[SI-LIST] Re: 0.8mm BGA routing

I have a Question on Laser VIA's

I have read through many articles that says Laser VIA cannot pass through
more than 2 layers.
Why is this so?

As i understand Fabrication process the drilling happens before plating and
stacking of layers.
what makes it impossible for a Laser Via to end at the 2nd layer.
or is it that Laser drilling is done after all layers are pressed together.

Prasad.S.A


----- Original Message -----
From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
To: "Tayyab Jamil" <tayyab@xxxxxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
Sent: Wednesday, December 24, 2003 11:50 PM
Subject: [SI-LIST] Re: 0.8mm BGA routing


> Tayyab,
>
> Ther are two possible applications of this pitch part that come to mind.
> The first is on tiny PCBs that are thin and the second is on multilayer
> PCBs that are thick.  Thin meaning less than 30 mils thick and thick being
> above that.
>
> For the thin PCBs, it is posible to drill 8 mil holes and plate them
> successfully.  This will allow a 3 mil trace to be routed, one between
each
> pin.
>
> If you are using this part on a thick PCB, it is not possible to drill and
> plate holes and at the same time maintain all of the clearances and
> tolerances, even with no traces between pins.  See my paper "Anatomy of A
> Drilled Hole" on my web site, www.speedingedge.com.
>
> Yes, there are people who will suggest that it is possible tor have
through
> holes and traces with this size package.  It is not, when yields and
> reliability are taken into account.
>
> When a .8 mm pitch part is used on a thick PCB, the vias are drilled with
a
> laser and are blind, meaning they reach only to layer 2.  Any leads that
> must reach fartner, will require through holes.  These through holes will
> need to be "fanned" out from under the part with traces on layer 1.  Then
a
> through hole can be drilled with proper clearances and tolerances.
>
> Hope this helps.
>
> Lee.
>
>
>
> > [Original Message]
> > From: Tayyab Jamil <tayyab@xxxxxxxxxxxxxx>
> > To: <si-list@xxxxxxxxxxxxx>
> > Date: 12/24/2003 1:50:29 PM
> > Subject: [SI-LIST] 0.8mm BGA routing
> >
> > Hello,
> >
> > I am currently working on a design where a 0.8 mm pitch BGA package is
> used.
> > Can any one suggest me the design configuration i.e trace width/spce and
> > specially the via size and via hole, which i should follow and
> manufacturers
> > can handle this.
> >
> >
> >
> > Regards
> >
> > Tayyab Jamil Qureshi
> > Sr. Design Engineer
> > CARE Pvt. Ltd.
> >
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