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 E-mail: [log in to unmask]