Brittle copper on the Surface:
    As several have mentioned: if you have brittle copper on the surface,
what happens in the holes; probably a lot more and the strain is a heck of
lot greater in the holes.  Suggest you test you copper plating per IPC-RB-276
par. 3..5..8.3 which specifies test method IPC-TM-650 Method 2.4.18.  Brittle
is inherent when there is either organic and/or metallic contamination of the
plating solution and it does not take very much, a few parts per billion of
certain compounds or elements will make the ductility go south.
Usual treatment is a peroxide/carbon treatment or dummying at a low current
density to remove unwanted metals, or send a sample to you chemical  supplier
for a better analysis.  You might pull of a conductor of of a board and
crease with the plating out and see what it does.  Some people use a bend
test in place of the tensile/elongation test, but the number of  bends before
breaking must be quantified with clad copper thickness, overplate thickness
and other conditions.  Suggest yoou attack contamination in the copper bath .

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