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Trace width & air gap for 85 ohm impedance

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sapphire_2010

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Hi Fellows,

I wonder if anybody could help me to verify the Trace Width to match 85Ohm impedance for Differential pairs (surface microstrip) for a two layer board. Currently I have used 12mil/0.3mm trace width with 5.5mil separation. TOP side would have these pairs & BOT would be a GND copper pour.

I am confused about the Lower Trace/Upper trace width as shown in the attachment. Do I need to consider them (underetch/over etch)? For now i have considered W1=W2. Core thicknes is 0.3mm; Final board thickness ~ 0.4 (including ENIG 2-3uin); 1/2 Oz outer layers.
Er=6.15 Rogers 6000


Thanks.
 

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