Hi Kathy! Sorry to hear that your Purchasing group is not supporting good
assembly practices. Any board that is subjected to the ANSIJ-STD-003
solderability test and fails should not be used for production assembly. I
doubt there is any published data demonstrating this in the manner you are
seeking - mainly because dewetting is an indication of a surface which will
not readily form a good metallurgical joint. If you can not reliably form a
good metallurgical joint then the reliability of the assembly is in
question. There is no need to conduct a study demonstrating this because it
is basic physics - no metallurgical bond, no reliable joint. I'll take a
look in my file to see if I find anything that may be of help. It is
possible to get solder to "solder over" dewetting surfaces - it looks good
but will result in latent field failures. And as for your Purchasing folks
believing that the IPC standards are not useful - let them explain why the
field failures are occurring! Good Luck.

Dave Hillman
Rockwell Collins/ ANSIJ-STD-003 Committee Chairman
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Kathy Palumbo <[log in to unmask]> on 07/29/99 12:00:07 PM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
      to Kathy Palumbo <[log in to unmask]>

To:   [log in to unmask]
cc:
Subject:  Re: [TN] Dewetting Issues




Werner,

I have sent modules out for thermal cycling from 0C to 100C.  500 cycles.
2
minute ramp. 1 minute transfer. 10 minutes hot.  10 minutes cold.  The
modules will be powered up during the cycling process.  The modules will be
pulled from the chamber every 25 cycles for full functional system testing.
The ones that fail will be analyzed and cross sectioned.  The modules that
pass will go back into the chamber for more cycling.

The modules selected for cycling were worse case (dewetting more than 40%
of
the pad), moderate (dewetting less than 30%), and mild (dewetting 5% TO
15%).  The dewetting is random in location, and some areas on the PCB are
worse than other areas.  Hopefully with this test we will get a good feel
as
to what level of dewetting is okay or not okay.  I expect the worse case
modules to fall out first, then the moderate, and then the mild.

As a side note the same exact design bulit on known good PCB's with no
signs
of dewetting were cycled and tested in the same exact way, and all made it
through cycling/testing with no failures.  This was a requirement for
qualifying in the design for our OEM customer.

I was really hoping someone might have the same kind of test data, so it
could back up the test data I will have.  So far there has only been the
metion of the studies done at china lake back in the 70's and 80's, which I
have not been successful in finding as of yet.  Anyone who has done any
temperature cycling/cross section with regards to dewetting and is willing
to share that data I will be forever greatful!

As always Thank You to everyone who responded for your wonderful support
and
information.

Sincerely,
Kathy Palumbo
Sr. Mfg. Engr.
Viking Components, Inc.

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