Hi Victor, Jeffrey & Roger,
IPC-2222, Page 19, section 9.2.2.1, and page 20, Table 9-4 is very helpful.
The aspect ratio as the PCB thickness divided by the drilled hole
diameter--of course, the critical aspect ratio of any PCB is the one with the
smallest drilled hole diameter. Thus, a 62mil thick PCB with a drilled hole
diameter of 20mil would have an aspect ratio of 3.1:1.
IPC-TR-579 "Round Robin Reliability Evaluation of Small Diameter Plated
Through Holes in Printed Wiring Boards" and  IPC-D-279 "Design Guidelines for
Reliable Surface Mount Technology Printed Board Assemblies" show that the
aspect ratio serves as a 'Red Flag;' it is not a physical parameter with a
single meaning. Hole diameter and PCB thickness act independent of each other
in  influencing via reliabilty.
If IPC-TR-579 and IPC-D-279 do not provide enough information, you may want
to attend my tutorial "Interconnect Failures and Design for Reliability for
Plated-Through Holes/Vias (PTHs/PTVs)"

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 386-437-8747, Fax: 386-437-8737, Cell: 386-316-5904
E-mail: [log in to unmask], Website: www.engelmaier.com

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