The comments by  Alan Cochrane need amplification:
Temperature measurements during profiling on PWBs at and in PTHs show that
the temperatures in and near the PTH copper barrel are as hot as anywhere on
the assembly (the PWB and the PTH lands are hotter than most solder joints
reflowed). The resin distant from the PTH barrel of course heats slower due
to its lower conductivity.
Tensile tests are not very good in determining ductility of foils, because of
the large aspect ratios of thickness:width and thickness:gage length. That is
why separate ductility tests (ASTM E-796, Standard Test Method for Ductility
Testing of Metallic Foil, and IPC-TM-650, Test Method 2.4.2.1, Flexural
Fatigue and Ductility, Foil) exist. One of the artifacts of the inadequate
elongation measure is the misleading result that elongation goes up with foil
thickness when in fact ductility does not. 
Even in full built electroless PWBs, that have uniform plating thicknesses
and properties throughout the PTH barrel, do PTH barrel cracks occur near the
center of the PWB (see IPC-TR-579, Round Robin Reliability Evaluation of
Small Diameter Plated Through Holes in Printed Wiring Boards). For
electrolytically plated PWBs the copper deposit near the center of the PWB
can indeed be thinner and of lower quality (strength, ductility) because of
the plating characteristics. 

Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 201-543-2747
E-Mail: [log in to unmask]

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