Steve,

I believe that is referring to the Minimum Standard Fabrication Allowance for
Interconnection Lands, as defined in Table 9-1 in IPC-2221.

"To meet the annular ring requirments, the minimum land surrounding a hole shall be
determined by the following. The worst-case land-to-hole relationship is established
by the equation:
Land size, minimum = a + 2b + c
where:
a= maximum diameter of the finished hole
b= minimum annular ring requirements
c= a standard fabrication allowance (table 9-1) that considers production master
tooling and process variations required to fabricate boards.

Table 9-1
Producibility Level A - 0.4 mm
Level B - 0.25 mm
Level C - 0.2 mm"

There are some additional notes on the table. But you've got the gist.

Lisa Williams
IPC
2215 Sanders Road
Northbrook, IL  60062
URL:  www.ipc.org

>>> Steve Hawn <[log in to unmask]> 07/20/98 09:47AM >>>
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 in.

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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