The Role of Fiducials in SMT Stencil Alignment

A lot of board designers that work on PCBs for mass production should already be familiar with fiducials. Automated imaging equipment uses the fiducials to measure the board's orientation and angular skew, which helps with precise component assembly and placement. Fiducials are found in other locations on the PCB, but their placement is crucial.

PCBs are printed circuit boards that are intended to be assembled using pick-and-place equipment and automated stencil printing. Additionally, fiducials can be positioned on a panel or a stencil, which are both utilized in computer vision systems for rotational alignment. Designers should normally follow the well-known PCB fiducial criteria, but what about panels and stencils? We'll examine their placement when getting a board ready for production. Know more, how laser can be used to manufacture the pcbs.

What is a Fiducial Mark and How Does it Work?

A circular pad or other exposed copper shape encircled by a clearance area is commonly known as a fiducial mark. It serves as a point of reference for automated devices, including pick-and-place machines, and for the alignment of PCB stencils, which guarantees precise component placement on the board.

The PCB and components are aligned during the assembly process by means of computer vision systems that look for the fiducial markers and use their placements. For surface-mount technology components, fiducial markings are especially helpful. It is recommended to place a mark next to packages with small pitches, such as BGA, QFN, and QFP.

Types of Fiducial Marks:

There are two types of fiducial marks commonly found in PCB designs:

• Global fiducial marks
• Local fiducial marks

Global fiducial marks: Global fiducial marks, often referred to as panel fiducials, are typically positioned on the penalization rails that hold several PCBs together during assembly or next to the PCB's corners.

Local fiducial marks: Usually smaller than global ones, these are positioned in regions with intricate routing or high component densities. The assembly machines can accurately determine a component's footprint and minimize fiducial placement errors by utilizing local fiducial markers.

Generally speaking, a three-point grid-based system should have a minimum of two global fiducial markings, one in the upper right corner and one in the bottom left corner.

Placement of Fiducials:

1. On the PCB:

It is common knowledge that fiducials should be placed on a PCB. On a PCB, the fiducials are usually positioned along the board's edge, usually next to mounting holes (because holes are usually positioned at the board's corners). These fiducials are positioned on the PCB as follows:

• Global fiducials : These are usually found in a PCB's lower left, lower right, and upper right corners.

• Local fiducials : They are found in some high-density components, usually with solder mask opening diameters of 2-3 mm and copper diameters of 1 mm.

2. On the Panel:

As the board moves through the fabrication process, fiducials are usually positioned on panels to provide an orientation measurement for the entire panel. Like on a PCB, the fiducials on a panel are usually positioned around the panel borders close to the tooling holes.

Panel fiducials can have the same dimensions as a standard board fiducial. But because of the way the boards are arranged inside the pane, the panel fiducials are only used on the panel itself. When each board is separated from the panel, it will have its own mark to ensure alignment.

3. On the Stencil:

A solder paste stencil must also be precisely aligned with a PCB reference edge or point. Solder paste stencils are used for the same purposes as panels and PCBs, and they may also contain fiducials. Global is usually placed on a stencil in the same location as it is on the PCB.

Structure of a Fiducial Mark :

Each fiducial mark should ideally be a filled, solid copper circle. Fiducial markings on the same board should have a minimum diameter of 1 mm and a maximum diameter of 3 mm, according to IPC rules. The size variation should not exceed 25 microns. A clear, mask-free region free of any other markings or circuit features should surround the fiducial mark. The size of the clean area should, at the very least, match the mark's radius. Nonetheless, assemblers like a mark clearance that matches the mark diameter.

Guidelines for Using Fiducial Marks in PCB Design

There are some crucial rules to follow when including fiducial markings in your PCB designs:

• A fiducial mark's diameter normally falls between 1 and 3 mm.
• It's crucial to add fiducial markings to both sides of any components on your PCB design.
• The fiducial mark's surface should be less than 15 micrometers flat for optimal results.

The distance between the fiducial mark and the PCB edge shall not be greater than the sum of the fiducial clearance and 7.62 mm. Designers frequently apply fiducial marks to the panel rails of boards that take the shape of panels. However, in order to facilitate assembly in both panel and individual board form, individual boards may also have their own fiducial markers.

How Fiducial Marks are used by Machines?

The machine can better identify the orientation and placement of the circuit layout on the board thanks to fiducial markings. Since each fiducial mark has a specific function, the majority of boards use three of these.

• The machine can identify the PCB thanks to the first fiducial, which also provides it with a reference point or locating X and Y coordinate.

• When the board is in its clamps, the machine can detect whether the circuit layout is distorted and how it is orientated thanks to the second fiducial.

• The circuit pattern has degraded since the third fiducial enables the machine to determine any shrinkage or stretching of the board. Because fluctuations of relatively small amounts over a long distance can add up to a huge number, this is especially useful for large boards.  

Conclusion :

In conclusion, as long as the automated machinery can use the fiducial marks in a unique way, there is no hard-and-fast limit on how many a designer can put on the board. The inclusion of fiducial markers in the PCB is a crucial step during design and a checkpoint for DFM reviews, since the majority of automated machines use them for orientation and registration.

These fiducial markers are used by surface mount assembly machines to guarantee that the copper pattern on the board is aligned perfectly and in the right direction. To locate fiducial markers on the board and make additional adjustments, the majority of assembly machines use cameras.