As per Toradex Carrier Board Layout Guide (Page 13, Table 4), for an Ethernet data pair microstrip on Layer 1 of an 8 Layer board, I’ve been using a trace width of 150um and a trace spacing of 165 um. However, when I plug those numbers (i.e. Trace Thickness 35um, Prepreg (2116) Thickness 112um, Trace Width 150um, Trace Spacing 165um and Er 4.5) into an impedance calculator (e.g. eeweb.com - tools/microstrip-impedance) I get a differential impedance of 115 ohms, rather than the 95 ohms required. What am I missing ??? Thanks !
There’s a better way. Consult with your board shop, and explain that you want a microstrip trace and space for 95ohm and ask for the stack up, along with tolerance(s). Then, when you order your board, include notes on special required impedance’s, and include the original stackup calcluations that they gave you. They will know exactly what to do because you asked about it ahead of time.
The tools on the web seem to never quite agree. The board shop should be able to account for the smallest things including the solder mask coating etc. I’d trust the board shop, since ultimately they have to meet your impedance requirements.
Also, on occasion, I’ve done microstrip on layer 1 referenced to layer 3, 4 o 5 etc if necessary. As long as you follow keep-outs in the layers in between, this can be a good strategy to relax the trace and space tolerances. Again, I usually consult with the board shop. For instance, I may ask what layer makes most sense to reference to if I want to target a 7mil trace.
Hope that helps.
Dear @jsm09a, @kswain provided the perfect answer (thanks a lot!). I would proceed exactly as recommended by him and rely on the calculation tool used by the PCB manufacturer. They know the material they will be using and therefore they can provide you the best answer.
Thanks again to @kswain, please let us know if you need any additional help.