Lowest possible PCB plane separation / maximize capacitance?

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JohnG300c

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I'm designing a four layer PCB and i would like to know what the lowest separation distance between the power and ground planes can be? Since the capacitance is 225 * er / t (where t is the dielectric thickness, and er = 4 for FR4) it seems like i should keep the plane separation between 3-5 mils. What is possible and a good economic choice?

I understand that there are specialty materials (ZBC) that can be used to separate the planes. Altium Designer has chosen prepreg distance of 13 mils but at that separation i only get 75 pF/sqin whereas at 3 mil i get over 200 pF/sqin.

Any general rules?
 

I have decided to upgrade to a six-layer PCB. The cost is 25% higher but i can control both the signal layer impedance as well as minimize the layer separation between the VCC/GND layers. The minimum layer separation seems to be 4 mil, with the extra layer pair i effectively double the plane area which is a great boon to EMI and ripple minimation.
 

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