I. Introduction
Choosing where to put the fiducial markers on the PCB may seem like a minor consideration in the great design. These indicators may seem little, yet they play a critical role in quality production. Many designers have no idea as to why they would include an unnecessary piece of copper on their board. While excellent fiducial markings provide uniformity and productivity, poor ones may necessitate costly recording and redoing of previous work.
Embedded on the PCB, fiducials serve as points of orientation for machines that rely on automation. The term “Fiducial Marker” refers to “an object placed in the field of view of an imaging system which appears in the image produced, for use as a point of reference or a measure.”
Automated equipment, such as Surface Mount Technology (SMT) and Automated Optical Inspection (AOI), employ a fiducial mark on a PCB as a reference point. In simple words, fiducial marker is used to provide assembly machines a point of reference while designing printed circuit boards. Machines can not function properly or appropriately assemble parts without fiducial markings. Fiducial PCB markings allow machines to determine the board’s orientation during SMT. Components may be rotated by a predetermined amount based on the PCB orientation and the previous orientation of components on reels, allowing for precise placement on their surface patterns.
Fiducial markers play a crucial role in enhancing manufacturing efficiency by facilitating precise and accurate alignment of components, facilitating quality control processes, optimizing inspections, coordinating multi-step manufacturing, and improving traceability. These markers are strategically placed reference points that are easily detectable by automated systems, helping streamline various stages of the manufacturing process without explicit AI involvement. When manufacturing intricate products, such as electronic devices or mechanical components. Fiducial markers act as visual guides, enabling automated machines to locate and position components accurately. By reducing manual intervention and increasing automation, the manufacturing process becomes faster and more efficient.
Figure 2 Manual Assembly of components of PCB
II. What Are PCB Fiducials?
There are normally two phases, fabrication and assembly, involved in the process of making circuit boards. The circuit board is made during manufacture by etching a design into a substrate, which will serve as the base onto which electronic components will be soldered.
After the fabrication process is finished, the assembly phase may begin, which requires the use of specialized tools. Accurate connections and performance of electrical components are ensured with the help of these equipment.
PCB fiducials are used often during assembly and are an important part of the process. These fiducials are critical for accurately directing the assembly machines along the circuit board. To guarantee correct assembly and peak performance in the final product, fiducials are strategically placed so that machines can readily determine the board’s orientation and align the components properly. PCB fiducials serve a crucial role in the assembly process, improving the quality and functioning of electronic products by facilitating the rapid and precise positioning of components on circuit board.
III. Understanding How Fiducial Marks Work
Fiducial marks are recognition marks that are made in the form of small pads on the PCB board to serve as reference points for aligning the PCB stencil and for automatic component placement by Pick and Place machines. Automated machines utilize these locations during PCB Assembly to place the board properly. These locations can be measured and used as a reference for placing SMT components.
The machine employs image recognition to pinpoint the precise location of the fiducial markers on the PCB board whenever an operator loads the board for printing or component installation. The machine will automatically adjust its position in relation to the fiducial markers in this way. Fiducial markings are often included in PCB circuit artwork. This means that the fiducial markings and PCB circuit layout should be engraved simultaneously.
Copper with an anti-oxidation coating is the ideal foundation for a fiducial mark. Having three fiducial markers on the PCB ensures that the automated machine will load the board in the correct orientation every time. It is also allowed to use two fiducials.
IV. Importance of using Fiducial PCB
Fiducial markings are essential for assembly machine selection and placement despite being small. Fiducial markers on the printed circuit board (PCB) are a necessity for several reasons, some of which are listed below.
Accuracy:
the automated machine can guarantee the components are positioned exactly in the right area on the PCB by using a reference point. When working with PCBs that include microscopic components or have limited surface area, this is very crucial.
Accurate inspection:
manufacturers may check for proper component alignment and soldering. The PCB can be visually inspected after construction thanks to the fiducial markings. For high-stakes uses like aerospace and military electronics, this can be a game-changer.
Cost Saving:
you may save money by using reference marks since they lower the possibility of mistakes and flaws, which in turn lowers your expenditures. Otherwise, the errors may necessitate throwing out the entire PCB. Eliminating mistakes using fiducial PCBs can boost production rates.
V. PCB Fiducial Size
PCB fiducial size is usually between 1 and 3 mm. This is either the size of the copper that is visible or the width of the solder mask opening. But the right size for you rests on how well your maker can put things together. Most of the time, three fiducials are placed at the corners of the board. This gives two measures of the angle of alignment, which when added together give the direction of the PCB. The pick-and-place machine can then figure out the right direction from this.
Some manufacturers will give a specific size, which also relies on what your manufacturer’s assembly tools can do. In general, the diameter of the hole in the solder mask should be twice as big as the diameter of the bare copper for the fiducial. However, some makers prefer that the diameter of the hole in the solder mask be three times as big as the diameter of the fiducial. Also, the global and local PCB fiducial sizes on the same board should be the same and shouldn’t change by more than 25 microns.
