Hi Richard,
Dogboneing typically occurs in high-aspect-ratio (small drilled diameters in
thick PWBs) plated-through vias. It is not so much "excessive plating build
up on sharp outside corners", even though some of this can occur, but inadequa
te plating near the center of the thickness of the PWB. Note that I did not
say inadequate plating thickness; while thin plating near the PWB center is
an obvious visual clue, the plating properties (strength and ductility) are
also significantly reduced. Thus, in the very part of the PTV where the
highest loads on the copper barrel occur, you have the smallest Cu
cross-section and the material properties are inferior.
This condition is the result of mass transport limited plating where the
copper ions are plated out faster then they are replenished by plating
solution flow, highly non-uniform plating current distribution, pushing the
existing plating chemistry beyond its capability, and/or improper plating
bath maintenance. It is the reason why techniques such as pulse plating,
reverse pulse plating, impingement plating, as well as high acid/low copper
plating chemistries have become popular.
Also, the plated hole diameter is irrelevant for vias, the important
parameter is the drilled hole diameter.

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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