Re: gEDA-user: Is it possible to route through ...

[Steve Meier <smeier@xxxxxxxxxxxxxxxxxxx>]

Gene,

For a 0.8 mm (31.5 mill) pitch device if half the space is for the pad
(16 mills) that leaves (15.5 mills) space between the pads. This is
plenty of room to put a 6 mill width trace through. 

For a 0.5 mm (19.7 mill) pitch device if half the space is for the pad
(10 mills) that leaves (9.7 mills) space between the pads. This is
plenty of room to put a 6 mill width trace through. In fact if you said
6 mills for the trace + 2 mills extra your pad could be 11.7 mills.

One thing that concerns me is that this vendor's standard is 6 mills
trace width and you are discussing a 0.5 mm pitch device. I would not be
comfortable. Have you asked them if they typically build fabs to these
dimensions?

The issue of dropping the vias through the board is that each vias
requires a pad around it. What will be the dimensions of these pads. Are
you going to stagger the vias so they don't interfere with each others
traces?

I am familiar with bga devices with pitches of 1mm and I have squeezed 4
mill pitch traces between rows to escape the device. More typically I
use a 4 mill width (now that pcb supports more then 8 layers)

Steve Meier

On Sun, 2008-12-14 at 21:30 -0500, gene wrote:
> Steve Meier wrote:
> > Gene,
> > 
> > My understanding is that the minimum solder mask is dimension is 5 mills
> > (check with your vendor).  So if you have a solder mask cover of 5.5
> > mills between pad openings then a trace of 4.5 mills or smaller should
> > not be a problem. I have used traces as small as 3 mills. I believe for
> > the copper layers this suggests you should use 0.5 oz copper thickness.
> > 
> > Steve Meier
> > 
> > 
> 
> I checked a couple of online fab outfits, and found at least one that 
> will  do 6 mil trace with 6 mil space. I think 3 mil is going to be a 
> little out of the ordinary. For now, in order to avoid any additional 
> cost, maybe 6/6 is a good size to use for those dense areas.   But then 
> again, that means minimum space between pins of 18 mils.  That won't fit 
> very well in the .8mm pitch part I described and looks pretty much 
> impossible on 0.50mm pitch part.
> 
> Maybe the better solution is to run the bus on one side of the card, and 
> drop vias from the other side.  It's just going to make a lot of swiss 
> cheese, I think, on the card.  I'm open to suggestions :)
> 
> gene
> 
> 
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