A couple of suggestions regarding this:

1) remember that on the vias you can work with a "max" hole size (in most cases).  In other
words, a .018" via would be drilled at .018" or less, depending on your comfort level (most shops
will go to .0125").  A component hole must be drilled .006" to .009" over the bottom end of the
tolerance (based on +-.003" tolerance) to insure lead fit.  This in itself should gain you
approx. .006" to .009" pad diameter reduction.

2) A teardrop placement towards the circuit will allow you to still maintain annular ring in the
event of hole movement towards the circuit.  Many design and CAM station software have this
feature available.

JOHN WAITE

Paul Gould wrote:

> 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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