Hi Roger,
Thicker copper deposits have indeed greater (but not better) elongation
values than thinner deposits. This, however, does not mean that thinner
deposits have less ductility; the lower elongation value is an artifact of
using a test method which is inappropriate for sample geometries like plated
copper foils. Elongation tests work well for specimen for circular and square
cross-section; foil specimen have a cross-sectional aspect ratio of 500 to
1000, which creates this type of artifact.
This is the reason test methods specific for foils (ASTM E 796 "Standard Test
Method for Ductility Testing of Metallic Foil," IPC-TM-650, TM 2.4.2.1
"Flexural Fatigue and Ductility, Foil" ) were developed. These test methods
show there is no difference in ductility for thin or thick foils from the
same batch.

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