Where are fiducials placed in PCB?

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What are PCB Fiducials and Why are They Important?

Fiducials are small copper circles etched onto printed circuit boards (PCBs) that serve as reference points for pick-and-place machines and other automated assembly equipment. They help align and orient the PCB properly so that components are placed accurately in their designated locations.

Fiducials are critically important in PCB Assembly for several reasons:

  1. Accurate component placement – Fiducials act as reference markers so pick-and-place machines know exactly where to put each component. This ensures components are positioned correctly.

  2. Proper PCB alignment – Fiducials allow the PCB to be aligned and oriented properly in the machine prior to placing components. Any rotational offset or shifting of the board can be detected and compensated for.

  3. Faster assembly – By using fiducials, automated equipment can quickly and reliably locate the PCB without manual alignment. This speeds up the assembly process.

  4. High precision – Fiducials enable very tight component placement tolerances, often within a few mils. This level of precision is essential for dense PCB designs.

In summary, fiducials are necessary reference markers that enable fast, precise, and reliable automated PCB assembly. Proper Fiducial Placement is key to realizing these benefits.

Fiducial Placement Guidelines and Best Practices

For fiducials to serve their purpose effectively, they must be placed on the PCB according to certain guidelines and best practices. Here are some of the most important considerations for optimal fiducial placement:

Fiducial Quantity

The general rule of thumb is to use at least three globally placed fiducials for each side of the PCB being assembled. However, the exact number needed may vary depending on the size of the board.

PCB Dimensions Recommended Fiducials
< 5 in2 2 (minimum)
5-20 in2 3
20-50 in2 4
50-100 in2 5-6
> 100 in2 1 per 25 in2

Additional local fiducials may be used near critical components such as fine-pitch BGAs or connectors. 1-2 local fiducials can aid in placement accuracy for these parts.

Fiducial Size

The ideal fiducial size depends on the capabilities of the vision system used by the assembly equipment. Most common fiducial diameters range from 25-125 mils (0.635-3.175 mm).

Fiducial Diameter (mils) Fiducial Diameter (mm)
25 0.635
40 1.016
50 1.27
80 2.032
100 2.54
125 3.175

If the PCB will be assembled by multiple manufacturers, it’s best to use larger fiducials (80-125 mils) to ensure compatibility with different machines. Smaller fiducials (25-50 mils) may be acceptable for newer, high-resolution systems.

Fiducial Shape

The vast majority of fiducials are circular, as this shape is easiest for vision systems to accurately detect the center point. The circle should be solid copper.

Some older equipment may require fiducials with a clear center, sometimes called “donut” or “ring” style. However, solid circles are preferred for compatibility with modern vision systems.

Fiducial Spacing and Clearance

Fiducials should be placed as far apart as practical near the edges or corners of the PCB. This provides the widest baseline for referencing component locations across the entire board.

A minimum spacing between fiducials of 0.5 in (12.7 mm) is recommended, preferably more. For large PCBs, fiducials should be placed several inches apart.

Adequate clearance must be provided around each fiducial. There should be an open area of bare PCB extending at least 3x the fiducial diameter on all sides. This clear space should be free of any other copper features, mask openings, or silkscreen marks.

Fiducial Diameter Minimum Clearance
25 mils 75 mils
40 mils 120 mils
50 mils 150 mils
80 mils 240 mils
100 mils 300 mils
125 mils 375 mils

Spacing fiducials too closely or failing to provide enough keep-out clearance can interfere with the vision system’s ability to locate the fiducials reliably.

Placement Symmetry

Whenever possible, global fiducials should be placed symmetrically around the center of the PCB. This helps compensate for any rotational misalignment of the board in the machine.

Avoid placing all the fiducials along one edge or in one corner of the PCB. A lack of symmetry reduces placement accuracy across the board.

Fiducial Location Priorities

The most important priority is to place fiducials where they have sufficient clearance from other features. Maintaining spacing and keep-out areas takes precedence over all other placement considerations.

The second priority is to maximize the distance between fiducials. Placing them far apart provides the best accuracy.

Symmetrical placement is the third priority. Optimize symmetry as much as possible while still maintaining required clearances and maximizing spacing.

Solder Mask Over Fiducials

In most cases, fiducials should NOT be covered by solder mask. The exposed copper circle contrasts with the PCB substrate, making it easy for vision systems to locate.

However, some equipment requires the opposite – fiducials MUST be covered by mask openings in order to be detected. If necessary, mask-defined fiducials can be used, as long as they have adequate clearance from other openings.

If unsure, leave fiducials exposed. This is compatible with the widest range of assembly equipment. Covered fiducials are the exception rather than the rule.

