Ken,
True position is a way of defining a tolerance as a circle as opposed to a
square.
The easiest way to describe this is to have you draw a square that
represents a 0.006" x and y tolerance.
Now draw a circle inside the square that is tangent to the four sides.
Color the area outside the circle but inside the square, IE the four
corners of the square. The colored area is outside the 0.006" true position
tolerance but is inside the x and y tolerance.  Since you are drilling a
round hole into a round pad this makes sense.
The 0.006 is what was considered allowable on the old design for whatever
reason, in circuit test, auto insertion.
The front to back registration is just what it implies.  When all is said
and done how far off are the centers of your pads from front to back.  This
could have alot to do with controlling annular ring.

John Laur
RA/AB
Milwaukee, Wi

-----Original Message-----
From:   Ken Patel [SMTP:[log in to unmask]]
Sent:   Wednesday, November 18, 1998 8:03 PM
To:     [log in to unmask]
Subject:        [TN] True Position and Registration on Fab

Fab Gurus,
We are designing board with a 40 mil BGAs first time and the fab will be
having 10 mil plated finished holes on it. I opened up an old existing
drawing of different non-BGA board and now I have couple of questions after
reading my fab drawing notes.

(1) Fab note: All holes to be within .006" true position on the diameter
with respect to original data.
Question: What is true position and why .006"? How to decide this number?

(2) Registration: Front to back to be within  +/-.005". Can anyone explain
what is registration and why .005"? Who can decide this #, is it fab
house's
processing capability?

re,
Ken Patel
______________________________________________________
Ken Patel                       Phone:  (408) 490-6804
1708 McCarthy Blvd.             Fax:    (408) 490-6859
Milpitas, CA 95035              Beeper: (888) 769-1808

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