This is an anonymized summary of a resolved support question.
Question
When manufacturing a board based on the UbiSwitch baseboard design, two PCB fabricators could not initially achieve the 100 Ω differential-impedance target on layer 3. One proposed 90 Ω and the other 66 Ω. How should this be handled?
Answer
BotBlox’s fabricators work to a 100 Ω differential target with a ±10% tolerance for this design. That puts 90 Ω at the lower end of the specified range; 66 Ω is outside it.
Work with the fabricator on the stackup rather than assuming a default overall board thickness must be retained. Different fabricators have different materials and manufacturing capabilities, so the stackup needed to achieve the target can differ. Agree any stackup and finished-thickness changes before fabrication, including checking that the resulting board still fits the mechanical design.
Resolution
Both fabricators were able to resolve the impedance issue by adjusting the total board thickness. The original thickness constraint had limited their options. The UbiSwitch baseboard’s relatively thick construction gives additional room to adjust the stackup.
The takeaway is to specify the impedance target and tolerance explicitly, and have the fabricator confirm a suitable stackup for the actual design before manufacture.
Public reference: UbiSwitch baseboard design files.