Re: gEDA-user: BGA package prototyping

[Steve Meier <smeier@xxxxxxxxxxxxxxxxxxx>]

John,

Via in pad is possible with PCB, micro-vias arn't yet. To avoid the
wicking or chimeny effect have the back of the board coated with an
epoxy material prior to assembly this prevents the heat from shooting up
the via and then sucking the solder down.

I have watched one diamand rated shop remove and replace BGAs. They use
a very hot bottom heater and a top heater focused around the vacum
grip/manipulator. Thus they are heating the device from bellow and
above. I think alignment might be tricky particullarly as we move to
roHs compliant components. It used to be that the solder balls would
actually via a capilary like effect would tweek the component into the
best position if you were reasonably close. I am not sure that roHs
balls will allow this.

Also, Flatness, flatness and flatness are critical. Did I mention that
you bneed to make sure your board is flat otherwise the balls will
puddle in places and not connect in others.

I think this also answers your question that the solder does melt or at
least get very close to melting though it could have been the preasure
difference near the center that caused the balls running together.

Here I am struggling to decide if it is ethecal to publish a vender
name. I think that we have in the past at least with respect to fab
shops. So the diamond rated fab and assembly shop that I know can handle
your project is www.hunterpcb.com If you want a specific contact person
drop me a line.

Best Wishes,

Steve Meier

On Thu, 2007-10-18 at 18:31 -0500, John Griessen wrote:
> Some feature packed chips are coming in 0.5mm pitch BGAs where there are
> four rings of balls to connect.   How do you connect those to vias so you can
> use middle layers to get out of the grid zone?  One could go to the outside formthe first two rows,
> and toward the inside from the 3rd and 4th rows, then to vias, then outside by way of middle layers.
> 
> Sounds like the center would get crammed.  Can vias in the BGA pads be made so they don't wick out
> BGA ball material?   Does a BGA ball melt during reflow, or just get near it?
> 
> What fabs can do
> 3 mils/3 mils (track/space) rules, cover the backof the vias with epoxy or soldermask?
> 
> Can you get one to assemble by toaster methods for prototyping?
> 
> Thanks,
> 
> John Griessen



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