If you are putting together a two-layer board, the fiducials for the top and bottom layers should be on top of each other. This may come as a surprise. You might think that the pattern of the fiducials should be the same on both sides, but I’ve never seen a maker say this in their instructions. The hole in the solder mask should be the same size on both the top and bottom layers of the PCB.
Figure 3 Recommended PCB fiducial size and solder mask opening.
The typical size of a local fiducial is 1 mm, while the size of the aperture in the solder mask is 2 mm. So that traces can be routed and other parts can fit, the local PCB fiducial size is typically no more than 1 mm. A local fiducial is unnecessary in the PCB layout when working with small components like a 0201 resistor or small BGA. In certain circumstances, the fiducial may even be bigger than the component itself.
Because fiducials are mechanical components that are not electrically linked to anything on your board, it is easy to alter the footprint and install a new fiducial if necessary. If the fiducial size of your printed circuit board (PCB) is incorrect, some manufacturers may adjust it for you. The cutting-edge PCB design and layout tools make it simple to create and position copper components like fiducial markers, pads, polygons, and more. To produce manufactured circuit boards, users can use a single integrated design platform that includes both circuit design and PCB layout tools.
VI. Types of PCB Fiducials Markers
There are usually three kinds of fiducial marks on PCB designs: panel fiducial marks, global fiducial marks, and local fiducial marks.
1. Panel Fiducial Marker
Panel points of reference are put on the rails of the panel. These marks are utilized to accelerate the manufacturing process. These rails will be used by automated tools to place the whole panel. Considering that this approach does not consider the specific printed circuit boards (PCBs) on a panel, incorporating fiducial marks on the panel is most suitable for designs featuring components of medium size or greater. To get optimum results, put the fiducial marks far apart as possible.
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2. Global Fiducial Markers
Each board is equipped with global fiducial markers that are securely attached. Utilize these to improve the manufacturing process’s precision. Global markers provide a significantly more dependable approach for precise triangulation compared to panel markers. During production, distinct calculations are performed for every single PCB location.
On all PCB layers containing components that will be affixed using automated equipment, global fiducials should be positioned. This is true even if there are no fine pitch (=.020″ pitch) components in the circuit design, as the majority of modern assembly equipment employs vision recognition for PCB alignment. While it is possible to use other circuit objects (via-holes, etc.), this can compromise the accuracy of component placement. The only disadvantage is that panels containing multiple PCBs, deciphering every global fiducial mark can significantly increase processing time. Global markings, like panel marks, works more effectively when placed at distance. Consider placing them in close proximity to the edges of the PCB.
3. Local Fiducial Indicator
Local fiducial markings, unlike previous marks, are typically considered optional rather than obligatory. Position these markers around particular elements that may be either too minuscule or excessively intricate for regular precision. Examples of such components encompass QFPs, BGAs, and QFNs featuring a fragile pitch. To attain utmost accuracy, kindly locate these reference points at diagonally opposite corners of the respective component.
Figure 5 Panel, Global and Local Fiducials
VII. Factors Influencing the Need for Fiducial Markers
In contemporary manufacturing practices, the utilization of fiducial markers remains a significant aspect for ensuring accurate and efficient production processes. These markers, typically small reference points or indicators, play a pivotal role in facilitating alignment, registration, and calibration procedures within various manufacturing technologies.
Modern manufacturing techniques encompass a wide array of advanced processes, such as computer numerical control (CNC) machining, automated assembly lines, additive manufacturing, and robotics. These cutting-edge technologies often involve intricate components and intricate operations that demand precise positioning and alignment.
Figure 6 Automatic Assembly of PCB
Fiducial markers are visual indicators that facilitate the alignment and placement of components or work pieces in the course of manufacturing operations. The markers have been intentionally positioned and configured to facilitate precise identification of particular reference points by either automated systems or human operators. Through this process, they guarantee that subsequent stages of production are carried out with the highest level of accuracy and consistency.
Fiducial markers are utilized in CNC machining to establish a reference frame that ensures precise part placement, tool alignment, and measurement calibration. This capability allows the machine to accurately carry out predetermined commands and produce uniform outcomes. Similarly, fiducial markers play a crucial role in automated assembly lines by aiding in the alignment of components, verifying their correct orientation, and facilitating efficient robotic manipulation.
Even in additive manufacturing, where 3D printers build objects layer by layer, fiducial markers aid in ensuring accurate placement and alignment of subsequent layers. This helps mitigate any cumulative errors that may occur during the printing process, resulting in high-quality and dimensionally accurate printed objects.
While modern manufacturing technologies have witnessed remarkable advancements, the implementation of fiducial markers remains indispensable. They provide a reliable means to establish reference points, achieve accurate alignment, and maintain consistency throughout the manufacturing process. By incorporating fiducial markers into manufacturing workflows, companies can enhance product quality, increase production efficiency, and ensure reliable performance in various industries ranging from electronics and automotive to aerospace and beyond.
Figure 7 Automated assembly machines put components using fiducial PCB.
