ENGINEERING ANSWER
Solder mask changes the dielectric environment above a microstrip and can affect impedance and loss. Its impact depends on material, thickness, coverage and frequency. Leaving a line uncovered also changes assembly and handling considerations.
What to decide before release
Specify RF mask openings intentionally and include them in the manufacturing data. If mask covers the line, use its properties and thickness in the final model. Keep abrupt covered-to-uncovered transitions in the simulation.
Checks to put in your design brief
- Define coverage along the RF path
- Confirm mask properties and process thickness
- Review assembly exposure requirements
Important distinction
The uncovered-line calculator on this site does not model solder mask. Do not interpret its precision as coverage-independent accuracy.
Sources & basis
Material facts follow the cited source. Design checks are preliminary guidance, not supplier process instructions.