Just my my 2 cents:

I think the tensile strength of the plated copper was reduced
maybe a lot of impurities at your copper plating bath.
Sludge from anodes that drops down maybe one impurity.
You bath need activated carbon treatment or your anodes need to be cleaned.
If the Plated copper at the corner will crack upon subjected to thermal
stress, this
means that the tensile strength of the copper is poor.
So i suggest to clean your plating bath.

thanks

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