Re: gEDA: Problems with connection scanning

[Magnus Danielson <cfmd@bredband.net>]

From: sdb@cloud9.net (Stuart Brorson)
Subject: Re: gEDA: Problems with connection scanning
Date: Sun, 8 Aug 2004 10:38:49 -0400 (EDT)
Message-ID: <20040808143849.82BE82AA07@earl-grey.cloud9.net>

> > For example some board have tons of tiny squares that are not connected
> > together to perform coverage equalization, copper savings etc. My manufacturer
> > didn't want any such thing. It's probably necessary only in very big series or
> > in some extra cheapo processes or I don't know what. I can see it on PCI cards.
> 
> That's called "thieving".  Usually, PCB manufacturers *do* want that
> -- they place it into the design themselves.  And not just cheapo
> boards use it.  My biggest board was a 20 layer SONET interface board
> for a major router (not done with gEDA, BTW  :-( ) which would retail
> for $20,000.00.  (Or maybe the BOM cost was $20K. . . .  I forget.)
> It had thieving. 
> 
> I would be curious to know why your mfr doesn't want thieving.  Maybe
> they will put it in themselves?
> 
> My understanding is that manufacturers want theiving because it means
> they need to dissolve less Cu in their etch baths, which means they
> have to change the bath less often.  Also, it helps with the
> mechanical strains on the board if you have an even copper balance on
> both sides of the board.  If you have a copper imbalance, your board
> can tend to warp.

The main reason I've seen it being done is the mechanical stress so that it is
needed to keep it equalent. One should recall that a PCB board shift in
temperature more during the manufacturing process than out in the field...
usually. A board flexing with small-pitch during the soldering-moment isn't the
best way to get good yield. Having do dissolve less Cu is just and added
benefit and is usually a good menefit for everyone since less chemicals is
needed and it is easier to keep things fresh since the decay is a tad slower.

For the mechanical design one must understand that the further out from the
center of the card, the more bending-strength does the mechanical stress
exercise, so it is more important to have the outer layers correct than the
inner layers.

But it is not a sign of el-cheapo design, it is a good design-rule.

Cheers,
Magnus