Hi Ken & Richard,
This has been a long ongoing debate.
The work done for IPC-TR-579, 'Round Robin Reliability Evaluation of Small
Diameter Plated Through Holes in Printed Wiring Boards', indicated a small
improvement in reliability with the presence od functional and non-functional
innerlayer lands. This was NOT a major finding of this study, nor was it a
recommendation for the use of non-functional innerlayer lands.
The results of the just released ITRI Report, 'PWB Hole-to-Land
Miregistration: Causes and Reliability', give the same indication (Note:
essentially all the failures come from one PWB manufacturer, primarily from
PTV barrel cracking, before the test was stopped at about 1100 cycles; the
other manufacturers show some interconnect separations of the innerlayer
trace from the barrel). The 1997 ITRI report also shows the vast improvement
in the quality of PTV structures since the 1988 IPC-TR-579 study.
The ITRI study was done with temperature cycling from -65<->+125C; thus, the
reliability results directly, on an accelerated basis, relate to the thermal
cycling PWBs see in product operation. However, because no excursions were
taken to soldering temperatures, where because of being above Tg of the epoxy
significantly more thermal expansion mismatch occurs and the epoxy is much
softer, the results can not be assumed to be representative of what might
happen during PWB assembly. It is during solder temperature excursions that
the compressive hydro-static hoop stresses act on the PTV barrel and that
surface and innerlayer land rotations occur. These represent quite different
loading conditions than -65<->+125C temperature cycling, and may produce
different conclusions.
CONCLUSIONS: Non-functional lands are not likely worth having, because the
small potential reliability gain for high-reliability PTV structures for most
applications in not worth the manufacturing difficulties and costs that have
been encountered. Misregistration and the absence of annular rings are no
significant reliability threat to PTV barrels. Assuming, that the behavior
during solder temperature excursions is not dramatically different and the
PTV interconnect structure survives assembly, misregistration and the absence
of annular rings are no significant reliability threat to PTV interconnects
(Interconnects can be protected from large loads by having a 90 degree trace
jog just before the PTV, i.e. do not approach the PTV with a long straight
trace).

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]