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Fiducial marks characteristicsPopa Marian - BogdanAdvanced electronics technologies in automotive industryPolitehnica University, Faculty of Electronics, Telecommunications and Information TechnologyBucharest, Romaniapopa.marianbogdan@gmail.com

Abstract - Most placement devices are fed boards for assembly by a rail conveyor, with the board being clamped down in the assembly area of the machine. Each board will clamp slightly differently than the others, and the variance which will generally be only tenths of a millimeter is sufficient to ruin a board without proper calibration. Fiducial marks allow automated assembly equipment to accurately locate and place parts on boards. These devices locate the circuit pattern by providing common measurable points. By measuring the location of the fids relative to the board plan stored in the machine's memory, the machine can reliably compute the degree to which parts must be moved relative to the plan, calledoffset, to ensure accurate placement.What is a fiducial mark?

FiducialMark - A printed board artwork feature (or features) that is created in the same process as the printed circuit board conductive pattern and that provides a common measurable point for component mounting with respect to a land pattern or land patterns.Inprinted circuit board(PCB) design, fiducial marks, also known as circuit pattern recognition marks or simply "fids," allow automated assembly equipment to accurately locate and place parts on boards. These devices locate the circuit pattern by providing common measurable points. They are usually made by leaving a circular area of the board bare from solder-stop coating (similar to clearcoat), in which a filled copper circle is placed. This center metallic disc can be solder-coated, gold-plated or otherwise treated, although bare copper is most common as it is not a current-carrying contact.

Why use fiducial marks?

Most placement devices are fed boards for assembly by a rail conveyor, with the board being clamped down in the assembly area of the machine. Each board will clamp slightly differently than the others, and the variance which will generally be only tenths of a millimeter is sufficient to ruin a board without proper calibration. Consequently, a typical PCB will have three fids to allow placement robots to precisely determine the board's orientation. By measuring the location of the fids relative to the board plan stored in the machine's memory, the machine can reliably compute the degree to which parts must be moved relative to the plan, calledoffset, to ensure accurate placement.Using three fids enables the machine to determine offset in both the X and Y axes, as well as to determine if the board has rotated during clamping, allowing the machine to rotate parts to be placed to match. Parts requiring a very high degree of placement precision, such asintegrated circuitchip packages with many fine leads, may have subsidiary fiducial marks near the package placement area of the board to further fine-tune the targeting.Conversely, low end, low-precision boards may only have two fiducials, or use fiducials applied as part of the screen printing process applied to most circuit boards. Some very low-end boards may use the plated mounting screw holes as ersatz fiducials, although this yields very low accuracy.For prototyping and small batch production runs, the use of a fiducial camera can greatly improve the process of board fabrication. By automatically locating fiducial markers, the camera automates board alignment. This helps with front to back and multilayer applications, eliminating the need for set pins.

Fiducial marks design

Fiducial Marks are a feature of the printed circuit board artwork, created in the same process as the circuit artwork. The circuit pattern and fiducial should be etched at the same step. 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: global, local, panel, image fiducials.Global Fiducials: fiducial marks 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 if used to locate the circuits from the panel datum.

Image Fiducials - Global fiducial marks on a multiple printed circuit board fabrication panel that are located within the perimeter of an end-product printed circuit board.Local Fiducial - A fiducial mark (or marks) used to locate the position of an individual land pattern on a printed circuit board. Three local fiducial marks should be provided when it is necessary to provide the most accurate correction for both translational and rotational offsets. For these applications the three local fiducial marks should be in a triangular pattern and should be located as far apart as possible within the perimeter of the land pattern.Panel Fiducials - Global fiducial marks on a multiple printed circuit board fabrication panel that are located outside the perimeters of the end-product printed circuit boards.

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. This is true even if the circuit design contains no fine pitch (