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Date: | Tue, 21 Nov 2006 17:31:37 +0000 |
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Vered
This is an old and thorny subject much debated on the TechNet in the
past.
The TCE of apretty much all conformal coatings is >300ppm/DegC but
the TCE of silicones are huge at ~300-350ppm/°C
Solder being only ~16 ppm/°C this is a pretty significant TCE mismatch.
As for silicones as debated by Brian Ellis and myself some time ago,
Brian said: They have a unique property that however soft they feel,
they are rock-hard when subjected to mechanical shock (hence super-
bouncing balls and bouncing putty) At only slightly elevated
temperatures there would be tension on a BGA-ball-pad combination,
that would increase by orders of magnitude should it be subjected to
a small shock at the same time
If you must under-fill, then a soft purpose developed epoxy is the
probable answer, check with Brian Toleno - wherever he is hiding
these days?
Having said all of that, I have had aerospace customers who apply a
very thin coating, diluting the coating and dipping at very slow
speeds, that can ensure (as far as one can without removing the
device to check!) coverage under the device. They then apply a
"conventional" spray coat to complete the job.
I hope this helps
Regards,
Graham Naisbitt
Gen3 Systems - Engineering Reliability in Electronics
[log in to unmask]
On 20 Nov 2006, at 16:40, Vered BS wrote:
> looking for reviews/articles or case studies regarding conformal
> coating on
> BGA
> for military/aerospace environment
> Appreciate any comment or sharing your experiance
> Thanks
> Vered
>
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