Re: gEDA: Problems with connection scanning

[Karel Kulhavý <clock@twibright.com>]

On Sun, Aug 08, 2004 at 04:58:59PM +0200, Magnus Danielson wrote:
> 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 think PCB should support 32 layers and not only 8 :)

> > 
> > I would be curious to know why your mfr doesn't want thieving.  Maybe
> > they will put it in themselves?

I didn't mean they don't want. They didn't require it.

> > 
> > 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