The "shock" that you describe is pretty low temp
excursion compared to what our US typical boards
must withstand. Our test is 550F for 10 seconds.
The fact that you say the cracks sometimes don't
go thru the copper into the laminate makes me think
the problem is coming from the solder mask
filled via and not the z-axis expansion of the laminate
substrate.
Have you tried baking longer?
How about holes which are not plugged with solder mask,
are they exhibiting the same problems?
Susan Mansilla
Robisan Lab
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