Simple
If you look at the percentage of copper fill with traces and pads, it's 20-35%
in many regions.
For example, for a 1 mm pitch BGA with a 0.5 mm pad, the circular pad occupies
about 19% of the space. Add one trace that is 4 mil wide, then the amount of
copper per unit square goes up to 29%. Once the traces breakout out of the BGA
region, Cu density goes down to maybe 20% with 1:4 trace/space ratio, and 33%
with 1:2 ratio. So, 25% is a pretty decent average.
Scott McMorrow, CTO Signal Integrity Group
Samtec
Office 401-284-1827 | +1-800-726-8329
www.samtec.com
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx <si-list-bounce@xxxxxxxxxxxxx> On Behalf Of
David Banas
Sent: Tuesday, July 30, 2019 3:46 PM
To: si-list@xxxxxxxxxxxxx
Subject: [EXTERNAL][SI-LIST] Re: Gnd pours
As long as we’re on this topic, I’ve wondered something for a while now:
Why does the industry seem to have settled on this technique of filling unused
space on the top and bottom layers with a rectilinear pattern of small isolated
copper squares with an approx. 25% fill factor? Is that optimum in some way?
Thanks,
-db
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