-retardant characteristics differ. The cited sheet lists design Dk of 3.66 for RO4350B and 3.55 for RO4003C; process Dk is a separate measurement.
What to decide before release
Check product safety requirements, loss budget and exact construction before considering a substitution. Retune line dimensions and electrical lengths when changing material rather than keeping the artwork unchanged.
Checks to put in your design brief
- Review the required flammability qualification
- Use each material’s correct design model
- Revalidate the RF circuit
Important distinction
RO4003C’s 3.55 design Dk should not be presented as its process Dk.
Sources & basis
Material facts follow the cited source. Design checks are preliminary guidance, not supplier process instructions.