Does anyone know the value of Er of FR4?
Between 3.5 and 4.5. It is not controlled well. Do you need a substrate with tighter tolerance?
If you do I recommend https://www.rogerscorp.com/index.aspx
I was able to talk them out of some their smanchy material for my kid's random-number-generator project for school.
I recommend Rogers as well.
I am not hip to them as a manufacturer of PCBs. Got a link?