ENGINEERING ANSWER
A microstrip places a signal conductor above a reference plane with fields distributed between the dielectric and the region above the board. Its effective permittivity is therefore lower than the substrate Dk in an uncovered air configuration.
What to decide before release
Keep the plane uninterrupted, avoid abrupt width changes and include the connector transition in the model. Nearby coplanar copper, mask and metal housings alter the fields, so the ideal isolated-line formula may stop representing the physical circuit.
Checks to put in your design brief
- Define the reference plane
- Check nearby metal and copper
- Use smooth, modeled transitions
Important distinction
Do not feed a microstrip calculator’s result into a stripline or grounded-coplanar layout without changing the model.
Sources & basis
Material facts follow the cited source. Design checks are preliminary guidance, not supplier process instructions.