What Are Fiducial Marks on PCB? Types, Purpose & Design Guide

What are Fiducials on PCB?

Fiducials are small, precisely placed round copper marks on a PCB, which serve like reference points for automatic assembly machines. These marks enable pick-and-place equipment and optical inspection systems to recognize the orientation of the board, thus placing components with accuracy. Normally, a fiducial would comprise a bare copper pad without any covering of はんだマスク or silkscreen, providing a high-contrast target for machine vision systems. 

They are especially useful in high-density PCB designs where a correct alignment must be ensured to avoid placement errors. Fiducial marks are essential in the SMTアセンブリ for the correct positioning of fine-pitch components such as BGAs and QFPs. Without fiducials, there is a possibility that automatic equipment will fail to align the PCB correctly, which may result in misplacement, soldering defects, and consequently, circuit failures.

What is the Purpose of Fiducial PCB?

Fiducial marks have become an essential part of modern PCB manufacturing due to their dependable reference points, which are provided to automated assembly machines. They help in maintaining precision, reducing errors, and improving production efficiency. Machines without fiducials may have difficulty correctly aligning components, leading to defects and operational failures. Below are some key reasons why fiducial marks are essential in PCB design and assembly.

Enabling the Accurate Placing of Components

Fiducials enable pick-and-place machines to work out the exact position and orientation of a PCB. This enables each component to be placed precisely where it needs to be, reducing errors and improving manufacturing consistency.

Misalignment Compensation

In the fabrication of PCBs or their assembly, there are slight variations in board dimensions or component positioning. Fiducial marks will help in compensating for such minor misalignments through a stable reference, hence the ability of the machines to make runtime adjustments with high accuracy for placements.

Increase in the Speed of Assembly

Machines without fiducials take extra time and require manual interference to locate and align the PCB. Fiducials make this process easier by giving reference points quite clearly, hence enabling faster automated assembly.

Supporting High-Density PCB Designs

While the electronics are getting smaller, the PCBs must be designed with closer and smaller components. Fiducial marks become more important in high-density designs where fine-pitch components like BGAs and QFPs have to be correctly aligned to avoid electrical faults or connectivity issues.

Types of Fiducial Marks on PCB

Fiducial marks can be categorized in relation to functionality and location on the PCB. Each of these types has a special purpose that will ensure precise alignment at higher levels in the アセンブリプロセス. The three most common types include global fiducials, local fiducials, and パネル fiducials; each has its role in improving precision and efficiency in manufacturing. Each of them is described below.

1. Global Fiducials (Board-Level Fiducials)

• This is for aligning the whole PCB within the pick-and-place machine’s vision system.
• Size: Normally, the global fiducials are 1.0 – 2.0 mm in diameter, but it must have a minimum 2.5 mm clear area around them.
• Placement: These are normally placed on three corners of the PCB to help define the orientation. A triangular arrangement will make the positioning accurate.
• Avoid Placement: It is undesirable to place global fiducials close to vias, pads, or silkscreen markings to avoid interfering with the optical recognition process.

2. Local Fiducials (Component Level Fiducials)

• Purpose: To be used for the fine-pitch components, like BGA and QFP, for the correct placement of such elements.
• Size: Usually 1.0 mm diameter, at least 1.5 mm away from other features.
• Placement: A number is found around fine-pitch components, one or two per component, to enable its accurate alignment.
• Avoid Placement: Do not place local fiducials close to the component’s solder pads as this will tend to give incorrect interpretation to the vision system.

3. Panel Fiducials (For Panelized Boards)

• Objective: When more numbers of PCBs are fabricated on a single panel, then used for correct alignment during depanelization.
• Dimension: It should be identical in size with the global ones that are between 1.0 -2.0mm in diameter size.
• Placement: To go on edges to locate and provide an overview and align an overall panelized assembly of PCB.
• Avoid Placement: Avoid placing them over tabs of break-away or in ‘V-grooves as that could cause damage when there’s separation from a panel.

Design Considerations of Fiducial PCB

Good fiducial design contributes to easy and accurate PCB assembly. Lousy fiducial designs result in misalignment, bad placements, and increased defects in manufacturing. Ideal fiducial design practices will enable the manufacturer to improve their alignment accuracy, hence reducing errors in production. Here are some important design considerations to remember:

Fiducial Placement Guidelines

• Global fiducials shall be located on three non-collinear points on the board. This provides the possibility of the pick-and-place machines getting the right angle and scaling properly to position a board correctly because of a triangle of fiducials.
• Local fiducials should be placed near fine-pitch components. High-precision components such as BGAs and QFPs require additional fiducials close to their pads. This helps machines make adjustments for slight misalignments at the component level.
• Leave at least 2.5mm clearance around the fiducials. There needs to be ample open area left around a fiducial and free from any traces, silk screen, or any other element on the board to avoid a possible error due to optical identification. 
• Place Symmetrically Fiducials for Uniformity. Symmetry in the fiducials will enhance accuracy through which machines should process and therefore align the correct PCB with their ease.
• Standardize the size of fiducials across all designs. Consistency in the design of the fiducials ensures that machines will be able to recognize the fiducials without needing reprogramming for different PCBs, hence reducing manufacturing setup time. 

Size, Shape, and Contrast

• The shape should be circular for uniform optical recognition. Circular fiducials prevent misinterpretation due to orientation, ensuring the machine identifies the correct reference mark.
• Should be 1.0 mm to 2.0 mm in diameter. The size of the fiducial should be such that the optical systems can find it, yet it shouldn’t be so large as to occupy too much PCB space.
• Contrast to be maximized by leaving the fiducial as exposed copper. Fiducials should not be covered by a solder mask or silkscreen. A high contrast between the fiducial and the PCB background ensures better visibility for machine vision systems. 

Layout Integration

• Fiducials have to be in a copper layer but not soldier-masked. This gives it good optical contrast and removes all possibilities of soldering or some other assembly distorting it.
• Standardize size and position for multiple PCB設計. Uniformity makes it easier to manufacture, with less possibility of machine misinterpretation.
• Avoid placing fiducials close to silkscreen markings or solder pads. The overlap of elements, or the proximity to them, might lead to disturbance in the recognition of the fiducials, causing problems with alignments.

These are general considerations in the design process to enable the fiducial marks to serve their intended purpose effectively and to increase assembly accuracy, therefore increasing production efficiency.

結論

Fiducials have become important in today’s プリント基板製造 for setting the position of components, thus reducing assembly errors. By following best practices in designing the size, placement, and clearance of fiducials, a manufacturer can optimize their automated assembly process for higher-quality and more reliable PCB production.

Applying the appropriate fiducial strategy will be instrumental in attaining not only better manufacturing efficiency but also help handle the increasing complexity of modern PCB設計.