Hi Les,
I don't know about the 'Ring and Ball' experiment, but I do know what happens
inside a PWB around the PTHs during heating to solder temperatures.
(1) At those temperatures the epoxy is way above its Tg, is very soft, and
should be considered a high-viscosity liquid as far as behavior is concerned.
(2) For analysis as to what is going on around a PTH, the PWB can be
considered an infinite plane in the x- and y-directions.
(3) While the epoxy tries to expand in all directions, it is constrained in
the x,y-directions by the glass reinforcements.
(4) Thus, the primary expansion is in the z-direction at roughly 3 times the
linear themal expansion, because all the volumetric expansion is funneled
into the z-direction.
(5) The PTH hole, however, allows the epoxy to also expand into the space
occupied by the PTH if the PTH barrel wall is thin enough to by compressed.
That is why in cross-sections of PTHs with 1 mil of copper plating and less
one sees the PTH barrel bowed inward between innerlayer lands, particularly
in prepreg layers (higher resin content, less cure) and when there are larger
distances between lands (no non-funcional lands), and sometimes a space
between the barrel wall and the reseded epoxy. If there were no copper
plating, the hole diameter would decrease on heating.
(6) Thus, the PTH barrel in in tension in the z-direction, with the largest
stresses near the PWB center, the PTH barrel experiences compressive hoop
stresses, innerlayer lands experience tensile stresses at their attachment to
the PTH barrel, and innerlayer lands near the PWB surface experience bending
moments due to land rotation resulting from the z-direction expansion of the
epoxy surrounding the PTH barrel.

Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 973-543-2747
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