Mark,
One more point I thought about (mind you, I'm certainly no expert) - vias, of
course, are excepted from the 20 mil annular ring rule. And, when you think
about it, conceptually, if you think of a pad as the placing of a component lead
in a trace then what you have is the trace splitting into two halves to go
around the lead. So, if a trace is 12 mils, and it splits to go around a 20 mil
lead then all you really need is 6 mils around the lead to preserve the trace
width (i.e. the current carrying capability of the trace). Now obviously, there
is a minimum "useful" annular ring around a lead so that servicing the board
(desoldering the part) is possible wiithout lifting the pad.
Susan said 20 mils (minimum) is what she uses and that seems about right to me
from a servicing point of view. Vias don't have to be "serviced" except when
you need to add a cut and jumper and jumper using a via.
regards,
Bob Landman
H&L Instruments
-----Original Message-----
From: Mark Holmes <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: April 16, 1999 4:48 AM
Subject: [DC] Pad sizes
>Hello all,
>
>Can anyone offer me any advice on where I can get hold of some guidelines
>regarding the size of the Pad with respect to the hole size, for through
>hole components?
>
>Ideally I would like to know how best to determine the correct pad size for
>a
>given pin size, taking into account current requirements, whether the part
>is to be wave soldered or hand soldered, and whether the board is double or
>single sided.
>
>I would like some sort of guideline for reference, but would also like to
>hear anyone
>else's 'rule of thumb' methods.
>
>Regards,
>
>Mark Holmes
>
>[log in to unmask]
>
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