Thanks Darrel for your thoughts... as an added note, the test group only
uses this method to help isolate faults, not as any part of some thermal
shock type of test.  These "faults" usually being of the intermittent
variety.  I am simply worried that the method of freeze mist/hot air gun
fault isolation is only needlessly stress solder joints (especially the BGA
joints) and that we should find another method to isolate there
intermittent board faults.

Bill...


At 07:57 AM 10/08/2001 -0700, you wrote:
>William,
>
>As an old test engineer, I used to use cold spray (freeze mist) to try and
>verify a component
>was defective that seemed to be running hot compared to other components
>temperature at the
>same ref des.  It was usually a focused blast of freeze mist, not a
>continuous stream and then
>only if a component seemed to be functioning and would blow up after some
>run time and seemed
>to be hotter to the touch then other components at that location.  I
>didn't use freeze mist to
>emulate thermal shock by blasting away at a component and certainly not a
>BGA contact area.
>
>I didn't use heat guns because they are pretty uncontrolled as you state
>and even if you stuck
>thermal couples on tops of the packages, it still is pretty uncontrolled
>and even if you have
>fixtures to focus the heat on the component in question, it still seems
>like a potential risk
>to the other components reliability so why take it.  I have seen people
>heat up BGAs to see if
>opens start to make contact, which probably says you then need to replace
>or reflow a certain
>BGA, but again, the risk to the reliability of other components can
>present a bigger problem
>with component/assy MTBF.
>
>In general, get your test coverage high through good DFM/DFT design and
>layout and you won't
>have to be so creative with heat guns and cold spray.
>
>DT

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