In PCB manufacturing and assembly, precise component placement is essential for high-quality products. This precise placement is achieved through the use of PCB fiducial marks, which serve as optical reference points, allowing automated equipment to accurately locate and orient the circuit board during the assembly process.
What are Fiducial Marks?
Fiducial marks are special copper features on a PCB that serve as optical targets for automated assembly equipment. These marks are typically circular pads with exposed copper surfaces, providing high contrast for easy identification by machine vision systems. In the surface mount technology (SMT) process, pick-and-place machines use these marks to determine the precise position and orientation of the circuit board before placing components.
The term "fiducial" comes from the Latin word meaning "trust," which accurately reflects their purpose: to provide a reliable reference system that machines can trust for precise assembly operations. Fiducial marks exist purely as mechanical reference points for manufacturing equipment.
Types of Fiducial Marks
PCB fiducial marks can be categorized into two main types based on their application and location on the circuit board.
Global Fiducial Marks
Global fiducial marks, also known as panel fiducial marks, are located at the edges of the entire PCB. These marks determine the overall position and rotation of the circuit board within the assembly machine's coordinate system. At least two global fiducial marks are required, but three are recommended to create a more reliable reference system, compensating for board warping or dimensional variations. A three-point configuration forms a stable plane, allowing the equipment to calculate the precise position even if the board is slightly deformed.
Local Fiducial Marks
Local fiducial marks, also known as component-level fiducial marks, are located near specific components that require extremely precise placement. These marks are particularly important for fine-pitch components such as ball grid arrays (BGAs), quad flat packages (QFPs), and chip-scale packages (CSPs). Local fiducial marks enable pick-and-place machines to achieve positioning accuracy of a few micrometers. Each critical component typically has two to three dedicated local fiducial marks around its package.
Fiducial Mark Design Specifications
Correct fiducial mark design is crucial for reliable machine recognition and precise assembly. Several key parameters must be carefully considered during the PCB design phase. Dimensions and Shape
The standard fiducial mark is a circular copper pad, typically 1 mm in diameter for most applications. For high-precision requirements or when using advanced assembly equipment, the fiducial mark size can range from 0.5 mm to 3 mm. A circular shape is preferred because it maintains consistent optical properties from any angle, making identification more reliable than square or rectangular marks. The copper surface must remain exposed and not covered by solder mask to ensure maximum contrast and reflectivity.
Keep-Out Area
A keep-out area must be maintained around each fiducial mark, free of copper features, solder mask, silkscreen, or other markings. This keep-out area is 2-3 times the diameter of the fiducial mark and prevents visual interference, avoiding misinterpretation by machine vision systems and ensuring accurate identification of the fiducial mark under various lighting conditions.
Materials and Surface Treatment
Fiducial marks should use bare copper or a non-oxidizing surface finish such as Electroless Nickel Immersion Gold (ENIG) or immersion silver. These surface treatments offer excellent reflectivity and maintain their optical properties throughout the manufacturing process. Solder finishes, such as Hot Air Solder Leveling (HASL), are generally not recommended for fiducial marks because they can create uneven surfaces that affect machine vision recognition.
Role in the Manufacturing Process
Throughout the PCB assembly process, fiducial marks serve several critical functions, extending far beyond simple positional reference.
Automated Optical Inspection (AOI)
In the AOI process, fiducial marks help establish the inspection coordinate system. AOI equipment uses these marks to align the actual circuit board image with the reference design data, enabling accurate defect detection.
Solder Paste Printing
During the stencil printing process, fiducial marks enable precise alignment between the PCB and the solder paste stencil. Stencil printers use vision systems to locate the fiducial marks on both the circuit board and the stencil, then adjust the position to achieve alignment accuracy within 25 micrometers.
Component Placement
Pick-and-place machines are a primary application for fiducial marks. Before placing components, the machine's vision system locates all relevant fiducial marks to calculate the precise position and any rotational offset of the circuit board. This information is then used to adjust the placement coordinates of each component, ensuring accurate positioning regardless of how the circuit board is placed on the machine's support system.
Conclusion
Fiducial marking is a critical step in PCB design, and its correct implementation ensures that the circuit board achieves the positioning accuracy required for reliable automated manufacturing.
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