Hi, Jason,
I think you've almost answered your own question. Fiducials are at opposite
corners so that both the x and the y tolerence errors can be reset for a
local component. If they were in line, only the x or the y would be reset.
All fine pitch devices (non-self-centring) should have fiducials, and
BGA's, especially micro BGA's, are still counted in that category by some.
I guess it's because, as you say, you can't see any of the contacts past
the first two rows, and if there are a lot of untented via holes next to
the solder pads, folks just might not want to take any chances in
misaligning the b.....s.
More a psychological answer than a technical one.
Pete Duncan
Jason Gregory
<Jason.Gregor To: [log in to unmask]
[log in to unmask]> cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST
Sent by: Group)
TechNet Subject: [TN] Dumb question
<[log in to unmask]
ORG>
05/10/01
08:59 PM
Please
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"TechNet
E-Mail
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Please
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Jason Gregory
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
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