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Fiducial Mark Design Guidelines
Scope: To provide a generic guideline for the design and placement of Fiducial marks. The guidelines adhere to design rules standardized by the Surface Mount Equipment Manufacturers Association and are supported by IPC.
FIDUCIAL MARKS
Fiducial Marks provide common measurable points for all steps in the assembly
process. This allows all automated assembly equipment to accurately locate the
circuit pattern. Fiducial Marks are generally categorized in two types:
Global Fiducials – Used to locate the position of all features on an individual
printed circuit board. When multiple boards are processed as a panel, the global fiducials may also be referred to as panel fiducials.
Local Fiducials- Used to locate the position of an individual component that
may require more precise location, such as a .020” pitch QFP.
Global and/or panel fiducials should ideally be located on a three point grid based
system, with the lower left fiducial located at the 0,0 datum point and the other two
fiducials located in the positive X and Y directions.
Global fiducials should be located on all PCB layers that contain components to be
mounted with automated equipment. While other circuit objects (via-holes, etc.) can be used, this can compromise accurate component placement regardless of the component type.
A minimum of two global fiducial marks is needed for correction of translational
offsets (x and y position) and rotational offsets (theta position). These should be
located diagonally opposed and as far apart as possible on the circuit or panel.
A minimum of three fiducial marks is required for correction of non-linear distortions (scaling, stretch and twist). These should be located in a triangular position as far apart as possible on the circuit or panel.
All Fine Pitch components should have two local fiducials designed into the land
pattern of the component. This will allow for correction of translational offsets (x and y position) and rotational offsets (theta position). This can be two marks located diagonally opposed within the perimeter of the land pattern.
If space is limited, a minimum of one local fiducial mark may be used to correct
translational offsets (x and y position). The single fiducial should be located in the
center of the land pattern.
FIDUCIAL MARK DESIGN SPECIFICATIONS
Shape- The optimal fiducial mark is a solid filled circle. Fiducials should have a
soldermask opening large enough to provide good contrast and be free of soldermask to enable accurate identification by all vision alignment systems.
Size- The minimum diameter of the fiducial mark should be 1mm (0.040 in) and the
maximum at 3mm (0.120 in).
Fiducial marks located on the same PCB should not vary in size by more than 25 microns (0.001 in). Some assembly equipment is less flexible in its ability to recognize different size marks on the same printed circuit board.
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 equal to the radius of the fiducial mark. A preferred clearance around the mark is equal to the mark diameter.
Material- The fiducial may be bare copper; protected by a clear anti-oxidation coating, nickel or tin plated, or solder coated (HASL).
The preferred thickness of the plating or solder coating is 5 to 10 microns (0.0002 to 0.0004 in). Solder coating should not exceed 25 microns (0.001 in). If solder mask is used, it should not cover the fiducial or the clearance area.
Flatness- The flatness of the surface of the fiducial mark should be within 15 microns (0.0006 in).
Edge Clearance- The fiducial marks should be located no closer to the PCB edge than 7.62mm (0.300 in) (SMEMA Standard Transport Clearance) and the minimum fiducial clearance required.
Contrast- Most machine vision recognition systems perform best when a consistent high
contrast is present between the fiducial mark and the PCB base material.