Hi Kenny,
It's a valid question and one that we regularly provide guidance to our
designers. There is more at issue than just the cracks, which really
isn't much of an issue anymore. The reason it's not an issue is that
anything larger than a 1206 is incredibly difficult to buy any more.
Even the 1206 is difficult to find a supply, and you might find yourself
sole sourced with a single supplier. Even that supply will dry up after
a while.
From a manufacturing perspective, it is desirable to reduce the number
of components that stick up (or have a height) greater than the rest of
the components on the board, simply to reduce breakage due to handling.
With the increased use of CSPs instead of tall QFPs or even tall BGAs,
smaller than 0805 becomes desirable.
Where you might be using the component for quite some time, my biggest
concern would be maintaining a supply of caps over that time period.
The cap suppliers should chime in, but in the mean time, my discussions
with several cap suppliers is that even the 0805 is currently phasing
out as demand drops significantly and it no longer becomes cost
effective for that supplier to maintain that product line. At the
moment, there is still a 'good' supply of 0805. But I would NOT bank on
that still being true another 4 years from now...
We have some old designs that still use 1210's with no problems with
cracking. However, we have made it clear to our designers since several
years, "anything larger than 0805s and we will come after them with
torches and pitch forks!".
-Ryan Grant
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bloomquist, Ken
Sent: Friday, March 16, 2007 8:49 AM
To: [log in to unmask]
Subject: [TN] Engineering Question
Since it's Friday this could be a joke but I don't really think so.
Design Engineering asked Manufacturing Engineering, (believe it or not),
"Do you have any guidance on the maximum size we want to use for ceramic
SMT caps?"
I believe their concern is coming from the potential for cracks during
processing. We currently and probably will for quite some time use SN63
for all our processing so the higher temperatures are not an issue.
Any guidance would be appreciated,
KennyB
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