Hi Chris! Part of the background Doug listed was ".......certain design
engineers who absolutely, positively believe that every ball on a BGA must
be coated with conformal coating......" and unfortunately these same
design engineers also believe that either (a) only a conformal coating
material stops humidity and/or (b) an underfill material costs too much
and takes too much time to apply to the BGA. We do successfully use
underfill in a number of applications where it is needed for solder joint
integrity purposes. However, there are times when a lack of understanding
of a material's characteristics and its application purpose seem to
befuddle a design team's actions. Except for specific use environment
applications, it is not necessary to coat every individual solderball on a
BGA or CSP - the standard process of applying conformal coating to a
printed circuit assembly is quite adequate for many, many use
environments. It is amazing how applying the laws of physics and
conducting a bit of testing can demonstrate the "bigger the glob, the
better the job" theory isn't advisable.
Dave
Chris Mahanna <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
11/30/2009 09:04 PM
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Re: [TN] BGAs & Conformal Coating
Hey Doug and Dave,
out of curiosity-
So why not use an underfill material as the coating? Do you not trust the
electrical properties of the underfill, or is it a processing issue (e.g.
wetting,voids...)?
Chris
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Douglas Pauls
Sent: Monday, November 30, 2009 11:52 AM
To: [log in to unmask]
Subject: Re: [TN] BGAs & Conformal Coating
Thank you Steve for posting the pictures. Here are the links.
http://stevezeva.homestead.com/files/deformed_Balls.jpg
http://stevezeva.homestead.com/files/Squished_Ball_1.jpg
http://stevezeva.homestead.com/files/Squished_Ball_2.jpg
To give a little background, we have certain design engineers who
absolutely, positively believe that every ball on a BGA must be coated
with conformal coating or it will fail in humidity. They put silly notes
on the engineering drawing to this effect, resulting in conformal coating
being used like an underfill or a potting material. Dave Hillman and I
did an internal study this last year to demonstrate the effects. We
forceably filled the area under the BGAs with conformal coating, one a
popular solvent based acrylic, the other a water based acrylic urethane.
We left the material come to a tack free condition, fully cured the
material, and then put it through thermal cycling, monitoring resistance.
The cycle was -55C to +125C, 15 min dwells at either extreme, 8-10C/min
ramp rates. The test ran for a little over 2100 cycles. The solvent
based acrylic had a 73% failure rate by the end of the test. The water
based acrylic urethane had none. The pictures are all of the solvent
based acrylic, which has a higher CTE than the acrylic-urethane. We
tested BGAs, QFPs, a couple of QFNs, and some SOICs. The only significant
failures were for the BGAs. Can't really say much more than this.
Doug Pauls
Rockwell Collins
"Steve Gregory" <[log in to unmask]>
11/30/2009 07:33 AM
To
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Subject
RE: [TN] BGAs & Conformal Coating
Hi Doug!
Send them here: [log in to unmask]
Steve
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Douglas Pauls
Sent: Monday, November 30, 2009 8:29 AM
To: [log in to unmask]
Subject: Re: [TN] BGAs & Conformal Coating
Good morning Juan,
I suspect that Xilinx wants you do do a thermal cycle study on solder
joint reliability when conformal coating gets under the BGA. The stresses
brought about by a mismatches in the coefficient of thermal expansion
(CTE) can rip a BGA off the board and can severely deform the balls.
Hillman and I did such a study here a few years ago. I can share a few
pictures of what happens, if Steve Gregory would be kind enough to forward
his present e-mail address for the photos. Not sure I have the right one.
Doug Pauls
"Juan T. Marugán" <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
11/26/2009 11:53 AM
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[TN] BGAs & Conformal Coating
Hi,
I've read in a Xilinx guide that: "Xilinx has no experience or reliability
data on
flip-chip BGA packages on board after exposure to conformal coating. It is
recommended that the end-user should characterize the board level
reliability
performance of Xilinx packages before production use."
Does anybody know any infomation or study related with this topic?
Thank you.
Juan T. Marugán
Indra Sistemas SA
Espańa
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