Hi Werner,
How is life in Florida? In response to your e-mail, I do agree that all things
being the same, a higher stand-off will decrease the magnitude of the localized
stress and thus increase the reliability of the joints according to published
data... but I do disagree with your statement about 'having nothing to do' with
the other input that people have given...increasing stand-off height may be the
most important, but it is not the only reason.... there is a lot that the
industry still do not know about reliability... reliability is application
dependent...and I wish that the only issue with using ceramic BGAs with high
temp solder (balls or columns) was stand-off height...but we know better now,
but we still need to keep conducting reliability tests to know even more...
marcelo
-----Original Message-----
From: Werner Engelmaier [SMTP:[log in to unmask]]
Sent: Tuesday, September 15, 1998 12:42 PM
To: [log in to unmask]
Subject: Re: [TN] X-ray for open joints on BGA's ?
Hi 'eholton', Marcelo, & Steve,
Lets remember why there are high-melt solder balls and columns on CBGAs;
it is
to reduce the strains on the solder joints due to the large thermal
expansion
mismatch between the ceramic and the FR-4 by assuring large solder joint
heights (stand-offs). It has nothing to do with cleaning, package
weight, or
devices generating heat. For BGAs, particularly CBGAs, a reflow profile
with a
peak temperature of 219C is too low even for eutectic Sn/Pb solder.
Even with high-melt solder balls you should get nice fillets because of
the
eutectic solder paste.
The reliability of the solder joints goes up roughly with the square of
the
stand-off height.
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask]
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