Have you ever wondered how robotic and vision systems in SMT machines fix electronic components at the exact desired place on tiny PCBs, and within very close tolerances?
There are multiple targets for SMT vision systems. Matching the artwork’s circuit pattern with either the fabricated PCB’s bottom or top, determining the PCB’s location along the planes x,y,z and then locating the circuit’s precise location to place or fix a component. With an indexing system, you can position the PCB with respect to the parameters of the program. However, the positioning isn’t properly exact for the placement of the component. Fiducial markers usually offer reference points for the imaging system.
What are Fiducials?
Fiducials are referred to as small pads, which you can place on a printed circuit board. This assists the pick and place machines anytime you are handling component placement. Also, they are added frequently to the PCB array’s rails. To get better results for assembly, it is advisable that you include fiducials at all times to the design of your printed circuit board.
You should place a fiducial in 3 corners or edges of the PCB design. Although, just 2 fiducials are useful, 3 will make sure that your circuit board isn’t incorrectly put into the machine. The locations for the fiducial pcb has to be included to your data file (Pick & Place file). Once this PCB is loaded, the machine locates your fiducials, and then utilizes image recognition in determining the right location. With this, the machine can orient your printed circuit board.
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PCB Fiducial Mark
These can be referred to as recognition marks that are created as small pads. These are present on the PCB board, and it serves as a point of reference for Stencil alignment of PCBs, as well as for the automatic placement of components.
During the process of PCB Assembly, automated machines use these points to make sure that the board is properly positioned. These reference points are measurable as well and serve as a reference frame during the placement of SMT components.
When the operator gets the PCB board and loads it for component placement or printing, the machine will be able to identify the PCB’s fiducial marks. It then utilizes image recognition to help in matching the right location. By doing so, this automatic machine can align itself with respect to the pcb fiducial marks.
Adding PCB fiducial marks is included as part of the artwork of PCB circuits. This means that the PCB fiducial marks and circuit pattern of the PCB has to have copper protection. This is made possible by anti-oxidation coating. These locations of the fiducial pcb are included in the pick & drop and stencil data file.
You should make three PCB fiducial marks on your printed circuit board. This is because it assists the automated machine in identifying if the location of the board is at the right side or not. It is also acceptable to use two pcb fiducial marks.
PCB Fiducial Marks: Types
PCB Fiducial marks come in two main types. This has to do with their location and placement on the printed circuit board.
Local Fiducial Marks
The local fiducial pcb marks are necessary for locating each component’s position. This happens when the component needs more precision.
Global Fiducial Marks
The global fiducial pcb marks are manufactured to help in differentiating the panel datum from the circuit pattern of the PCB. The location could be done with respect to the grid system having three points. The lower left fiducial can be seen at the point of origin (0,0), while the remaining two take the positive axis X and Y.
If there’s a need to process multiple boards at once, then it is referred to as panel fiducial.
When should you use PCB Fiducial Marks?
All the time, high volume assembly needs fiducial pcb marks to make sure there’s accurate parts placement and registration. Low volume assembly, which you will see at a prototype or low-volume specialist, doesn’t always need them.
However, because there are shops of low volume that rely solely on them, you should ask before you assume. Although PCB fiducial marks may not be necessary for the assembly of any product, they are always a great idea and they still play a role. The time when you can be sure of not needing PCB fiducial marks is when hand placing these parts.
However, due to the fact that the copies of the board are manufactured in an assembly line, then there’s a need that the machines understand the difference between the board’s sides and top/bottom. You wouldn’t want the placing of your electronics to be done in a way that there’s no special pattern it follows.
Constructing a PCB Fiducial Mark
Regarding the specific PCB fiducial marks construction, there are two very important things. These are contrast and accuracy.
Contrast
The machine’s cameras require good contrast. This is one good reason why this mark must be bare copper. The solder mask found on a copper pad top will reduce the contrast up to a point that it may not be seen by the camera.
Just like silkscreen, solder masks have similar registration issues. Therefore, you need to be sure that none of the pad’s parts are obscured. Ensure the diameter of the copper pad falls within 1 – 2 mm. The opening of the mask has to be twice to five times larger compared to copper. There are fiducial marks in some CAD packages. This route is the easiest.
Position Accuracy
You must create a fiducial in the top layer made of copper. Now, the question is, why make use of copper when in theory, a drill hole or a silk screen would be fine? Registration is the answer. In just one operation, the top copper as a whole is put down. Therefore, a surface mount pad’s positional accuracy relative to the copper fiducial remains the same all the time. Drill holes and silk screens are included in different operations. This means that there’s a chance this registration varies for different boards.
