This might help:
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Werner Engelmaier
Sent: Friday, May 07, 1999 4:41 PM
To: [log in to unmask]
Subject: Re: [TN] Unconnected inner-layer pads
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 of 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 1997 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. This is particularly true for todays smaller diameter
PTVs.
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.
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com
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