Fiducials on PCBs, Panels, and Stencils
Many board designers who build for high-volume production should be familiar with fiducials, which are placed around the edge of the PCB. The fiducials are used in automated imaging equipment to measure the orientation and angular skew of the board, which will aid accurate assembly and placement of components. Fiducials are important for placement on the PCB, but they are not the only place they are located.
Fiducials can also be placed on a panel and on a stencil, both of which are used for purposes of rotational alignment in a computer vision system. The rules for PCB fiducials are well-known and designers should generally implement these, but what about stencils and panels? We’ll look at where these are placed when preparing a board for production.
Where Fiducials Are Placed
On the PCB
Placement of fiducials on a PCB is a well-known practice. The fiducials on a PCB are generally placed around the edge of the board, typically near mounting holes (assuming holes are placed in the corners of the board). The fiducials are placed as copper dots that poke through a solder mask opening, typically with copper diameter of 1 mm and solder mask opening diameter of 2-3 mm.
These markings are what most designers refer to when using the word “fiducial”. The technical terminology for the above markings is “global fiducials” in that they are meant to examine the orientation of the entire PCB. Local fiducials may also be placed around specific components (e.g., large BGAs) or groups of very dense components. This gives an additional verification of orientation of the board during processing.
Global fiducials typically appear in the corner of a PCB.
These could be placed by a designer or they could be placed by the fabrication house. If they are present, the assembly house will also use these for alignment and inspection, although there is sometimes a debate as to whether these are still required given the more advanced capabilities of today’s assemblers.
On the Panel
Fiducials are typically placed on panels to provide an orientation measurement for an entire panel as the board proceeds through fabrication processing. The fiducials in a panel are typically placed along the panel edges near the tooling holes, similar to what would be done on a PCB.
Panel fiducials can be the same size as a typical board fiducial. However, given the board arrangement in the interior of the pane, the panel fiducials are reserved for the panel and on the individual board in the panel. The individual board will have their own mark to provide alignment once detached from the panel.
Squeegees are used for manual application of solder paste through a stencil.
Next comes the matter of stencils. A solder paste stencil used in PCB assembly must line up precisely with the pads in the PCB in order to ensure solder paste is applied in the correct areas. Fiducials are an important part of this process as they enable orientation measurement and alignment with the copper features in the PCB.
On the Stencil
A solder paste stencil also requires accurate alignment with respect to some reference edge or point on the PCB. Solder paste stencils can also have fiducials, and they are placed for the same purposes as in panels and PCBs. The difference is in the number and use of localized fiducials; typically solder paste stencils only need 3 global fiducials (same as a board) without any local fiducials. Placement of global on a stencil typically matches the placement in the PCB.
Solder paste printing with a blade and stencil.
Not all designers will be designing stencils, and instead they will rely on their fabrication and assembly vendors to provide stencils. However, engineers who work at an EMS company do sometimes need to design stencils for fabrication, or at least they will need to provide specifications for stencils to an outside vendor. In either case, they should select where and how many fiducials to place on the stencil to aid in angular alignment of the stencil along the board.
Beyond Stencil Fiducials With Automated Dispensing
If you want an alternative solder application option that does not require a stencil, contract with an assembler that uses an automated paste dispensing machine. These machines can use your copper fiducials for alignment and orientation determination. By eliminating the stencil, you remove a tooling step and you eliminate the need to clean any stencils.
While these machines might seem like the best option for your assembly, the downside to using automated dispensing is the lower throughput of these machines. In an automated process, nozzles dispense solder paste on pads individually, which takes much more time than application with a blade or squeegee through a stencil. For short-run, quick-turn assembly jobs, automated dispensing is probably the best option.
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