The aspect ratio is definitely the ratio of PWB thickness and drilled hole
diameter. While specifying the finished hole diameter of a PTH that needed to
accommodate a component lead made sense, it is much more logical to specify
the drilled hole diameter for PTVs. The finished hole diameter for a PTV is
an irrelevant parameter; what is important for the quality and reliability of
a PTV is drilled hole diameter (it is more difficult to plate into holes with
smaller diameters in terms of plating uniformity and copper properties) and
PWB thickness (it is more difficult to plate into longer holes in terms of
plating uniformity and copper properties and longer holes also create larger
stresses and strains on the PTV copper barrel, everything else being equal).
Thus, aspect ratio is not a determining parameter for reliability, but PTV
diameter and PWB thickness are; the aspect ratio only serves as a 'red flag'
when it is larger than 3:1, that a 'Design for Reliability' may be necessary.

In this context my workshop "Design for Reliability and Quality Manufacturing
of Plated-Through-Holes and Vias", next given at NEPCON West'97 may be of
interest. 

The design information presented in this workshop has been included in the
industry document IPC-D-279, 'Design Guidelines for Reliable Surface Mount
Technology Printed Board Assemblies'. 

Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 201-543-2747
E-mail: [log in to unmask] 

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