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6 Vision functions User Manual HS-50 6.2 PCB vision system Software-Version 5.01 Edition 01/99 190 and coor dinates of the fiduci al ar e determ ined in each cas e by app lyin g the correl ation pr oce- dure to th e colu…
User Manual HS-50 6 Vision functions
Software-Version 5.01Edition 01/99 6.2 PCB vision system
189
The 1-D pattern search procedure is used for precisely determining the pattern and position of the
fiducial. The fiducial image is broken up into rows and columns and the gray scale values in each
row and column added up. The next diagram illustrates this process using a double cross.
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(1) Fiducial
(2) Sum of the grey scale values in one column: column profile
(3) Sum of the grey scale values in one row: row profile
– The position of the fiducial is precisely determined from the horizontal and vertical profiles.
After teaching, the fiducial structure parameters obtained are saved to the line computer.
– The saved pattern is now tested. The gantry moves the PCB camera over the board to all 4
corners of the search area (worst case). During this test the vision system must re-identify the
fiducial four times.
– Finally, the coordinates of each individual fiducial (at least two) are manually added to the NU
file or transferred from the CAD file to the NU file. In this way, the coordinates and fiducial struc-
ture parameters for the board are defined as a pattern in the system.
– During the placement process, the fiducial parameters will be determined once again using the
1-D and 2-D image processing methods described above. The template window is moved
moxel by moxel over the search area searching for the best possible agreement of the gray
scale values of the reference and board search fiducials (correlation procedure). Maximum
correlation is attained when the reference and search fiducials agree.
– Once the fiducial has been found, the 1-D pattern search process begins. The precise shape
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6 Vision functions User Manual HS-50
6.2 PCB vision system Software-Version 5.01 Edition 01/99
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and coordinates of the fiducial are determined in each case by applying the correlation proce-
dure to the column and row profiles (see Fig. 6.2 - 2). From the coordinates obtained in this
way, the location, skew and shear of the board are determined.
Reject fiducials (= ink dots) are also detected and evaluated using the methods described
above.
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Basically the same criteria apply to both fiducials and reject fiducials (ink dots): uniqueness of fi-
ducial shapes and readily detectable structures which stand out from their surroundings.
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Instead of fiducials, you can also use uniquely identifiable structures within the PCB layout. It
should, however, be remembered that solder stop lacquer is accompanied by a loss in contrast.
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Position the fiducial where there are as few structures as possible and where the fiducial will stand
out well from its surroundings. Measuring outwards from the center of the fiducial, there should be
a clearance on each side equal to at least one fiducial size plus 1 mm.
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There are 2 types of fiducials:
– Positive fiducials
The fiducial extends above the base material of the board.
– Negative fiducials
The fiducial is etched into the base material of the board.
User Manual HS-50 6 Vision functions
Software-Version 5.01Edition 01/99 6.2 PCB vision system
191
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Always choose a well-structured, axially parallel, distinct shape as fiducial shape:
Recommended fiducial shapes:
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Properties
– Low informational content (fiducials can easily be confused with test dots).
NOTE
Make sure that there are no similar structures in the fiducial search area.
– Low space requirements in the layout
– Very robust with respect to different tinning procedures (e.g. hot-tinning).
Recommended fiducial dimensions
– for square and rectangles: Side length 1.2 mm - 2.2 mm
– for the circle: Diameter 1.2 mm - 2.2 mm
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Properties of the double cross
– Higher informational content
– More space required in the layout
– Sensitivity with respect to high tin-coatings (bare copper is preferable)
– Poor fiducial quality may result in incorrect position recognition.
Properties of the simple cross
– The informational content is somewhat lower than with the double cross
– Less space required in the layout than with the double cross.
– Less sensitive to high tin-coatings than with the double cross.