Gary,
Thank you very much for your prompt response.  I was hoping that of 655
subscribers to this list, someone would be able to shed some light on this
dilemma.

I understand the process and the purpose of the fabrication allowance and
the purpose of doubling the annular ring amount when calculating the minimum
pad size.  What I don't understand is this - notice that the fabrication
allowance from the table is to be included ONE time when calculating for the
minimum pad size, but is to be included TWICE, same as the electrical
clearance, when calculating for the clearance pad size (see Figure 5-18 - it
indicates the full fab allowance to be applied to each side of the hole).
This makes no sense and I believe it to be an error.

Notice that, in the example, the total fabrication allowance added to the
minimum pad size formula is 10 mils, giving a +/- 5-mil range.  However, in
calculating for the clearance pad, it would be 20 mils (10 on each side of
the hole), giving a +/- 10-mil range, twice the required tolerance.  The
purpose of the tolerance is to insure that there is at least the electrical
clearance provided, even if the drill was off by the full fabrication
allowance.  In doing it the way presented, you would not only have the
electrical clearance but also an additional 5 mils (the 10 allowed minus
what was required).  Not right.

Ray Humphrey
DynaCad Design Services

----- Original Message -----
From: Gary Ferrari <[log in to unmask]>
To: <[log in to unmask]>
Sent: Monday, September 20, 1999 6:48 PM
Subject: Re: [DC] IPC Specs - Errors?


> Ray,
>
> The fabrication allowance in the table is a single number that the
designer
> uses to adjust his calculations to compensate for fabrication process
> variation. The numbers in table 5-10 (IPC-D-275) are there as a guide to
cost
> comparison and what is achievable. They hope to illustrate when one would
be
> moving into a lower yield situation. The numbers are a statistical
summation
> of all the variations in the process (which tend to be far greater than
the
> numbers in the table).
>
> As a designer you may choose the exact numbers in the tables or select any
> number that you wish. Critical is that you place SOMETHING in the formula.
>
> There is no further tolerance applied to a standard fabrication allowance.
> You need to double your annular ring requirement since you have a ring on
> both sides of the land. But only double the annular ring, not the standard
> fabrication allowance. The same is true for the clearance in a plane.
>
> Regards,
>
> Gary Ferrari
> Executive Director
> IPC Designers Council
> 860-350-9300
>