"Dorothy M. Lush" wrote:

> 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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Jason ,

Here we use local fiducials for all fine pitch components be it ssop's, tssops, qfp, ...
and all BGA's . And as you pointed out it is crucial to have them (fiducials) for better
placement.

--




Shahed Safavi-Bayat
PCB Designer, C.I.D.
Harris, Microwave Communication Division
3 Hotel de Ville, Dollard-Des-Ormeaux, QUE, H9B 3G4 Canada