Hi Steve - yes, the BGA void papers we did don't directly apply to your
case but there are some parallels. When you say "must pass 15000 thermal
cycles", do you mean having the thermistor electrically function correctly
or are you describing solder joint integrity?

Dave Hillman
Rockwell Collins
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On Fri, Feb 13, 2015 at 1:31 PM, Steven Kelly <[log in to unmask]> wrote:

> Hi All,
> I have some queries that is a little off the wall- at least for us. We are
> soldering an aluminum nitride thermistor onto a 2 layer flex. Under the
> thermistor there is mostly a ground plane and there are vias through the
> ground to the other ground layer (basically thermals) . We also reflow a
> .015 solid copper stiffener/heat sink on the other ground layer  of the
> flex.
> I have read Mr. Hillman’s papers on voids but in this case we seem to find
> if we “smash” the solder paste to reduce the voids when we run the finished
> unit we pass more thermal cycles – I think in reading Mr. Hillman’s papers
> that makes sense ( I hope).
> Solder paste is water soluble SAC 305.
> We also find if we glue the thermistor down and not “smash” the solder
> paste we pass more thermal cycles- I think this also makes sense to me.
> We are going to run one more set of tests doing both the smashing and
> gluing to see what happens.
> We need to pass 15000 thermal cycles - -40 to + 85 but we think we are
> seeing deterioration in the thermistor- any thoughts on how we look for
> that – what kind of analysis can be done for this?
>
> Thanks. Steve Kelly
>
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