Hello Tom,
Provided there are no other mechanical flexure or severe thermal
stresses causing the secondary failures, then I suspect that the
underfill itself may be the cause.
Matching of the CTE properties of the underfill to the PWB material is
very critical. If the CTE of the underfill is not very, very close to a
match with the BGA, you could very well be creating a worse situation
than no underfill at all.
Also, you may wish to check to see if there was any hot gas rework done
on other components near the underfilled BGA. If there was, you may very
well find a correlation to units that failed. This is because if there
is a CTE mismatch, even a slight mismatch, the stresses placed on the
solder joints during a rework cycle due to the mismatch will crack or
shear the solder joints. This can happen due to the entire bottomside of
the assembly being pre-heated during a hot-gas rework, causing it to
expand rapidly, but the BGA is not being reworked and thus remains cold
or nearly so. This delta T can cause a substantial CTE delta between the
component and the PWB/underfill combination. This can happen from other
thermal excursions as well, such as something as simple as the component
getting hotter than the board during operation. Or vice versa. The fact
that you can turn the part off and on with a heat gun is a pretty good
indicator that there are significant CTE changes occuring with your
particular combination of PWB material, BGA material, and underfill.
Werner Engelmaier wrote an excellent article on this subject. Here is
the link:
http://www.globalsmt.net/index.php?option=com_content&task=view&id=33&It
emid=65
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gervascio, Thomas
Sent: Monday, November 12, 2007 2:34 PM
To: [log in to unmask]
Subject: [TN] BGA failures after OMPAC underfill
We have a problem - we are using Loctite 3568 to underfill a 19mm
plastic BGA- 256 pin 1 mm pitch eutectic solder BGA. We test the board
before underfill and after (after underfill and conformal coat). We are
seeing about 10 percent of the BGAs failing after the second functional
test- the same part passes the first test but will fail the second. The
boards fail for open solder joints- when heated with a heat gun they
work- intermittent open that seems to work when the part is heated
(expands)
I have reviewed the process but can find no smoking guns that might
point to the root cause of the problem. THe BGAs are all baked out prior
to reflow, THe reflow profile peak temp is adequate- a little on the
cool side for me- but the time above reflow doesn't seem excessive. THe
boards are not subjected to any other mechancial fastening operations
that might subject the BGA joints to strain. The board is not routed out
after assembly.
U10 is the BGA in question. THe peak temp is 212 C and is above reflow
for 71 seconds. Note the part is MSL Class 3 but the parts are baked out
and then stored in sealed Moisture Barrier Bags purged with nitrogen.
Anyone else run into something like this before? I have run into
situations where conformal coat under BGAs would expand and stress and
crack the solder joints but I would think that an underfill would be
designed to avoid this situation.
Thanks
Tom
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