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In 2009 IPC (Dieter
Bergman, John Perry and Tom Hausherr) started progress on a land pattern
standard for through-hole components called IPC-7251. There were meetings where
Gary Ferrari attended by webcast. The 3-Tier pad stack annular ring for
through-hole was based off the same precedent for SMD where the Least, Nominal
and Most environments had a single value for Toe, Heel and Side. IPC-7251 never
got published and progress was discontinued on October 1, 2014 by the IPC 1-13
Land Pattern committee as they decided to instead add it to IPC-7351C which
we’re currently working on.
The IPC-7251
through-hole pad stack was inherently flawed due to the 3-Tier annular ring
concept that the same Annular Ring Value could be used for any hole size. An
example of this would be: The Nominal Environment annular ring (AR) was set to
8 mils. That means that a 10 mil hole had a 8 mil AR and a 20 mil hole had a 8
mil AR and the 40 mil hole had a 8 mil AR and a 80 mil hole had a 8 mil AR. The
flaws are apparent in that small holes require small AR and large holes require
a larger AR for solder volume, strength, voltage current, heat, etc. Even
though LP Wizard and Library Expert supported IPC-7251, the through-hole
default in both programs was “Proportional Pad Stacks” due to the fact that
IPC-7251 was an unreleased standard.
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