FWIW I've been using 0.2mm (8mil) drilled vias all the time on 0.8 pitch
BGA's - dogbone from the pads to the offset encroached-mask vias.  On
0.5 pitch I have used a filled 0.35mm pad via-in-pad, 0.15mm drill,
plated flat & flush.  Needs .05mm traces to get out.  But those are on
40mil thick boards.  6mil drills might be a problem on thicker.  And, I
don't know the cost trade-off between microvias and filled vias.  Also,
I am not using either of these methods on production-quantity boards.
But I can get them fabbed that way, routinely, no problem.  It's worth
discussing with your fabricator/assembler as an alternative to
microvias.

Gary Crowell, CID+
Micron Technology

> -----Original Message-----
> From: piclist-bounces@mit.edu 
> [mailto:piclist-bounces@mit.edu] On Behalf Of Vasile Surducan
> Sent: Wednesday, September 24, 2008 11:33 AM
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] BGA board processes
> 
> DDi is good but a little bit expensive.
> You didn't say what is the ball diameter so we can compute the
> reduction size of the footprint. If the ball  is 0.4mm diameter the
> pad could be 0.35 to 0.4mm.
> Which means you need no microvia, just a 8mil (0.2mm) through via with
> 18mil (0.45mm) outer ring.
> Microvias is for 0.5mm pitch or lower.
> 
> Vasile
> 
> On 9/22/08, Zik Saleeba <zik@zikzak.net> wrote:
> > TQFP is just too big for the project I'm working on so I'm 
> looking at going
> > to BGA for the first time. The process of breaking out 
> those signals seems a
> > little tricky. The package I'm looking at has 100 balls 
> with 0.8mm spacing
> > and I've been having some difficulty finding vias small 
> enough to route the
> > signals out using conventional board processes. The vias 
> are just too large
> > too allow enough clearance between the pads. A little 
> reading around the web
> > seems to indicate that at this density I should be looking 
> at microvias
> > rather than conventional vias.
> >
> > Have I got this right? Do I really need to go to a pcb fab 
> which can do
> > microvias? It seems like this counts out the cheaper-end 
> fabs I'd normally
> > use. Any suggestions on where I should go for fabrication? 
> I'm only looking
> > at prototypes and short runs and my budget is tight.
> >
> > Thanks,
> > Zik

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