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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.

  

Posted by Sebastian: Thu Oct 20, 2005 4:35 pm    Post subject: Precon to Bias HAST

 

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.

 

Posted by Guest: Fri Oct 21, 2005 7:41 am    Post subject:

 

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.

 

Posted by Guest: Fri Oct 23, 2005 9:53 am    Post subject:

 

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.

 

Posted by Sook Fong: Tue Oct 25, 2005 5:23 pm    Post subject:

 

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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