In the PCBA manufacturing process, precise component placement is critical to ensure optimal circuit board performance and reliability. Fiducial marks (optical alignment points) are essential reference points on PCBs, enabling accurate positioning throughout the pick-and-place process. This article provides a comprehensive analysis of fiducial marks, including their definition, application scenarios, types, and specific functions, to help audience delve into the understanding of their role in SMT processes.
What Is Fiducial Mark?
Fiducial marks are optical alignment points on a printed circuit board (PCB) primarily used to ensure precise component placement during pick-and-place process.
Main functions include:
- Determining the placement position of the PCB on the pick-and-place machine.
- Compensating for slight deformations of the PCB caused by manufacturing processes (e.g., thermal deformation).
- Correcting angular deviations of the PCB.
Relying solely on pads or vias as reference points may significantly increase placement deviation:
- When using vias, placement deviation can reach up to 0.15 mm.
- When using standard fiducial marks, placement deviation is less than 0.05 mm.
Application Scenarios for Fiducial Mark
- Single-Sided Assembly: If the circuit board contains surface-mounted components on one side only, it does not require fiducial marks.
- Double-Sided Assembly: For boards with components on both the top and bottom layers, Fiducial marks must be set on each side.
- Panelized Production: In panelized designs, additional fiducial mark must be placed on the panel frame to facilitate the positioning of the entire panel.
Note: If using PCBA services, it is unnecessary to configure process edges or fiducial markers for prototype PCBs.
Types of Fiducial Marks
Fiducial marks can be categorized into three types based on their function and placement:
Type | Function | Position |
Local Fiducial Mark | Used for precise positioning of specific components, especially for high-pin-count or fine-pitch packages (e.g., QFP, BGA). | Typically placed around these components. |
Global Fiducial Mark | Provides a reference point for the entire PCB. | Usually located on the diagonal corners of the PCB to ensure accurate alignment during placement. |
Panel Fiducial Mark | Used for positioning the entire panel, ensuring proper placement in the pick-and-place machine. | Typically located on the tooling strip or rail edge of the panel frame. Even if individual boards have Mark points, they require additional panel Mark points. |
Specific Functions of Fiducial Marks
Typically, 3-4 Fiducial Mark points need setting on a PCB to achieve the following functions:
Position Calibration
The first fiducial mark calibrates the XY coordinate position of the PCB; furthermore, it ensures the correct placement on the track of pick-and-place machine.
Angle Correction
The second fiducial mark point corrects any minor angular deviation of the PCB. Even a slight shift of just 0.1° can cause significant deviation. This fiducial mark enables the pick-and-place machine to correct the angular error to within 1%.
Deformation Compensation
The third fiducial mark primarily compensates for deformation in large-sized or double-sided assembly PCBs:
- Large PCBs: They may experience slight distortion due to gravity or thermal stress.
- Double-Sided PCBs: After the first reflow soldering, deformation may occur due to high temperatures.
If four fiducial marks are in use, they should not form a perfect rectangle, with one fiducial mark intentionally offset. This design prevents PCB misplacement when rotated 180° to ensure correct positioning (an idiot-proof/ foolproof design).
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