Hi Earl,
ILS was previously referred to 'post-separation.'
Actually, the forces are not exerted diagonally to the trace/pad to conductor
interface, but radially relative to the PTH barrel. This is the result of the
PTH barrel experiencing the same compression deformation as a submarine on
diving; except, of course, that a submarine has its reinforcement ribs on the
inside and the PTH barrel on the outside [that's the functional and
non-functional lands]. The traces resist the compressive movement of the
barrel and failure can occur at the weakest of 3 links [ plating/flash
interface, flash/foil interface, foil]. When the compressive deformation is
plastic for the copper barrel, hole separation occurs as the resin returns to
its former position on cooling.

Werner

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