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





William Raymond wrote:

> Hello all...
>
> I’m in need of some opinions. Our test and repair department (functional
> test) uses a cold spray (from a can) and hot air guns (heat-shrink heat
> guns) to heat and cool BGA devices (plus other IC devices like QFP’s, etc)
> to help diagnose boards that fail functional test.  I’m trying to make the
> point that this practice is very risky, especially to BGA type devices
> because of the severe stress applied to the solder ball connections.  The
> application of cold spray and/or hot air from a hand held heat gun are both
> VERY uncontrolled processes and should be eliminated from our procedures.
> Would anyone out there be willing to share their views and/or experiences
> with this?
>
> Thanks in advance,  Bill
>
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--
Darrel Therriault
VP, Mfg. Operations
INCEP Technologies, Inc
(858)547-9925 223
[log in to unmask]

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