A padless PTH is not a blind via unless it is closed off at one end; than it
would be unplateable in an .093" PWB. Occlusion of either PTH opening would
indeed result in mass transport limited plating were the Cu ions in
inadequately replenished. The quality of the copper deposit would suffer in
terms of greatly diminished physical properties (strength, ductility) and
plating thickness uniformity (dog-boning) with reduced reliability
consequences. 

A padless PTH may require for processing reasons a small pad of a size
dictated by the registration accuracy. A padless PTH by definition has a rim
void on the padless side and therefor there is no weak 'knee' or shoulder;
without any connections on the O/L that is of no concern. I would not
recommend reducing the PTH diameter; smaller diameter PTHs have lower
reliability (see IPC-TR-579). 

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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