Bob,
Do I understand that you want to use an underfill style material to
help heat flow away from BGA's which are running hot during operation?
Should be ok (in theory) there are a wide range of underfill
materials available, and the usuall suppliers should be able to help
you with data sheets and samples (Emmerson and Cummins, Alpha, Dexter,
Loctite etc) The thing to be aware of is that these "traditional"
underfills are designed to be used with flip chips, and in small
volumes, you are proposing underfilling three BGA per board, and that
could be potentially expensive, so expensive it may be prohibative
Loctite were working on an underfill specifically for CBGA, and that
mey help, or be more cost effective. I suppose the other potential
advantage could be an improved reliability, the epoxy should be
sharing the strains with the solder balls, similar to flip chip
designs, and this could give an improved lifetime. On the downside,
if the BGA is running real hot, then the underfill will need a high Tg
to minimise thermal mismatch during service, and stop an accelerated
failure of the balls. In order to get this, you may need a pretty high
cure schedule, which could exceed the Tg of your FR4, and give
problems elsewhere. I guess the way round that would be spec a high
Tg board (160C+) but that will cost!!
In terms of processing, I would assume its much the same as a flip
chip, attach the part, possible clean depending on which flux
technology is being used, and underfill. (this will take longer than a
flip chip, and a multi pass dispense may be needed to ensure 100%
coverage under the BGA) Scanning Accoustic Microscopy, or possibly
Xray could be used to check the underfill flow. One major point to
consider is the need for rework. I know we shouldnt mention it, but
we all need to rework something, some time for some reason, and if its
gunked up with epoxy, "you is gunna have a problem" there are
reworkable underfills available, but they can need a two stage cure,
with rework being carried out before the final cure, again suppliers
should be able to help with that aspect.
All in all, if there is a real need to underfill a BGA try it out,
but beware, the potential cost is high, in terms of additional process
routes, equipment and also material cost. If you want to talk more,
contact me anyway you can.
Roger
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______________________________ Reply Separator _________________________________
Subject: [TN] Underfill, BGA???
Author: "TechNet E-Mail Forum." <[log in to unmask]> at #email
Date: 01/07/99 14:12
Hi all..
Have a question relating to cooling a HOT 225-ball
BGA. I tried to look in the archieves, but my search
technique must be lacking or I am using the wrong
"words". Can I use an "underfill" to help cool a hot
BGA or is that the wrong term? I am considering using
a thermally conductive, electrically insulating
material under the part (3parts/assy), but have no
hands-on experience with "what material, how do I apply
it and what are the related war stories". Any help out
there in TechNet land???? Thanks in advance, have a
great day and a super/safe 4th holiday!!!!!!!!!
Regards,
Bob Vanech Bob
Mango Computers
Norwalk, Ct.
(203) 857-4008 x 108
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