Ok, just to throw my 2 cents worth on top of this to
make 4 cents.... I started running BGA's in September
of 1995 and studdied VERY hard to understand all these
issues. I talked to Motorola and IBM extensively
about the issues. The way Motorola engineers explained
it to me was....any 3 balls dictate a plane. So, if
you have 3 balls that are slightly larger then all the
other balls, the other balls will not be touching the
board. Now, when those balls reduce to 2/3 of the
original size of the ball, the ones not touching the
board may never touch the board. By putting solder
paste down you can assure that all the balls are
touching making contact is to put solder paste down.
The question is, why wouldn't you put paste down for
rework? Would you run it on just flux in production?
Try to copy the production process for rework.
This all applies to course pitch BGA. When you get
into micro BGA and CSP, the pitch is too small for
paste and flux is used.
Brad Kendall
--- Jim Kittel <[log in to unmask]> wrote:
> Dan,
>
> Just to throw my 2 cents in....we found that with
> solder paste we get much
> better coverage of the PWB pad with solder,
> resulting in a more consistent
> solder connection. With just flux we were getting a
> small percentage of the
> connections with a thin solder contact to the pad.
> Although percentage was
> small, it was frequent enough on a large ball count
> to reduce our rework
> yields. Paste also helps to cancel any co-planarity
> problems.
>
> Jim Kittel
>
>
> -----Original Message-----
> From: d. terstegge [mailto:[log in to unmask]]
> Sent: Monday, February 05, 2001 9:56 AM
> To: [log in to unmask]
> Subject: Re: [TN] BGA REWORK
>
> Hi Robert,
>
> Are there any data that support your statement ? If
> you calculate the
> amount of solder from the solderpaste and compare
> this with the volume of a
> ball, you'll see that about 90% of the solder
> joint's volume comes from the
> ball. 10 percent more or less of total solder volume
> shouldn't make out the
> difference between a short and a long product life.
>
> Kind regards,
>
> Daan Terstegge
> SMT Centre
> Thales Communications
> Unclassified mail
> Personal Website: http://www.smtinfo.net
>
> >>> "Furrow, Robert Gordon (Bob)"
> <[log in to unmask]> 02/05 4:27 pm >>>
> Trevor,
>
> I think it will depend on the expected useful life
> of the repair. If you are
> looking at short life product with eutectic solder
> balls, and coplanarity is
> not an issue, then using just flux should not be an
> issue. If you require
> long term reliability then in most cases paste is
> preferable.
>
> Thanks,
> Robert Furrow
> New Product Engineering
> Lucent Technologies
> 978-960-3224 [log in to unmask]
>
> > -----Original Message-----
> > From: Trevor Goddard [SMTP:[log in to unmask]]
> > Sent: Monday, February 05, 2001 9:44 AM
> > To: [log in to unmask]
> > Subject: [TN] BGA REWORK
> >
> > Hi all.
> > I have a question about reworing BGAs. Do you have
> to reprint the solder
> > paste before placing the component or can you lay
> down some flux and rely
> > on
> > the solder balls of the BGA?
> > Thanks in advance
> >
> > Trevor Goddard,
> > SMT Supervisor
> > XLTEK
> > (905) 829-5300 x 348
> > [log in to unmask] <mailto:[log in to unmask]>
> >
> >
>
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