The electronics manufacturing industry is composed of hundreds of different kinds of equipment, from pcb fabrication machinery to electronic component placement and soldering ones. But, when there is a very complex circuit full of different components, how to guide a sophisticated machine assisted vision system to place a part with great accuracy? PCB Fiducial marks have been widely used among the industry, but the term not only relies for the electronics, but for many other industrial and research areas as common use to assist different machines and algorithms. In this article we will have a deep dive on the markers for the electronics industry.
What is fiducial marks in PCB
When you take a closer look to a PCB, have you ever noticed small golden colored dots on special or specific places that might not be connected to any other net or component within the board? Moreover, they seem to have no specific function at all. These special dots are known as PCB fiducial marks and they have the task to assist component placement machines in the PCBA process, they come in specific sizes and there is some to them for the stencil printing as well. PCB fiducial marks are made of copper, more on this below.
What are PCB fiducial marks used for
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How do PCB fiducial markers work
The general recommendation for the PCB fiducial markers is to place two or three, being better the latest. The purpose of the first one, as commented previously, is to orient the board within the X-Y space. The second marker or fiducial helps to understand the board orientation and skews. It is important to ensure that the board rotation does not go above a tenth of a degree, hence the continuous visual feedback will generate compensation whenever slight rotation is detected. Finally, the last fiducial marker would help understand if the board had changes in standard size, since some boards actually do change in size depending on their fabrication and overall length.
Where do you place fiducial markers in PCB
Since the PCB fiducial marks are used as references, it is better to have them placed as outside as possible of the PCB, and then it is better to follow a specific PCB manufacturer/machine designation for padding, being five millimeters the standard one. Also, depending on the number PCB layers, there might be a need to place fiducials on both sides of the board. For component or local fiducials, it is recommended to place them for components with small pitch, diagonally and have them to be visible after the component placement, meaning that proper component spacing is advised. Most designers shall place them on outside areas without surrounding components, on the corners of the board, but there is also additional guidance on their size and placement depending of a specific PCB manufacturer’s requirements.
What are fiducial markers made of
PCB fiducial marks are not part of the board’s silk or mask but outside copper layers of the design, hence they are made of copper. However, since they are lasered from the bottom side when the SMT stencil is produced, they should be indicated in the solder paste data.
What is the size of the fiducial marks on a PCB
Typically, a standard PCB fiducial mark will have a diameter of one millimeter, with three millimeters of exposition in the PCB solder mask, which can be round or squared pattern. Depending on the number of components and spacing, these markers can have a minimum diameter of around 0.5 mm, with a minimal open area of one millimeter; if there is similar spacing from the outer solder mask spacing and marker size ratio, it should be fine.
Generally, a PCB manufacturer shall provide their recommended size according to their own equipment capabilities, although the size of the exposed area should be twice the copper size as a “rule of thumb”. Finally, the average size of the markers withing the board should not vary 25 micrometers to ensure quality of the markers; ensure that there are no surrounding blank PCB (regional pours) within the exposed area to ensure proper visibility.
How many fiducials do you need in PCB
As stated above, the number of PCB fiducial marks has to do with the number of layers. While a single slayer PCB will require at least one fiducial, three markers are recommended for the already stated reasons. Hence, up to six global fiducials shall be placed on a two-layered board.
As for the local fiducials, it is important to have the diagonal markers, so as a general formula, it should be double the number of total components with small pitch (less than 0.5 mm). Since the fiducial markers are usually named with the F literals, number 1 is used for position, number 2 for orientation and number 3 for size. Therefore, the marks F1 through F6 shall be better understood by the stated numbering and naming convention.
Conclusion
In this article we presented a way to understand the purpose, main rules of thumb and functionality of fiducial markers for PCB fabrication. Although these markers are becoming out of use with latest pick and place machinery, most PCB manufacturers still require them and may charge an extra amount for placing them whenever they have not been placed by the designer or no panel was done at all. Try to utilize standard fiducial sizes and appropriate placement to ensure board acceptance by your manufacturer.
These special dots are known as PCB Fiducial Marks and they have the task to assist component placement machines in the PCBA process, they come in specific sizes and there is some to them for the stencil printing as well.
PCB Fiducial Marks are not part of the board’s silk or mask but outside copper layers of the design, hence they are made of copper.
Typically, a standard PCB Fiducial Mark will have a diameter of one millimeter, with three millimeters of exposition in the solder mask, which can be round or squared pattern.
