Outer Layer are harder to etch than inner layer because the copper thickness
varies a lot after thru plating, so manufacturers like to balance the copper
fill to be equal on top and bottom for best yield. This is especially important
for thin traces and impedance control. For very dense designs, when the
tracewidth and spacing are identical, you will mathematically have a maximum of
50% fill, but on outer Layers there is much more solder pads with larger
spacing in between and between pads and traces, so high density Designs often
reach 30% fill.
That’s why the best compromise for the manufacturer to keep the etching process
stable for the majority of designs is in the area of 30%.
Using a pattern rather than solid copper fill also helps to stabilize the
process. The etching fluid saturates locally and thus is kept flowing. Large
areas without copper will have the flowing media saturating quicker, thus the
etching around these areas isn't as aggressive. The media over copper filled
areas will stay very aggressive, and when it hits the next copper feature it
will be etched much quicker. A pattern helps to balance the saturation level
all across the PCB for balanced etching.
Mit freundlichen Grüßen / Kind regards.
Gert Havermann
Signal Integrity Engineer
Dept.: Corporate Technology Services
HARTING Stiftung & Co. KG
Marienwerderstraße 3
D-32339 Espelkamp
Phone +49 (0)5772 47 857
Fax +49 (0)5772 47 90857
mailto:gert.havermann@xxxxxxxxxxx
http://www.HARTING.com
----------------------------------------
HARTING Stiftung & Co. KG | Postfach 11 33, 32325 Espelkamp | www.HARTING.com
Persönlich haftende Gesellschafterin:
HARTING Führungsstiftung | Amtsgericht München | HRA 108479 | München
Vorstand: Dipl.-Kfm. Philip F. W. Harting (Vorsitzender), Dipl.-Kffr. Maresa W.
M. Harting-Hertz, Dipl.-Kfm. Dr.-Ing. E. h. Dietmar Harting,
Dipl.-Hdl. Margrit Harting, Dipl.-Ing. (FH) Dipl.-Wirtsch.-Ing. (FH) Andreas
Conrad, Dipl.-Ing. Uwe Gräff, Dr. iur. Michael Pütz
Sitz der Gesellschaft: Espelkamp | Amtsgericht Bad Oeynhausen | HRA 9021 |
UST-ld Nr. DE812136745
-----Ursprüngliche Nachricht-----
Von: si-list-bounce@xxxxxxxxxxxxx <si-list-bounce@xxxxxxxxxxxxx> Im Auftrag von
David Banas
Gesendet: Dienstag, 30. Juli 2019 21:46
An: si-list@xxxxxxxxxxxxx
Betreff: [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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