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May 1998

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Subject:
From:
Paul Gould <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 15 May 1998 13:29:04 +0100
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Can anyone help with a query on land around via holes. I have a copy of
IPC-A-600 Rev E Aug 1995 which states for plated through holes the
minimum land between the track and pad and hole must not be less than
0.002" as a minimum standard. This means that misregistration towards
the track is not acceptable if the pad is reduced below 0.002" even if
there is no actual breakout. On the other hand, the hole is allowed to
break  out of the pad in any other direction by as much as half the
diameter. Clearly the two are incompatible with each other since with
registration that bad there is bound to be breakout towards tracks in
some areas.

Has there been a revision specifically for via holes only, or is there
any other specification relating to landless via holes as this criteria
is impossible to meet on some designs. The implications of meeting this
spec are to increase via pad size and/or reduce hole size which has
implications for design density and manufacturing cost.

Any views on this would be greatly appreciated. For production drilling
on large panels, what minimum land do you think would be needed at the
outset to guarantee minimum annular 0.002" on the finished panel taking
account of drill size and etch reduction also?

Best Regards
--
Paul Gould
[log in to unmask]
Isle of Wight,UK

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