Fiducials on Both Sides

If components will be placed on both sides of the PCB, a full set of fiducials should be provided for each side. Double-sided placement requires side-specific fiducials for referencing components on the top and bottom.

Fiducial locations should be mirrored on the top and bottom sides. This allows the same placements to be used when the board is flipped for placing the opposite side.

PCB Panelization Fiducials

When PCBs are fabricated or assembled in multi-board panels, additional panel fiducials must be provided. These panel fiducials are used to accurately locate and register the position of the entire panel.

Panel fiducials are typically placed in the corners of the panel frame surrounding the individual PCBs. At least three panel fiducials are required, though four (one in each corner) is most common.

Panel fiducials use the same size, shape, and clearance guidelines as PCB fiducials. However, panelization may impose additional minimum spacing requirements from features like tooling holes, V-score lines, or board edges.

Common Fiducial Placement Mistakes and How to Avoid Them

Mistakes in fiducial placement can lead to assembly issues and reduced manufacturing yield. Here are some of the most common pitfalls and how to avoid them:

Inadequate or Missing Fiducials

One of the most frequent mistakes is not providing enough fiducials on the PCB. Less than three fiducials per side does not provide sufficient referencing to accurately locate the entire board.

In some cases, fiducials may be missing entirely. This is common on PCBs that have been hand-assembled in the past but are migrating to automated assembly.

To avoid this mistake, always include at least three fiducials per side, even if the board is small. For larger PCBs, add more fiducials according to the guidelines in the table above.

Fiducials Too Close Together

Another common issue is placing fiducials too close to each other or too close to other features on the board. This violates clearance and keep-out requirements.

Fiducials placed in close proximity don’t provide a wide enough baseline for accurately locating the PCB. They may also interfere with the vision system’s ability to distinguish the fiducials from nearby copper elements.

To fix this, space fiducials at least 0.5 in apart, and farther if possible. Make sure fiducials have the proper keep-out clearance on all sides, relative to their diameter.

Placing Fiducials Under Components

Designers may be tempted to put fiducials wherever there is open space on the PCB, including under larger components. This placement is a problem because those components will obscure the fiducials during assembly.

Fiducials must be visible to the pick-and-place machine’s camera system. If they are hidden under parts, the PCB can’t be located properly.

Instead, place fiducials only in areas that will be unobstructed and visible throughout the assembly process. Adhere to the recommended clearance guidelines to ensure fiducials are sufficiently isolated from components.

Incompatible Fiducial Shapes and Sizes

Some PCB designers may be accustomed to outdated fiducial styles like donuts or crosshairs. While these shapes were once necessary for older vision systems, they are no longer compatible with modern equipment.

Incorrectly sized fiducials also cause recognition issues. Fiducials that are too large or too small may not be detected reliably.

Use solid copper circles for all fiducials unless otherwise required. Stick to common fiducial sizes between 40-100 mils in diameter. When in doubt, larger is generally better.

Asymmetrical or Skewed Fiducial Layout

Placing fiducials asymmetrically or skewed relative to the center of the PCB can reduce placement accuracy. The fiducials may accurately locate one portion of the board while misaligning other areas.

Avoid bunching fiducials along one edge or in one quadrant of the PCB. Don’t place them in a straight line unless the PCB is also straight and narrow.

Instead, place fiducials as symmetrically as possible while prioritizing appropriate spacing. Fiducials should be distributed evenly near the PCB corners or around its perimeter.

Frequently Asked Questions (FAQ) About PCB Fiducial Placement

Q: What is the minimum number of fiducials required on a PCB?

A: At least two fiducials per side are required at a bare minimum, though three or more is strongly preferred. A single fiducial does not provide enough referencing to accurately locate the PCB. Use 2-3 fiducials for very small boards under 5 in2, and more for larger PCBs.

Q: How big should PCB fiducials be?

A: For most PCBs, fiducials should be 50-100 mils (1.27-2.54 mm) in diameter. Smaller 25-40 mil fiducials may be acceptable for newer equipment or high-density designs. When in doubt, larger fiducials offer the best compatibility with a wide range of assembly machines.

Q: What shape should PCB fiducials be?

A: The vast majority of fiducials should be solid round copper circles. This shape is ideal for modern vision systems. Only use ring-style or crosshair fiducials if the assembly equipment specifically requires them.

Q: How much space should be left around fiducials?

A: Fiducials need sufficient clearance from all other PCB elements. At a minimum, provide open space at least 3x the fiducial diameter on all sides. More keep-out spacing is better whenever possible. Make sure no other copper, mask opening, or silkscreen marking encroaches on this clear area.

Q: Can fiducials be placed under components?

A: No, fiducials should never be placed underneath components. They must remain visible to the pick-and-place camera system throughout the entire assembly process. Fiducials obscured by components cannot serve their purpose of board registration and alignment.