Forums

Ground Fill and track impedance

Started by steve January 27, 2014
I am designing a impedance controlled PCB and have ended up with 0.1mm tracks over ground plane spaced 0.1mm away. My PCB vendor has asked me to ground 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 much effect on my track impedance, my feeling is that it will reduce the impedance by a small amount

Thanks for any advice


Steve
On Mon, 27 Jan 2014 03:16:25 -0800, steve wrote:

> I am designing a impedance controlled PCB and have ended up with 0.1mm > tracks over ground plane spaced 0.1mm away. My PCB vendor has asked me > to ground 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 much effect on my track impedance, my feeling is that it > will reduce the impedance by a small amount
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
On Monday, January 27, 2014 7:28:00 PM UTC, Tim Wescott wrote:
> On Mon, 27 Jan 2014 03:16:25 -0800, steve wrote: > > > > > I am designing a impedance controlled PCB and have ended up with 0.1mm > > > tracks over ground plane spaced 0.1mm away. My PCB vendor has asked me > > > to ground 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 much effect on my track impedance, my feeling is that it > > > will reduce the impedance by a small amount > > > > 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
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 copper and was worried about impedance change. My saturns PCB microstrip calculator give 0.1mm track z0 as 60R on outer layer, if I use saturns PCB coplanar waveguide over ground calculator it gives z0 as 74R for 0.5mm clearance ground fill 0.1mm over ground plane, however as ground fill clearance increases 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 and not worry about impedances changes due to ground fill. Thanks again