Fiducial Markers – What They Mean For A PCB - Miracle Electronics

With PCBs being vital components of almost every electronic and electrical device today, it is important to take care of every detailing when designing a PCB. One way to get PCB designing right is by adding reference points to the design, which helps to keep the orientation right. Fiducial markers can help to provide such reference points for imaging systems.

What are fiducial markers?

The term “fiducial” means a fixed-point basis of reference; its purpose being to compare. Fiducial markers are surface marks that designers use as directional measures in indexing systems. These markers work as alignment marks in indexing systems of electronic assembly industries that use automation, helping to orient the PCBs in the right skews and directions. They allow the vision system to insert components precisely on the circuits. Fiducial PCB markers thus prove to be a vital aspect of the entire automated process. These fiducial markers consist of three parts – a solid copper ring on the top or bottom of the copper layer, the circle in the solder mark which is the target that needs to be aligned, and the option text label on the side.

The three benchmarks of a fiducial PCB

First datum – The first datum, depicted by X and Y directions, helps mounting machines to recognize the PCB location.

Second fiducial benchmark – This second benchmark helps the mounting machine to recognize the PCB orientation, and reveals the PCB’s skew in the clamps. Fiducial markers ensure that the PCB is fixed in place, because even a slight rotation of the PCB can result in complete damage of the entire assembly. The machine can measure the angle of the board’s rotation to even the nearest 0.01 degrees, and then compensates all the placements accordingly.

Third fiducial benchmark – This third benchmark assists the mounting machine to compensate for shrinks and stretches present in the PCB.

Importance of fiducial PCBs

Precision is absolutely vital, especially in processes that use automation. Hence, machines need to know the board’s exact location and where to put the components. Fiducial markers not only make this task easy, but also ensure that the orientation of the board is in the right direction and alignment. Assembly machines can locate the precise location of the board by using high-tech cameras. The entire process thus needs not one, but several fiducial marks in an irreversible pattern.

When to use fiducial markers?

Fiducial markers are highly crucial in high-volume PCB assemblies to ensure correct parts placement and registration. Low-volume assemblies don’t necessarily require these marks, but it is good if you use them in such situations too. However, if you are placing your parts manually, then such fiducial markers are not required at all.

When designing fiducial markers

There are four factors you need to consider when designing fiducial markers.

Shape – It is best to stick to circular marks because this shape is recognized universally by every machine. Other shapes like squares, diamonds, and hourglasses may or may not be recognized by a machine.

Size – The basic size of fiducial pads ranges between 1 and 3 mm, but you could experiment out of the range too. However, the basic range makes it easy for the camera and equipment to recognize places as fiducial markers.

Material – The fiducial pad should be of the same material as the rest of the PCB board, ensuring that the place is free of solder mask or silkscreen that might cover the markers.

Clearance – There should be appropriate clearance around the pad, because the open space around the PCB makes it easy for the camera to pick up the fiducial markers without interference. In addition, the diameter of the clearance should be at least twice the size of the pad.

While using fiducial markers

When you plan on using fiducial markers for your PCB assemblies, it is important that you follow certain essential guidelines.

• The optimal size of fiducial markers ranges between 1 and 3 mm. You need to maintain a clearance area similar to the marker’s diameter.
• The fiducial marker must maintain a distance of 0.3 inches from the board’s edge, the distance excluding the fiducial marker’s clearance area.
• You need to maintain a circular shape when placing the non-drilled copper layer, keeping fiducial markers free from solder mask.

With PCB assembling being so critical, using the fiducial markers the right way can help you have a seamless assembly line, as compared to when you have to make continuous adjustments. It is thus wise to leave the task to experts, one like Miracle Electronics, the best PCB assembly manufacturer , who can correctly incorporate fiducials in the PCB design, along with taking care of every other designing and production aspect of a PCB assembly from India.