Romeo,

Be very careful of aspect ratios if you go this way, the use of a
microvia will generally limit the layer count due to this factor.  Which
in turn, will limit the available surface area for escape routes.  In a
lower I/O count BGA, the impact may be lessened.  If not, to overcome
this, you may need to go to blind segments, this tradeoff is not one
that I would accept.

We have had much success keeping it simple and utilizing teardropped
lands to the via.  In this approach,  the routing limitations are
therefore only felt on the primary layer.  In general practice, we have
found that the more exotic the solution, the lower the yield rates.

Scott E. Lee
Manufacturing Engineer
Mercury Computer Systems

> -----Original Message-----
> From: Kubes, Romeo (NM75) [SMTP:[log in to unmask]]
> Sent: Monday, October 05, 1998 3:03 PM
> To:   [log in to unmask]
> Subject:      Re: [TN] Via's under BGA's
>
> Chad,
>
> Is using blind Microvias acceptable, since their cavity is so small it
> would
> rob a very small % of the solder? (I know this is expensive, but
> besides
> this factor).
>
> R. Kubes
> Honeywell DAS Albq
>
> > ----------
> > From:         Thibodeau, Chad[SMTP:[log in to unmask]]
> > Sent:         Monday, October 05, 1998 11:41 AM
> > To:   [log in to unmask]
> > Subject:      Re: [TN] Via's under BGA's
> >
> > Bridget,
> >
> > There was an excellent paper written by Paul Mescher of Amkor Anam
> > addressign "...Surface Mount Assembly of Amkor/Anam BGA Packages".
> In
> > this
> > paper Paul describes the importance of PTH isolation (via isolation)
> and
> > to
> > paraphrase him: "No matter what shape or definition technique is
> used for
> > the mounting pad, the isolation of the PTH from the mounting pad is
> an
> > important feature.  If the PTH is contained within the mounting pad,
> > solder
> > can and will wick down the PTH.  The amount of solder that wicks
> depends
> > on
> > many factors, including PTH finish and coating variations.  Because
> of
> > this,
> > the results are somewhat unpredictable.  Some solder joints may be
> > unaffected, while others will be starved to the point of creating
> opens.
> > The worst result is a partially starved joint with severely reduced
> cross
> > section.  This joint can have significantly lower fatigue life and
> result
> > in
> > early system failure.  Because the quality of the solder joint is
> > guaranteed
> > by control, rather than inspection, designs/processes that result in
> > random
> > distributions are generally considered unacceptable, and the
> PTH-in-pad
> > design is not recommended."
> >
> > If you require additional information you may want to talk with Paul
> > himself.  I do not have his number, but he is located in Chandler,
> AZ.
> >
> > Chad Thibodeau
> > EF Data
> > Component Engineer
> > 602 333-2138 Phone
> > 602 921-9012 Fax
> > [log in to unmask]
> >
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