Why Fiducial Marker Placement is Crucial For Automated Assembly of PCBs

Did you ever wonder how vision and robotic systems in surface mount assembly machines (SMTs) are able to situate electronic components exactly where they need to go on a tiny printed circuit board, within extremely close tolerances?

The targets for an SMT vision system are multiple: matching the circuit pattern of the artwork with the top or the bottom of the fabricated printed circuit board (PCB), determining the location of the PCB along the x, y, and z planes, and locating the precise position on the circuit to place a specific component. With the help of the indexing (positioning) system, the PCB is positioned per the program parameters, however, this positioning is not exacting enough for component placement. Fiducial markers provide reference points for imaging systems.

What are Fiducial Markers?

Although it sounds like a financial term, the term "fiducial" means a fixed basis of reference for comparison purposes. In general, a fiducial marker is an embedded object or a surface mark, used as a directional measure or reference point for indexing and imaging systems. 

Fiducial markers are used in multiple industries and applications, such as in physics, 3D computer graphics, optics, and photography. They are used in geophysical airborne flights to reference survey measurements, in augmented reality media to pinpoint locations to activate synthetic images or game board pieces, and in medical imaging by correlating body scans with locations in the body.

In automated electronic assembly applications, fiducial markers are alignment marks for the indexing system to orient the PCB in the correct direction and skew. The vision system uses fiducial marker placement to precisely insert components on a circuit. Also known as "circuit pattern recognition marks", fiducial markers are a crucial aspect of the entire automated manufacturing process all the way through testing and packaging. The importance of fiducial marker placement cannot be underestimated, because if at any point during assembly the exact location of the board or the circuit does not measure true, there is an automation "train wreck"! 

How Does Fiducial Marker placement affect component insertion? 

Precise fiducial marker placement ensures that your PCB is oriented in the correct direction and aligned as close to perfect as possible. Assembly machines use cameras to locate fiducial marks, and then adjust component placement based on the exact position of the board. 

The downside to robotic assembly arms is they won’t complain with you about how long the work day is. 

SMT machines are fed PCBs for assembly from a conveyor. Each PCB is secured or clamped in a minutely different orientation than the others, and this tiny variance is sufficient to ruin a board unless it is recalibrated. To achieve this, PCBs are marked with two or three fiducials to allow placement robots to precisely determine the board's orientation. By comparing the actual location of the fiducial markers relative to the board plan stored in the machine's memory, the vision system can reliably compute the "offset" to which parts must be moved to accurately place the components. 

How Are Fiducial Markers Set in Place on Printed Circuit Boards? 

Fiducial markers are usually made by leaving a circular area of the printed circuit board bare from solder-mask coating. Inside this circular area the copper plating beneath is exposed. Good image recognition requires that there must not be anything around the fiducial marker, so care must be taken in the assembly that there is a "matte" background around the fiducial marker. Alternatively, it is possible to use clear solder-mask lacquer to cover the fiducials, because the vision system can still detect the copper circle. 

From the first two fiducials (placed diagonally from each other) the machine can recognize where the PCB is at by its X and Y position, and also how skewed the PCB is in the conveyor clamps. Finally, the third fiducial helps the machine to compensate for any shrink or stretch of the PCB. 

If components are placed on double-sided PCB designs, it is necessary to place fiducials on both sides of the PCB. Multiple fiducial marker placements for PCB orientation are called "global fiducials”. Components which require a high degree of placement precision may have singular "local fiducials" placed near that component location to afford a greater degree of precision targeting.    

What happens if no fiducial markers are utilized?

Not utilizing fiducial markers is fine if you are manually assembling single or prototype boards. If your intent is to go into automated production, however, you will find that the SMT placement machines will not be an option, as their operation requires points of reference on the PCBs to align them properly. Also, manual assembly results in a higher percentage of boards failing test and quality standards, and many hours will be spent fixing imperfect manual soldering.

Don’t stress over the soldering, take care of your design beforehand.

Fiducial marker placement tips:

Here are some tips for placing fiducial markers on your printed circuit boards:

1. The optimal size of a fiducial marker should be between 1 mm and 3 mm. Target for a matte "buffer" area around the marker of about the same size as the marker diameter. 

2. The fiducial marker must be free from solder mask, unless the mask is clear.

3. For global fiducials, there should be at least 2 and preferably 3 markers for precision accuracy in component placement. The markers should be positioned at opposite ends from one another and separated as far as possible on the board. If you have 3 fiducial markers, position them in a triangular shape, keeping as wide a separation as possible on the board.

4. Where there is insufficient space around the edges of the board, one global fiducial marker is required.

5. The fiducial marker should have a distance of 0.3 inches to the edge of the board, excluding the buffer area of the fiducial marker.

6. For local fiducials, place at least two fiducial markers diagonally on the outside edge around the specific component. To correct for placement errors on components, local fiducial marks may also be utilized. 

Fiducial marker placement can make the difference between a smooth assembly line and one that is continually requiring adjustments. It is a wise practice to always incorporate fiducial markers in printed circuit board designs, even if suppliers tell you that they have upgraded their machines to work without the markers. Chances are high you may need those markers during testing or possible future rework. 

Need more tips in placing fiducial markers on your design? Cadence can provide expert guidance and innovative suggestions for your PCB design. Be sure to look into Cadence's OrCAD PCB Designer, which assists designers in analyzing for manufacturing readiness and adherence to manufacturer's rules. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.