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July 1998

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Subject:
From:
Gary Ferrari <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 20 Jul 1998 18:49:09 -0500
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Steve,

The note in question relates to the standard fabrication allowance in IPC-D-275,
table 5-10. This number represents process variations which include drill size,
registration, drill positional tolerance, film variation, etc. In short it is a
statistical number that compensates for the variations found in the fabrication
process. You need to include this number in your calculation for minimum land size,
to prevent hole breakout during manufacture. Some fabricators require a larger number
than others. It is a good reflection of their capability, and should be used to match
a supplier with a specific design technology.

If a fabricator requires a larger number than you provide, then he will experience a
poorer yield. If he requires less than you have provided, you will get the best
pricing since it is in his highest yield category.

By placing it in a master drawing note, you are communicating to the fabricator your
design requirements, leaving no confusion or misinterpretation.

Regards,
>>> Steve Hawn <[log in to unmask]> 07/20/98 10:47AM >>>
Hello,

I'll try sending this one out again since I didn't see any responses
last week.

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.

Steve Hawn
Precision Digital Images

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Gary Ferrari
Executive Director
IPC Designers Council
(860) 350-9300 Fax (860) 350-9309
[log in to unmask]

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