No published references, just experience. I have seen recent issues
associated with reworking a board where components were removed from a board
to correct a surface mount problem. The board was not baked out adequately
to dry the component to be reworked. In this particular case, it was an
expensive, functional, flip chip component that was being reworked. The
component was a large BGA with a lot of Cu planes in the substrate,
including nearly solid P and G planes. And the die was very large.
Moisture desorption from such a package demands water to diffuse a long
distance lateral to the die, or through the thick, multi-plane substrate.
The inadequate bakeout induced popcorn delamination of the underfill to die
interface, and destroyed the device. I suspect this type of rework-induced
popcorn will occur with many types of components that are intended to be
reballed or reworked in some way and then re-placed back on the motherboard.
If the rework heat is not tightly localized, it may also be possible to
popcorn adjacent components that may also have saturated with water in a
high humidity environment. If the popcorn does not cause component failure,
a partial delamination due to the popcorn effect could lead to subsequent
field failure if the area of delamination increases over time in the
application environment, causing wire or solder bump breaks, or leading to
device overheating. So proper bake out of the entire board is important,
especially if the assembled board and components equilibrated with moisture
absorption in a high relative humidity environment prior to rework.
Best regards,
Leo
Director of Applications Engineering
ASAT, Inc.
3755 Capital of Texas Highway, Suite 100
Austin, Texas 78704
ph 512-383-4593
fx 512-383-1590
[log in to unmask]
www.asat.com
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-----Original Message-----
From: EMTC Ireland [mailto:[log in to unmask]]
Sent: Thursday, November 18, 2004 8:21 AM
To: [log in to unmask]
Subject: [TN] Field Failures and reworked PCAs?
Hi all,
I am working on a presentation to highlight the risks associated with
excessive rework cycles on a PCA for SnPb and ultimately Lead free
processes.
I was wondering if anyone from the Reliability world has any
pointers/references to show the actual rework process contribution as the
root cause of failure on field returns???? Just rough estimates/breakdowns
would be helpful (I wouldn't dream of asking you to stand over the data!!!)
All assitance appreciated
Rgds
Moss
Technical Chair
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SMART Group Ireland
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