Ground Fill and track impedance

steve 12 years ago

I am designing a impedance controlled PCB and have ended up with 0.1mm trac ks over ground plane spaced 0.1mm away. My PCB vendor has asked me to groun d fill on inner layers of the PCB to balance the copper. I am going to use

0.2mm copper clearance (0.017 thick copper foil). Is this liable to have mu ch effect on my track impedance, my feeling is that it will reduce the impe dance by a small amount

Thanks for any advice

Steve

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Tim Wescott 12 years ago

Hey Steve. Just to make sure, your stackup is signal on top, then a ground plane, then a middle plane that's either empty or has "stuff" -- yes?

And you're going to infill that middle plane with copper -- yes?

If so, then your "small amount" becomes "vanishingly small, of use only as an exercise in an advanced E&M class".

Tim Wescott Wescott Design Services http://www.wescottdesign.com

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steve 12 years ago

Thanks, Yes, I have Top L1 signal+Ground place

0.1mm Prepreg L2 (Ground Plane) 0.13mm Core L3 Signal1 0.9mm (Prepreg+Core) L4 Signal2 0.13mm Core L5 Ground Plane/Power Plane 0.1mm Prepreg L6 Signal+Ground Plane.

PCB manufacture wants me to put ground fill on layers 3 and 4 to balance co pper and was worried about impedance change. My saturns PCB microstrip calc ulator give 0.1mm track z0 as 60R on outer layer, if I use saturns PCB copl anar waveguide over ground calculator it gives z0 as 74R for 0.5mm clearanc e ground fill 0.1mm over ground plane, however as ground fill clearance inc reases impedance goes up which to me is illogical, if ground fill is a long way away it should have the same z0 as microstrip. Will take your advice a nd not worry about impedances changes due to ground fill.

Thanks again

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