I suppose that you can say a null result does not prove anything in the
sense that if God doesn't strike you dead for a sin done doesn't prove that
God does not exist nor that the sin was not a sin.
However in the terms of the test plan, there was no observable change over
a period of time does prove that nothing happens under those conditions. I
think that to prove that the test is invalid it may be necessary to prove
that a sample that always fails under these conditions did not fail. Then
it would be a null result.
I believe that tests are now being considered which do generate the
failures looked for in some fairly reproducible fashion and these should be
considered controls for such experiments.
James P. Canner
Senior Applications Engineer
Murata Electronics North America
1900 W. College Ave
State College PA 16801-2799
(814)-237-1431 x 2032
[log in to unmask]
Werner Engelmaier
<[log in to unmask] To: [log in to unmask]
OM> cc:
Sent by: Leadfree Subject: Re: [LF] Lead free migration info
<Leadfree@listser
v.ipc.org>
07/09/2003 09:33
AM
Please respond to
"(Leadfree
Electronics
Assembly Forum)";
Please respond to
Engelmaier
Hi Doug,
My comments were indeed directed to TI app note SZZA026 titled "Evaluation
of
Nickel/Palladium/Gold-Finished Surface-Mount Integrated Circuits".
The full quote is "The specific comment you reference is on page 4 of app
note SZZA026 and is
as follows: "Those studies showed that NiPd and NiPdAu finishes achieved
equivalent, or better, lead-pull and temperature cycle results versus SnPb
plated
component leads (control). Any difference in performance of the different
lead finishes (SnPb, NiPd, NiPdAu) was merely visual."
I have no problems with anything but the underlined words--a null-result is
a
null-result, no conclusions can be drawn from it. If you did not get any
damage, let alone failures, in 3000 cycles of -65 deg C to +150 deg C, then
there
is something seriously wrong with your experiment. Removing test vehicles
from the temp cycle chamber at various read points and tested electrically
them
is a serious no-no--see IPC-SM-785 and IPC-9701; this may be part or all of
your problem.
To say: "Yes, you're correct that there were NO failures on temp cycle
testing, but
the statement is also correct in that the NiPd and NiPdAu units achieved
equivalent temp cycle results at the 3K read point compared with SnPb
plated
components -- none failed." is at best misleading. Given both the difficult
nature of LF-solder reliability and the controversial nature of LF-solders
per
se, one cannot treat this issue this cavalierly.
Further, there are no industry standards for lead pull data--there is not
even industry standard for a lead pull test; whether or not lead-pull
results
before and after temperature cycling
were acceptable, or in fact what they actually mean for the performance of
the product is an open question.
Regards,
Werner Engelmaier
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