VIII. Optimizing the Fiducial Design
In many disciplines, including electronics, robotics, and computer vision, optimizing the fiducial design is a crucial step. By optimizing their design, we can enhance the precision, resiliency, and productivity of these processes. Given are some considerations for optimizing fiducial design:
1. Shape
After selecting fiducial marking locations, it is time to determine their appearance. While certain manufacturing equipment has been programmed to identify various geometrical shapes like diamond, hourglass, and squares, not all equipment possess this capability. Because of this, utilize the universally acknowledged circular fiducial mark.
2. Size
The mark size is somewhat more flexible. Range for average diameter of fiducial pad lands between 1 to 3 mm. This small size helps the camera and software quickly identify the pad as a special mark. Numerous designers have discovered that a 1mm round fiducial with a 2mm masking area around it is the most effective.
3. Material
Fiducial pad must be made from the same metal as the remainder of board. To make sure everything works properly, avoid covering the pad with any materials like silkscreen, solder mask, or anything else.
4. Markdown / Clearance
Make sure there is enough empty space around the fiducial device. Leave an area without any solder mask, copper, or silkscreen around the electrode. This space helps the camera see the mark clearly. The empty area’s diameter should be at least twice the size of the cushion. For example, if the cushion is 2mm, the empty area should be 4mm. Square and circular empty areas are commonly used shapes.
Figure 8 Different PCB Fiducials Size.
IX. Tips for Using PCB Fiducials Effectively
When considering the type of PCB fiducial to use, it’s important to note that global, panel, and local fiducials serve similar purposes but may differ in their shape, location, and quantity. Therefore, selecting the appropriate type must be the primary concern.
- To determine the placement of fiducials, it is advisable to position global and panel fiducials at the panel’s or board corners to ensure sufficient edge clearance. Additionally, it is important to place fiducial marker next to every Surface Mount Technology (SMT) component.
- Choosing the optimal shape and size for fiducial markings can be crucial. A commonly used PCB fiducials size is 2mm diameter for the opening of solder mask, having center copper marking of 1mm. However, for precise specifications regarding shape, size, and clearance of fiducials, referring to the IPC-751 guidelines is advisable.
- The number of PCB fiducials to employ is not fixed, but some general guidelines can be followed. For panel markings, it is generally recommended to use at least three fiducials. For global marking, two fiducials are often sufficient, while one fiducial is typically suitable for local markings.
X. Incorporating the Fiducial Marks
Where should the fiducial symbols be placed for optimal results? The answer will vary based on the overall design of your PCB panel, but there are some general guidelines you can use to get it right every time. When incorporating fiducial marks into a printed circuit board (PCB), it is essential to discreetly consider the following factors:
Placement on Copper Layers:
Fiducial marks should be added exclusively to the top or bottom copper layers of the PCB.
Avoiding Solder Mask Coverage:
It is crucial to ensure that the fiducial marks are not covered by the solder mask. The automated system relies on identifying these marks, and covering them with solder mask would render them untraceable.
Contrast Enhancement:
To optimize performance, it is desirable to achieve a significant contrast between the surface of the fiducial mark and the adjacent PCB base material.
Shape Selection:
While round-shaped fiducial marks are commonly used, square or diamond shapes are also acceptable alternatives.
Precision Considerations:
Placing the fiducial marks as far apart from each other as possible on the PCB contributes to better precision.
Diameter Range:
Fiducial marks typically possess a diameter ranging from 1 mm (0.040 in) to 3 mm (0.120 in). It is important to ensure that the size of the marks on the same PCB does not vary by more than 25 microns (0.001 in).
By attentively adhering to these guidelines, the integration of fiducial marks into a PCB can be optimized.
XI. Advantages of Using Fiducial Marks for PCB Assembly
Fiducial marks offer several advantages when it comes to PCB assembly.
First, they guarantee regularity and precision while installing parts. In order to ensure that components are correctly positioned and aligned during assembly, fiducial markings are etched onto the PCB to serve as distinct reference points. The constructed PCB benefits from this uniformity in quality and dependability.
Second, fiducial markings stand out against the background of a printed circuit board (PCB). Both automated optical inspection systems and human operators benefit from the contrast since the fiducials are simpler to identify and track. The significant difference aids in mistake prevention and streamlines the assembly procedure.
In addition, fiducial markers are consistently sized, shaped, and placed. Because of this uniformity, producers may set up concrete rules and benchmarks for PCB assembly. Regardless of the PCB design or production run, the efficiency and accuracy of the assembly process may be guaranteed by using these universal fiducials. Because fiducial markings are standardized, everyone engaged in making and assembling PCBs may easily communicate with one another.
Because of these benefits, the PCB assembly process is faster, less error-prone, and of higher quality.
XII. Conclusion
Fiducial markers are small yet indispensable components in the design and fabrication of printed circuit boards. The markers function as reference points for automated machinery and aid in achieving accurate alignment and placement of components in the course of assembly. The integration of fiducial markers in printed circuit board (PCB) designs can potentially enhance precision, minimize inaccuracies, optimize manufacturing efficiency, and streamline inspection procedures for manufacturers. Comprehending the significance of fiducial markers and adhering to optimal procedures for their positioning and configuration can significantly enhance the general caliber and efficacy of electronic commodities.
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