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LASERpoints
Fiducials: how to be where you need to be
Panel Fiducial
Global Fiducial
14 | MARCH/APRIL 2010 | MICROmanufacturing
When running a job on a laser—or any
other tool—it is frequently required
that the processing toolpath be aligned to existing
features. Generally, the sharper and
smaller the alignment feature the more accurate
the alignment.
A camera- or optical-based vision system
usually is used. The alignment process can be
performed manually or automatically. Accuracies
on the order of microns are possible with
systems that incorporate high-resolution motion
stages and cameras, and crisp fiducials.
A fiducial, or fiduciary marker, is an object
in an imaging system’s field of view that appears
in the image produced and serves as a
point of reference or a measure. One fiducial
is usually designated “primary” and is used to
adjust the X/Y offset. Secondary fiducials are
used for other operations. Fiducials are not all
All images: PhotoMachining
Figure 1: Commonly used fiducials.
Local Fiducial
Figure 2: Global fiducials should be positioned
as far apart as possible. When high accuracy is
required, it’s best to use local fiducials.
the same, requiring operators or automated
vision systems to take these differences into
account. Figure 1 shows several common
markers.
Some general characteristics of fiducials are:
Shape. Choosing an optimal fiducial mark
for a specific application depends on the requirement.
For instance, the crosshair-shape
that is easy for a human to employ may not
be appropriate for a machine vision approach
capable of edge detection. Some fiducials
contain a combination of features, which allows
the same unit to work well with different
detection methods. For example, the bow-tie
shape has two rounded edges on the outside
to generate a circle, as well as a crosshair in
the middle for manual alignment.
Size. Smaller and crisper fiducials provide
better alignment. Minimum diameter is determined
by the optical system and the resolution
of the motion hardware. Maximum
size is determined by the field of view of the
imaging system. Fiducial marks located on
the same part should not vary in size by more
than a few percentage points.
Clearance. A clear area, devoid of any
other circuit features or markings, should be
maintained around the fiducial mark. The size
of the clear area should be, minimally, equal to
the radius of the fiducial mark. When possible,
the amount of clearance around the mark
should equal the mark’s diameter.
Edge Clearance. A fiducial mark should
be located no closer to the edge than 7.62mm
(0.300"), per Surface Mount Equipment Manufacturers
Association Standard Transport
Clearance.
Material. The material will be dictated by
the part, but etched metals, organics and raw
ceramics are common. The main points are
mark resolution, permanence and contrast.
Contrast. Most machine vision recognition
systems perform best when a consistent
high contrast is present between the fiducial
mark and the base material.
Flatness. The flatness of the fiducial surface
should be 15μm (0.0006") or flatter.
To illustrate how fiducials are used, consider
a simple 2-D printed circuit board (Figure
2). The number of alignment points