EESemi.com Forum Archives
Storage of Precon
Samples Prior to Rel Testing
Preconditioning is a step
performed on reliability samples to simulate board mounting prior to
reliability testing. It is basically a Solder Heat
Resistance Test (SHRT) done before certain reliability tests like
THB, HAST,
TST, and TCT. Ideally,
the reliability test must immediately follow after preconditioning of
the samples. Unfortunately, this is not always possible,
especially for a busy reliability lab where loading priorities can
change in an instant. Thus, it is sometimes necessary to store
preconditioned samples for a while before they can be
reliability-tested. If such a case arises, it is important to
store these samples properly to ensure that environmental factors will
not affect them in any way and result in invalid reliability failures.
The archived forum thread below discusses this.
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Posted by Sebastian:
Thu Oct 20, 2005 4:35 pm
Post subject: Precon to Bias HAST |
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Guys,
Just curious...
If I have two sets of samples coming from the same lot
and subjected in the following conditions:
1. Precon/MRT and staged for a month inside N2 cabinet,
then HAST.
2. Precon/MRT and immediately do HAST.
After HAST, both lots were subjected to
Test/OS/Parametric.
Would there be a bearing in test results, esp. OS?
My point of argument is that the staging of units after
precon (as in Condtion 1) may have aggravated the
moisture that seeped through the package during precon
and with prolonged time, degraded the adhesion of the
ball to the bonding pad which will result to "Opens".
Condition 2, which virtually t=0 from precon to HAST
then Test may be better.
I understand too that there is no governing specs
defining the staging time from precon to HAST?
Appreciate some inputs.
Thanks. |
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Posted by Guest: Fri
Oct 21, 2005 7:41 am Post
subject: |
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The idea for precon is to
simulate the stress during board mounting of the parts. And
supposedly after precon the packages have been saturated
with moisture and subjected to thermomechanical shock.
So no matter how many days or condition the units are
staged there should be no more additional effect because
the worst is over. Thus, I
believe the staging should not affect the
result. |
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Posted by Guest: Fri
Oct 23, 2005 9:53 am Post
subject: |
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Just make sure the staging is
done properly to prevent contamination or oxidation of
leads and package because these can lead to invalid
failures and confuse the rel results. |
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Posted by Sook Fong:
Tue Oct 25, 2005 5:23 pm
Post subject: |
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Hi Sebastian,
The moisture absorbed during precon is to simulate
popcorn failure during reflow. If the packages pass precon, it is fine.
I'd recommend to keep the parts in dessicator or seal
them in Moisture Barrier Bags with dessicant before
HAST. I did that once and the packages passed just
great, yielding same results as the sister lot that went
straight to HAST without delay.
Your concern about ball adhesion to the bond pad applies
more in High Temp Storage test.
Good luck,
SF |
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