Jason,
Diagonal BGA fiducials are nice to have just in case you need them. Like
Dorothy mentioned, these fiducials are typically part of the component
drawing. At my shop we use 2 global fiducials, and have never really used
any local fiducials- even for an 0201 process. It depends on your
equipment, and very much on the planarity of the board. All of are products
use custom underside tooling to ensure the board is flat, so locals are not
as important to us.
If we did run into a problem where we needed local fiducials, I would want
to have them near the components that hold the largest penalty for
misplacement. The BGA is typically the most expensive and hardest to rework
component on the board. Once reworked, the BGA must be replaced.
Also do not buy into the whole self alignment of BGAs! While their large
surface contact versus weight improve alignment, there are other factors
such as wetting speed of paste, reflow profile, etc.. All components have
the ability to self align. Your BGA will self align better with a fast
wetting OA paste than in would with most no clean formulas. I have seen
many BGA cross sections, and believe me, self alignment is a relative term.
Steve Abrahamson
> -----Original Message-----
> From: Dorothy M. Lush [SMTP:[log in to unmask]]
> Sent: Thursday,June 28,2001 5:01 PM
> To: [log in to unmask]
> Subject: Re: [TN] Dumb question
>
> On Thu, 10 May 2001 07:59:58 -0500, Jason Gregory <[log in to unmask]>
> wrote:
>
> >OK, guys, here's one.
> >Why is it so important for local fiducials to be placed near diagonal
> corners of BGA's. The keyword here is "SO". I've performed studies in the
> past detailing the natural tendency of BGA's to self-align during reflow.
> It's even lead to me to work on a patent concerning a BGA rework preform.
> What I'm wondering is....ssop's, tssops, qfp's, etc. have greater impact
> on
> placement quality when misplaced. And they do not self-align!!! I do,
> however, understand that BGA's balls are not easily seen once placed (you
> can only see the edges). But my point that I'm getting to is.....wouldn't
> you want local fiducials more at the parts that will not "self-correct"
> themselves during reflow than at the parts are more forgiving? Just
> wondering.
> >
> >
> >Jason Gregory
> >Software Specialist - NPI Group
> >SCI Systems/Plant 2
> >13000 S. Memorial Pkwy.
> >Huntsville, AL. 35803
> >(256) 882-4107 x3728
> >[log in to unmask]
> >
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> Jason,
>
> Global and local fiducials are a requirement of the machines that place
> them. You need to design/layout fiducials for ALL the machinery in all the
> locations and countries that your PCB will be loaded from prototype to
> full
> production. Global fiducials will have a error tolerance per length so any
> distance over 12 inches should have another global fiducial inserted
> between them including fiducials that are diagonally opposite. Global
> fiducials are also used on the stencil for solder printing. On Fuji
> machines a fiducial is good for 2 inches in the X and Y direction so every
> 4 inches is recommended for local-to-local-to global fiducials. Chip
> placement machines do not refer to local fiducials as often as IC placing
> machines. But Fuji does not make all placement machines. Every time a
> placement machine resets itself it is to a fiducial and if it has to go to
> the global ones then it is sometimes a longer distance than to a local
> one.
>
> The convenience of giving every fine pitch part and/or BGA its own
> fiducials is in the layout library hence no thinking and you cannot forget
> to do it. It is definitely not necessary to have two fiducials diagonally
> opposite for every part with modern equipment butn we all don't have
> modern
> equipment.
>
> Dorothy Lush
> Manufacturing Engineer
>
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