PCB Technical - PCB fiducial markers in the circuit boards design

In PCB design, the pcb fiducial mark is the round shape of copper,which serves as a reference point for picking and placing assembly machines.The fiducial pcb mark can help the machine identify the direction of the PCB and its surface-mounted components,using packages with small pitch,such as quad flat package (QFP),ball grid array (BGA) or quad flat no-lead (QFN).

There are two common fiducial marks in printed circuit board design:global fiducial marks and local fiducial marks.The global fiducial mark is a copper reference placed on the edge of the PCB, allowing the machine to determine the direction of the board relative to the X-Y axis. Fiducial marks are also used when placing the machine to compensate for any deflection when the PCB is clamped.

The local fiducial mark is a copper mark,located outside the corner of the surface mount component of the quad package.Assembling machines use it to accurately locate the footprint of parts and reduce errors in part placement.This is especially important when you use detailed large quad components in your design.

Fiducial markin PCBs have multiple functions,mainly including:

Positioning and Alignment:Mark points are an important basis for machine and equipment to identify and position the PCB, ensuring that the equipment can quickly find the correct position of the PCB when soldering or patching.

Calibration Reference:In high volume production,Mark dots serve as a key reference for calibration, ensuring consistency and stability of PCBs in continuous production.

Improve Production Efficiency:Accurate positioning of markers reduces machine alignment time and improves the overall efficiency of the production line.

Does modern manufacturing technology require fiducial marks?

I have been designing my printed PCB circuit boards to have both global and local fiducial marks.However,when I saw an article explaining the possibility of omitting local benchmarks,I was very interested.It makes sense to remove the fiducial marks on the smaller PCB to maximize the signal trace space.

Due to advances in manufacturing technology,local fiducial pcb marks can be omitted under certain conditions.On smaller circuit boards,modern assembly machines can use only global reference points to place SMT components.For components with larger spacing,the fiducial mark can also be omitted.For example,the latest machines can accurately place surface mount components with a pitch greater than 1.0mm and above.

That being said,before removing local fiducial marks in your design,it is important to discuss the scope of the manufacturer's machine functions.I have learned that not all manufacturers are equipped with machines powered by the latest technology.On the other hand,never ignore the global fiducial mark in your design.Even if you are using some of the most advanced manufacturing capabilities.

Best practices for using fiducial marks in PCB design

If you want to get the best machine assembly,you need to get your fiducial marks correct. There are few important guidelines when placing fiducial marks in a design.

The fiducial mark is made by placing an undrilled copper layer into a circle.The fiducial mark must have no solder mask.

The optimal size of the fiducial mark should be between 1mm and 3mm.A gap area similar to the diameter of the mark must be maintained.

For the global reference point,3 markers are placed on the edge of the board for best accuracy.In the case of insufficient space, at least one global fiducials pcb mark is required.

The fiducial mark must maintain a distance of 0.3 inches from the edge of the board, excluding the clearance area of the fiducial mark.

For local fiducials, place at least two fiducial marks diagonally on the outer edge of the surface mount component.

Improperly designed  fiducial marks can seriously affect the PCB (Printed Circuit Board) production process.

1.Assembly Errors

Improperly positioned markers can cause equipment to fail to accurately identify the position of the PCB during the assembly process, resulting in misplacement of components. Such errors not only affect the functionality of the components,but may also lead to short or open circuit problems throughout the board, thus increasing the complexity and cost of repair.

2.Soldering defects

If the positioning marks are not set properly, it will directly lead to alignment problems during the soldering process. For example, solder pads may have poor soldering due to misalignment. This can lead to poor contact at the solder joints, reducing the stability and reliability of the circuit and thus potentially affecting the performance of the product.

3.Reduced productivity

Productivity can be significantly reduced due to assembly and soldering errors caused by misalignment. Mislabelling can cause the machine to require multiple readjustments, increasing production time. At the same time, misalignment may lead to additional retesting and repair work, thus increasing production costs and cycle time.

4.Difficulty in testing and repair

Improperly positioned markings can also have a negative impact on subsequent testing and repair. Test equipment relies on accurate marking points to assess board functionality. Improper design can lead to inaccurate test results, which can make troubleshooting and repair difficult.

5.Product quality degradation

Overall, improperly designed location markers can lead to a degradation in the quality of the final product, increasing failure and rework rates. End users may face substandard performance or poor reliability, which can have a negative impact on brand reputation and market competitiveness.

PCB manufacturers are using professional PCB design software,such as Altium's CircuitStudio®, which can insert the pad, change the pad size to zero, and set the correct diameter value to place the pcb fiducial mark.