Could it be that the corner ball is a redundant circuit (or an unused I/O),
also being cracked but not detected in test?
Jim Marsico
Senior Engineer
Production Engineering
AIL/Electronics Systems Group
An EDO Company
[log in to unmask] <mailto:[log in to unmask]>
-----Original Message-----
From: <Peter George Duncan> [SMTP:[log in to unmask]]
Sent: Wednesday, March 28, 2001 7:25 PM
To: [log in to unmask]
Subject: Re: [TN] BGA crack- major headache
It is the fact that it's always the 2nd last ball on the top outside
row
that I find interesting. Is there no damage to the adjacent corner
ball, as
this is often the first one to go in CTE differences/creep fatigue
situations? Thinking aloud for possible causes:
Is the substrate material used for the failing boards different
from the
other products on which the BGA is used?
Is it of a substantially different thickness?
Is the board reaching high temperature in use, or is it subjected
to
wide temperature varience?
Is this product subjected to greater levels of vibration and/or
flexing
in the field than other products?
Is there a feature on or in the board that is acting as a very
good
heatsink just under or close to the failing pin? If everything
else is
heating up and this ball isn't, it's relative lack of expansion
might
pull the joint apart at its weakest point.
Is there an inherent weakness in the BGA design or manufacture at
this
particular ball position? Is there anything different about the
attachment point of this ball in comparison with the others if
this
device?
Have you microsectioned any of the failed boards? Is the form of
the
solder joint significantly different from the others - shape,
size,
cleanliness, grain structure, etc?
Have you tried doing a peel test on a sample board that hasn't
failed?
Do you suffer from Poltergeists?
Any combination of the above?
I can't give you a definitive answer as to what causes the problem,
but do
you use any underfill material with your BGA's? If not, you could
try using
a urethane or epoxy underfill to support the solder joints - enough,
maybe,
to hold the thing together. There are thermally conductive types of
which
there are types that flow under the component and others that don't
flow
under the component. I suggest you use a type that flows under the
component. Although it makes it more difficult to remove the BGA
again
later, its thermal conduction properties may serve to even out any
differences in joint expansion and preserve the weak one.
Hope something here inspires some better ideas for you.
Regards
Pete Duncan
Asst Principal Engineer
ST Aerospace
Rudolph Yu
<ylt@EARTHLIN To: [log in to unmask]
K.NET> cc:
Sent by: Subject: [TN] BGA
crack- major headache
TechNet
<[log in to unmask]
ORG>
03/29/01
12:48 AM
Please
respond to
"TechNet
E-Mail
Forum.";
Please
respond to
Rudolph Yu
Here are the facts:
PBGA-272 balls
FR4 with HASL finishes 8 layers
The failure point always happens at the same I/O which is the 2nd
last ball
of the top outmost row of the package. It is not located near the
edge of
the board or any breakaway point.
Failure mode
Micro fracture found near the intermetallic layer between the BGA
package
and the solder ball attached to it.
Around 0.001% of the products we built failed in the field because
of this.
None of these were caught during the ICT or Functional test.
The same ASIC is also used on several other Products and have never
seen an
issue like this. Somehow this failure mode with this ASIC only
occurs in
one particular product /design.
The ASIC / fab lot-related , ICT pin interference, stress by the
breakaway
tab, and stencil cleanliness assumptions had already ruled out after
a
controlled lot was built few weeks back. All boards passed the
tests. But
now some boards started failing in the field.
Why the crack always happen to one single location(ball) with the
same
product we built??
We have run out all the possibilities that we can think of. I hope
all the
experts in TechNet can share their opinions on this. Customer kept
asking
for the root cause analysis. Right now we just cannot came up with a
reasonable one.
Thanks
Rudolph Yu
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