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February 2015

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From:
Steven Kelly <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Steven Kelly <[log in to unmask]>
Date:
Fri, 13 Feb 2015 14:31:36 -0500
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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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