User guide

7-14 Programming Basics
Corner Fiducials
Corner Finder is an alternate method provided by the Asymtek Vision System to identify
fiducials that are part edges or lines or edge pairs, such as the edge of a die. In some cases
accuracy is greatly improved by identifying the part to be dispensed on as a fiducial.
The position of parts can be slightly rotated by automatic placement systems, or the parts can
be dislodged during the solder reflow process. To compensate for the possibility of part
misplacement, Corner Finder offers a feature called Orthogonality Tolerance. You will notice
this feature as one of the options when defining a corner fiducial. Orthogonality Tolerance is
a plus or minus value that FMW uses to determine if a part is square. When defining a corner
fiducial, FMW measures the angle of the corner. If the corner is within 90° plus or minus the
Orthogonality Tolerance, the part is considered to be square. If the angle is outside of the
Orthogonality Tolerance, FMW issues a Fiducial Error message which gives the operator the
option of skipping the part or redefining the fiducial.
The Corner Finder method assumes that edge images are a change in gray value from light to
dark or dark to light. Edges are identified by locating these types of gray changes and
analyzing them statistically to see if they meet a given criteria. The programmer can set the
search area just like with Gray Scale. Corner Finder uses a vision tool called a Caliper
designed to find edges or pairs of edges.
Selecting Good Edges
The same rules for size, uniqueness and contrast in selecting model fiducials applies to edges
and corners. Edges must be easily distinguishable from the background so that
dark-to-light or light-to-dark transitions are maximized. The Light Level and Gain settings can
be adjusted to increase this contrast. Edges should also be as “clean” and straight as possible,
even though the software uses an averaging algorithm.