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Establishment of an Artwork Datum

IP.com Disclosure Number: IPCOM000108137D
Original Publication Date: 1992-Apr-01
Included in the Prior Art Database: 2005-Mar-22

Publishing Venue

IBM

Related People

Larnerd, JM: AUTHOR [+2]

Abstract

Many of the operations that are performed on printed circuit boards in today's manufacturing environment must be performed relative to the artwork that exists on the surface of the circuit board. To do this, we must first establish a Part Coordinate System on the circuit board that is positioned relative to the artwork in question. Once positioned, the Part Coordinate System becomes the reference for all machining activities to follow. This report deals with the techniques involved in correctly placing a Part Coordinate System on a circuit board relative to artwork that resides on the surface of the circuit board.

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Establishment of an Artwork Datum

       Many of the operations that are performed on printed
circuit boards in today's manufacturing environment must be performed
relative to the artwork that exists on the surface of the circuit
board.  To do this, we must first establish a Part Coordinate System
on the circuit board that is positioned relative to the artwork in
question.  Once positioned, the Part Coordinate System becomes the
reference for all machining activities to follow.  This report deals
with the techniques involved in correctly placing a Part Coordinate
System on a circuit board relative to artwork that resides on the
surface of the circuit board.

      The remainder of this report is a description of the
mathematical algorithm and associated software that provides the best
fit of the Part Coordinate System relative to artwork features that
reside on a printed circuit board.

      Before we can machine features into a circuit board relative to
the artwork, we must first establish a Part Coordinate System which
is defined as a function of the artwork.  To do this, we must first
have a drawing which shows the basic or nominal relationships between
the artwork features in question.

      Assume we have a drawing of a circuit board, as shown in Fig.
1.  The artwork is represented by four fiducials, one each in the
four corners of the board.  Included on the drawing are two holes to
be drilled.  What this drawing shows is the "nominal" or "basic"
relationships between the four fiducials and the two holes.  These
are not dimensions per se, but rather general relationships between
six points on a plane.

      Knowing these basic relationships allows us to establish an X-Y
coordinate system, as shown in Fig. 2. This is called the "Part
Coordinate System", and we could have placed it anywhere we wish.

      There are two distinct sets of relationships here that should
not be confused.  The first set contains the relationships between
the four fiducials and the Part Coordinate System, and the second
between the Part Coordinate System and the two holes to be machined.
The first set of relationships between the four fiducials and the
Part Coordinate System can be seen as "rules" for the placement of
the Part Coordinate System on an actual circuit board given the
locations of the four fiducials.  Said another way, given the actual
locations of four corner fiducials on an actual circuit board, the
Part Coordinate System could be penciled in according to these rules.
The Part Coordinate System will simply be placed onto the actual
circuit board such that the relationship between the four fiducials
and the Part Coordinate System matches those shown in Fig. 2. Once
placed, the fiducials are no longer needed and the Part Coordinate
System becomes the basis for machining the two holes, H1 and H2.

      In general, however, due to growth and shrinkage of the actual
circuit board, the relationships between t...