I'll try and join this tag. This may be in no way accurate, but may be a
good guess.

Potential answer: History

Just a few years ago the minimum annular ring, minimum external annular ring
was specified to be 0.005". It is then possible that this kind of
referencing dates back to before multi-layer, when boards were two-sided and
annular ring was measured by non-destructive methods. When multi-layer came
along, the contact with the land was still good with 0.001" to 0.002" copper
plating when you had a 0.005" plating.

We, humans I mean, do a lot of things out of habit, this could be one of
them.

I asked the question myself to quite a few people in the past, without ever
getting a straight answer.

Just a potential answer, hoping to finally get the real proper answer.

Have a nice day,                 :-)

Alain Savard, B.Sc.
Chemical Process Analyst
CAE Electronics Ltd.
e-mail: [log in to unmask]


-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]]

I will try and answer your question regarding the difference in measuring
lands, then maybe others will join in the fun.

It was always explained that
1)  on the external lands the plating was in contact with the entire surface
of the land - so if there was disattachement of the plating from the very
end
of the foil, there would still be an intact connection from the land to the
end of the plating in the hole.

2)  with inner layers - if the plating becomes disattached the inner layer
only
consists of the inner layer itself

Susan Mansilla

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