Hi Phil,
The devil is in the details—however, 2-to-2.5mils standoffs = 2-to-2.5mils 
solder joint height are of concern even with small delta-CTEs [I am not even 
going near the cleaning issues].
With large thermal slugs, these packages may also be thermally non-symmetric, 
which means that they warp out-of-plane both during soldering operations and 
operation with any kind of power dissipation.
Ask them if their "reliability claims" are based on IPC-9701 testing.

Werner



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