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Date: | Mon, 16 Mar 1998 15:03:51 -0600 |
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Three comments.
First, a positional tolerance of .006" TP is normally toleranced
from a datum point. The datum point is typically established by a
non-plated through hole with no pads ( to assure that it is not
drilled after plating [also called second drilled]). Your call out
of within .003" of pad center is not a true position call out in the
sense many (most?) use it.
Second, a .006" TP becomes increasingly difficult to maintain as the
distance from datum increases. One reason for this is that the
laminate moves during processing as it sees heat cycles and it can
move (especially if heavy copper is used on the outer layers) after
etching.
Lastly, the printing process (which locates the pad and is
performed after drilling) requires more fabrication tolerance than
does drilling the hole. In my opinion, registration of the hole to
the pad should be controlled by the minimum annular ring call out,
not by a positional tolerance to the pad. This is the approach most
fabrication specifications take.
> Date: Fri, 13 Mar 1998 07:25:50 -0600
> From: Richard G Smith <[log in to unmask]>
> Subject: PTH Positional Hole Tolerance
> Hi All,
>
> Here's the issue:
>
> We specify in our fab spec that all holes are to be within .003" of
> their corresponding pad center (true position of .006"). We are being
> told by our fab house that this tolerance is unreasonably tight.
>
> Any comments???
>
> Thanks,
> Rick
>
>
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