Dear Dr. Bloomquist, Werner, and Bev,
First of all, I don't care where MIT is. The best education you can get
is still in the Institute of Hard Knocks on the University of Minnesota
campus.
Werner is correct in stating there is no standard for determining solder
joint reliability through sheer testing. There are too many variables
that affect solder joint reliability, and there is no way to provide
testing for every single combination of pad size, solder joint shape,
pad finish, component finish, etc, etc.
The best method would be something that is done from a peel strength
standpoint, but there would be no point in attempting to put a pass/fail
spec on it. How would you possibly determine that?
You can perform shear testing from the perspective of determining which
combination gives you the best shear-through rate, but only for the
purpose of comparing it with another average (variation from mean) of a
different combination of factors.
For example, you can solder two identical PWBs with different surface
finishes such as IAg and ENIG, using the same print process, place
process, and reflow profile, and solder alloy. Then you can perform
shear testing using apparatus such as a Dage shear tester to compare,
for example, how many of a given set of solder balls soldered to pads
shear away without fracture, how many break off, and how many tear off
of the board with the pad still soldered on. You will find that it takes
about 1800 kg to fracture away a group of balls soldered to ENIG. You
will also note that it requires 2200 kg to shear through the same set of
balls soldered to IAg, and finally you will notice that nearly 100% of
the balls actually fractured prior to completely shearing off in the
ENIG group (no pad pullaways), as opposed to 90% fully shearing off (no
fracture of the solder joint) and 10% of the pads being pulled in the
IAg group. These ratios change with different variables. With SAC alloy,
for example, the number of full-shears goes way down on the IAg group,
but the number of pad pullaways does not, instead the number of
fractures increases proportionally.
And so forth.
I can refer you to IPC-9701A Par. 4.2.1.2 for instructions in performing
this type of test. I can also refer you to JESD22-B117 where you will
find more information on how to do this type of analysis. Please note
that the rate of shear (500um/sec) is critical. Variation either way
skews the results.
But just because more of the balls in the ENIG sample(s) break off does
not necessarily mean that combination is less reliable, or more reliable
than the IAg combination. It only means for high initial force
application in a lateral direction the one solder joint is less likely
to fracture immediately than the other. It does not mean that the IAg
combination will survive longer than some other type of reliability
testing, such as thermal cycling to failure. There are too many other
variables that come into play.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
Sent: Friday, March 16, 2007 1:54 PM
To: [log in to unmask]
Subject: Re: [TN] SMT Solder Joint Strength
Hi Gregg,
Last I looked MIT was in Cambridge.
Solder joint strength is never the issue with a properly 'wetted' solder
joint. Under sufficient force you will rip the soldering pad out of the
resin matrix, but not separate the solder joint.
The strength, or more accurately the yield strength, comes into the
picture when thermal cycling, because the yield strength is highly
temperature-dependent [as are almost all other properties of solder].
When T-cycling leadless solder joints the cyclic stress range will be
bounded by the yield strength of the solder at the prevailing T; the
reason why compliant leads work is that they do not allow the stress
range to come close to the yield strength=f(T).
The high variability of lead geometries, and some of the above, make SJ
strength standards impractical to say the least, and even if you could
do it, what would you do with the information?
What is much more important is the failure mode-either brittle
interfacial
showing there is something wrong with the SJ arrangement, or pad
rip-out
meaning the SJ is good.
Werner
P.S.: Bev, which part of the above do you disagree with?
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