Hi Inge - I'll take the baton from Doug for this explanation. I know this
seems "weird" but the combination of differences in expansion rates and
compliancy between the solderballs, the BGA package and the conformal
coating material result in the solderballs literally being squished to
death. I didn't believe this to be possible until one of my mentors showed
me an example of a solder joint and potting adhesive combination that
pushed a TO-18 thru hole component right out of its plated thru hole
solder joints. Solder has reasonable tensile strength in electronic
applications but if you put in under a cyclic compression load, it will
creep over time. As other TechNet contributors have mentioned, if you get
the wrong combination of materials, you will actually do more damage to
the solder joint that protect it. The underfill material formulators spend
a considerable amount of time making sure their underfill materials work
in unison with the BGA. Hope that make sense.
Dave Hillman
Rockwell Collins
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Inge <[log in to unmask]>
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11/30/2009 03:15 PM
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Re: [TN] BGAs & Conformal Coating
Doug,
how can that be possible? When temperature rises, the coating ought to
soften and should expand even horisontally, and when at lower
temperatures,
the coating should crimp. The only way I can see is if you have the
situation of a closed in liquid, which one cannot be compressed.
BGAs use to be very hard to rip from the board. I measured some 350 kg
force
necessary to pull off a very large BGA.
Inge
----- Original Message -----
From: "Douglas Pauls" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Monday, November 30, 2009 2:29 PM
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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