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December 2001

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Subject:
From:
"Roger M. Stoops" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 7 Dec 2001 15:38:19 -0500
Content-Type:
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text/plain (86 lines)
1. How does the diameter about come into play with the true position?

Table 9-3 defines the diameter about true position, or tolerance around
true position expressed as a circular value, compared to the square
tolerance area with non-GD&T dimensioning.  IPC-2615 does a pretty good job
explaining how GD&T works (for the curious).  Figure 6-8 on page 28 of
IPC-2615 helps answer your question.

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

Though Table 9-3 specifies 0.2mm for Level B boards, I typically specify
0.15mm, or 0.006, MMC.  As the drill/hole size gets larger, the tolerance
zone increases in size.  For non-critical features, I specify 0.2mm or
0.25mm.

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

I'll leave this to one of our fabricator friends.

Roger M. Stoops, C.I.D., PCB Designer
[log in to unmask]


Trimble
Engineering and Construction Division
5475 Kellenburger Rd.
Dayton, OH 45424-1099 USA
Ph: +01 937.233.8921 or +01 937.233.4574 ext 288
Fax: +01 937.233.7511



                    Scott Kauling
                    <skauling@TRI-        To:     [log in to unmask]
                    ONICS.COM>            cc:
                    Sent by:              Subject:     [TN] PWB Fab Note Question - Diameter True Position
                    TechNet
                    <[log in to unmask]
                    RG>


                    12/07/01 03:01
                    PM
                    Please respond
                    to "TechNet
                    E-Mail Forum."





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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