Hi Bob,
I think you are forgetting the plating in the hole to also carry
current.
The annular ring is primarily for the board vendor to hit the inner
layer
pads on a multi layer board when drilling. The outer layer pad can use
less annular ring because the outside imaging is done after the drilling
process. This is why the manufacturing tolerance is larger for inner
layers
than outer.
John Shaub
Honeywell IAC.
> -----Original Message-----
> From: Bob Landman [SMTP:[log in to unmask]]
> Sent: Friday, April 16, 1999 10:34 AM
> To: [log in to unmask]
> Subject: Re: [DC] Pad sizes
>
> 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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