At 07:55 AM 2/18/99 -0800, Jack Olson wrote:
>The problem was that the vendor says its not their fault if the board came
>in too thick, because the callout was ".093 nominal" with no tolerance
>callout. I say if there is no callout then they have to use the one in the
>title block "all three-place dimensions shall be .xxx unless otherwise
>noted", right?

But there *is* a callout. The word "nominal" means that the thickness shall
be what .093 material routinely implies in the industry, and it releases
the fabricator from being held to a specific tolerance like the +/- .005
common for three place dimensions. The purpose of saying "nominal" is to
indicate that there is no *exact* tolerance specification; but from
industry practice, it would be clear that a thickness closer to 0.063 or
0.125 would not be acceptable, and one might argue for even tighter
tolerance than that, especially given the prodominance of a +/- 10%
tolerance on this specification.

Bottom line is that it would be easy to argue that the board was out of
spec if it was outside of the range of 0.078 to 0.108. One could *not*
argue that it was to be held to +/- 0.005. The word "nominal" specifically
releases the fabricator from that. If not, then what does it mean?

It means that the fabricator shall use a process intended to produce a
board approximately 0.093 thick. If this were a two-sided board, that would
be a standard material which is nominally 0.093, and the fabricator would
have no difficulty, with keeping within 10% of that. This is the reason for
the use of the word "nominal." In fact, if I am correct, the normal "0.093"
board would be a bit thicker than that. But with a multi-layer board,
things are not so easy.

It is not common that a board being, say, 20 mils oversize would cause any
problem *unless* it has connector fingers on it. Any competent fabricator
should flag and question board tolerances which seem non-standard, unless
they are explicit and clear, particularly if an edge connector is involved.

If there is an edge connector fitting a standard edge connector, there
would be a good case for arguing that the fabricator is at fault. If not,
and the board is less than about .108 thick, I would think that it would be
pretty hard to force the issue.

How thick *was* the board? And why was it a problem that it was too thick?

Normally, if the thickness of the board is a critical issue the thickness
should be explicitly specified such that acceptability matches usability.
"Nominal" implies that it is not critical; this is why a fabricator should
question the matter if a board with connector fingers has a "nominal"
specification; certainly if the layup is likely to produce a thickness
outside of the 10% range.


[log in to unmask]
Abdulrahman Lomax
P.O. Box 690
El Verano, CA 95433

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