Hello Scott,

The 'True Position' dimension, (boxed, sometimes also called 'basic') is
without tolerance.
The positional tolerance (in the case for example of a drilled hole) is
expressed as a diameter, within which, the centre of the actual hole must
lie. This positional tolerance diameter is 'normal' to a reference datum as
well.

I usually specify a diameter positional tolerance of 0.08mm for the pattern
of drilled holes on the board, and use a diameter positional tolerance of
0.2mm to position the drilled hole pattern on the PCB profile.

These values are based on our manufacturers' typical capabilities for good
yield.

hope this helps a bit..

regards,

Phil.



-----Original Message-----
From: Scott Kauling [mailto:[log in to unmask]]
Sent: Saturday, 8 December 2001 06:31
To: [log in to unmask]
Subject: [TN] PWB Fab Note Question - Diameter True Position


In IPC 2221 section 9.2.2 describes hole location tolerance, it is
expressed as "diameter about true position".  IPC T-50 defines "true
position", not diameter about true position.

My questions are:

        1. How does the diameter about come into play with the true
position?

        2. What tolerance of diameter about true position do you specify on
your
fab prints

        3. What is the typical capability of diameter about true position
that PWB
fabricators hold?

Thanks in advance for your input.

Have a good weekend!

Scott Kauling

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