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At 02:22 PM 10/4/01 -0400, [EMAIL PROTECTED] wrote:
>My experience with fabricators has been quite the opposite, that I want to
>specify the actual drill size, after having made an allowance for plating
>thickness where I need to mount things in the holes. If you use finished
>size, the fabricator must bump up the drill size to account for plating
>thickness, and then all your carefully checked clearances go out the window.

They go out the window anyway, since you don't have so much control over 
the plating.

Specifying the actual drill size can have some value, because the hole then 
represents the limits of the metal; but the norm in fabrication drawings is 
"finished size," which is pretty much all you can easily check. 
Metalization limit is only going to help you if you section boards or use 
other hi-tech inspection methods (because metalization only matters on 
inner layers when you remove the pads, otherwise we are talking about a 1.5 
mil annular ring, so it is normally swallowed in the pad.

It is then up to the fabricator to take the rest of your specifications 
into account and choose the appropriate drill size to meet your tolerances 
on the finished size.

[EMAIL PROTECTED]
Abdulrahman Lomax
Easthampton, Massachusetts USA

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