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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