Amol
I published a paper on this issue in this months PC Design and Fabrication.
In the paper, I go through a rigorous mathematical analysis of the peel
strength measurement. Essentially, I found it does not measure the bond
strength of the cooper, but rather a vector combination of shear and tensile
stresses. For thermal failures the desired measurement is the shear strength
of the bond. The plane and magnitude of the vector is highly dependent upon
the radius of curvature and thickness of the copper being peeled from the
substrate as well as the peel angle. In fact the dependence of the peel
strength is exponential dependent upon all three variables. So by very minor
adjustments you can rotate the stress vector direction and magnitude and get
a significant modification in the result (peel strength). We have all known
this to be the case for years, I have only quantified the analysis. Keep in
mind the ultimate tensile strength and the ultimate shear strength vary by
an order of magnitude. So minor changes in the variables mentioned above
produce significant changes in the peel strength.
What I proposed in the article is the use of a double lap joint that is
pulled to failure. The test vehicle I proposed creates only a shear stress.
This should be a huge improvement over the peel strength measurement.
----- Original Message -----
From: "Kane, Amol (349)" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Friday, August 25, 2006 7:29 AM
Subject: [LF] Copper Pad Peel Strength
Dear Technetters,
We are having instances where copper pads (Under BGAs) are being lifted off
ENIG Boards almost too easily during rework. We use the same thermal profile
on each board (for the same component removal) and only some show this
phenomenon. Is there a test or standard to test the copper adhesion @
incoming to see whether it's the fab (i.e. are the boards coming to us with
poor copper adhesion strength to being with) or the process that's doing
this. The board is heated upto 120 C when the component is reflowed for
removal. Also, is there data to show the decrease in Cu adhesion strength
with temp?
Thanks,
Amol Kane
M.S (Industrial Eng.)
Process Engineer
Harvard Custom Manufacturing, Inc.
941 Route 38 Owego, NY 13827
Phone: (607) 687-7669 x349
[log in to unmask]
www.harvardgrp.com
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