Perhaps a self healing material could be coated over the ceramic
caps such is done with tires that self plug after they are punctured.
Just a thought.
Ramon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Friday, March 16, 2007 11:22 AM
To: [log in to unmask]
Subject: Re: [TN] Engineering Question
Here is good information. Your design group is doing you a big favor. If
they read the following information it will save you a lot of headaches
down the road. Werner Engelmaier can provide you a wealth of information
on pad aspect ratios that will also prevent you from "anchoring" the
larger MLCCCs to the PWB, as this is one known cause of the cracking.
The CTE factor for the MLCCCs is very different from the standard PWB
materials. The larger the PWB, and the larger the MLCCC body, the
greater the stresses applied. It is not entirely a constant delta. There
are two concerns, the cracking of the MLCCC body and body/termination
disconnect, and the solder joint reliability. Both can be alleviated by
properly designing the pads to optimize the amount of solder under the
termination, assuming you use 63/37 solder. The propensity for problems
increases rapidly if you use Pb-free solder, both due to the higher
temperature transitions during reflow (especially initially during
cooling after the solder solidifies), and because of less ductility of
the Pb-free solder alloy. It is well worth the time spent to determine
all of the design rules to prevent this issue from happening. I have
seen two companies crying in their beer because they were either
ignorant of these issues or chose not to worry about them.
http://www.avxcorp.com/docs/techinfo/mlccflex.pdf
http://www.avxcorp.com/techinfo_documentdetail.asp?ID=117&Heading=AVX+ML
CC+FLEXITERM%99+Guarding+Against+Capacitor+Crack+Failures&Category=Ceram
ic+Capacitors
There are a number of good papers for crack cause and prevention
information at the AVX website.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bloomquist, Ken
Sent: Friday, March 16, 2007 9:49 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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