How to calculate Surface and Coated Structures on the same stack up

In some instances it may be required to have a coated and non-coated structure on the same stack up.
To achieve this in Speedstack the following procedure can be deployed to achieve this.

However if you have Speedstack HDI you can achieve a similar result by utilising a sub stack - please see AP524.

Open Speedstack and then under Tools | Options |  Rebuild and Calculate Structures, uncheck Coating Dielectric (CEr)**.

** Note changing this option will prompt to restart Speedstack.

Once Speedstack has restarted open a stack up and add a Coated Microstrip structure and set the CEr = 4.00.
(a value suitable for solder mask)

Now add a second Coated Microstrip structure, same W1 / W2 and target Impedance parameters but this time set CEr = 1.00.
(a value suitable for air to mimic a surface structure)

Comparing the above to a structure with no solder mask and using a Surface Microstrip 1B we see favourable results for Zo.
(Zo = 81.56 compared to 81.66)