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July 1998

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Subject:
From:
JoAnn Amerson <[log in to unmask]>
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Date:
Mon, 20 Jul 1998 11:11:40 -0400
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On 20 Jul 98, Steve Hawn wrote:

> On all of our PCB fab drawings there is a specification which reads:
>
>   Land/Hole fabrication allowance shall be 0.012.
>
> What does this spec mean?
>
> Our PCB designer and two board houses I've spoken to do not know what this
> spec is referring to.  Our PCB designer states that this wording can be
> found in IPC-D-325A, Documentation Requirements for Printed Boards, in an
> example fab drawing.

Hi Steve -

I've been dealing with this issue for the last week and I think I can explain it.
The .012 is the artwork to drill hole tolerance that your board fabricator is
expected to comply with.  We use .014.  The following is from a wonderful book
I got from a Hadco rep.

"The tolerance for true artwork is +/-.002".  The hole location is +/-.003".  The
board material itself moves during thermal excusions.  Etch loss occurs.  The
inner manufacturing adder of .014" takes all of these potential misalignments
into account.  By taking 'the square root of the sum of the squares' (RMS), we
arrive at .014" inner layer manufactuing adder."

I don't know which figure your designer is talking about but I understand what
he means.  We just don't specify a given dimension on our drawings but it is
taken into consideration when I lay out a footprint.

Hope this helped.

Jo

JoAnn L. Amerson
Design Librarian
Red Lion Controls, Inc.
E-mail: [log in to unmask]
Voice: (717) 767-6961 ext 6308

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