Shapes of PCB Fiducial Marks
Most PCB fiducial marks utilize a round cutout. However, this shape isn’t that important. With this, the copper pad ought to be round as well. If this board is from a fab house and in a panel, then the PCB fiducial marks can be placed on the rails of the panel. Also, if these boards will be received by you as individuals, all you should do is to place these PCB fiducial marks on these boards. If you doubt, then you can do both. By doing so, whether you assemble the boards as a panel or there’s a need to depanelize the board before the assembly, then there’s no issue.
Take note that, at low volumes, fiducial marks aren’t always necessary. However, they remain useful. Electronics with great quality are tied closely to risk reduction. This begins with a great design for your PCB.
PCB Fiducial Size
Generally, the PCB Fiducial size is around 1 to 3 mm. However, the right size for you will depend on the type of assembly machines the manufacturer you are working with makes use of. There are manufacturers that advise the addition of 3 fiducials placed at the board’s corners. This is because it provides 2 angular measurements. Also, it permits the machine that picks and places to infer the right orientation.
There are manufacturers that give a desired size. This depends also on the type of assembly equipment your manufacturer utilizes. Generally, the solder mask opening’s diameter should be twice the size of the bare copper’s diameter for the fiducial. Though there are manufacturers that prefer that the opening of the solder mask is triple the diameter of the fiducial. Asides from this, the PCB fiducial size present on one board (both local and global) has to be consistent. Also, it must not vary by over ~25 microns.
Let’s say you are putting together a two-layer board, the bottom and the top layer fiducials have to sit on each other. One may feel that the fiducials’ layout has to direct mirror images. However, we have not heard of a time a manufacturer included this in the guidelines. The bottom and top layer of the PCB fiducial size has to be exact; this includes the opening of the solder mask.
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PCB Fiducial Size: Local Fiducials
Local Fiducials must be as little as 1 mm coupled with a solder mask opening of 2mm. You also must be attentive to the D-3D rule, because your manufacturer may prefer a larger opening of solder mask for the PCB fiducial size.
Most times, the PCB fiducial size for local fiducials is not higher than 1mm. This is to ensure that trace routing is possible. It will also create space for some other components. Small components like the chip-size BGA or 0201 resistor, the machine for assembly will be very accurate. This means it will not need a local fiducial pcb. Also, the machine will know where exactly your components have to sit.
It shouldn’t take you anything to check the correctness of your solder mask and PCB fiducial size before sending the design to your producer or manufacturer. As expected, fiducial pcbs are grouped under mechanicals and don’t have any connection to anything. This is why it is very easy to adjust the footprint to achieve a customized fiducial. You can also place a new one if necessary. There are manufacturers that will adjust the PCB fiducial size if the earlier size is not proper.
Users can use a single platform of integrated design with PCB layout and circuit design features to create circuit boards that are manufacturable.
Fiducial Point in PCB
Concerning fiducial pcb points, there are no strict rules. It is a great idea if you make your fiducial point is at the PCB’s opposite corners, close to the corner edges. Let’s say you have QFP, QFN, BGA packages, with the layout having a small pitch, and then it is advisable that each fiducial mark is produced close to each of those footprints/packages.
We recommend that this fiducial pcb point is placed on your panel. Also, it is necessary and helpful to place about four fiducial pcb points close to the corners of the panel frame. This depends on this machine for assembly.
PCB Fiducial Guidelines
Before going ahead to add fiducial pcb marks, you should consider these points.
- You should only add the marks to the PCB’s bottom or top copper layers
- Don’t cover them with solder masks. This is because the automated system has to identify them. So, covering it with a solder mask will not make it possible to track.
- To ensure better performance, there must be much contrast between the fiducial mark’s surface and the base material of the adjacent PCB board.
- Fiducial marks are usually round in shape. However, diamond and square shapes are acceptable too.
- To ensure better precision, place the fiducial marks distant from themselves on the printed circuit board.
- The fiducial mark’s diameter should fall between 0.040 in (1 mm) to 120 in(3 mm). This PCB’s marks shouldn’t change in size by 0.001 in (25 microns).
Without Fiducial Markers, What Happens?
It’s fine if you decide to stop using fiducial markers. This will be the case if you are assembling prototype or single boards manually. If you intend using automated production, you will discover that the machines for SMT placement won’t be an alternative. This is because their operation deals with reference points on the printed circuit boards to help in aligning them well.
Asides from this, manual assembly causes a large fraction of boards to fail the quality standards and tests. Also, you will have to spend many hours fixing any wrong manual soldering.
Conclusion
Placement of fiducial markers can make much difference between smooth assembly lines, as well as one that requires adjustments continually. This practice is a wise one, as it integrates fiducial markers always in PCB designs. There is a high chance that you may require those markers when testing or for future rework. Searching for more tips on how to place fiducial markers? RayMing PCB can offer innovative suggestions and expert guidance for PCB design.
