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What are Fiducial Marks on a PCB?

erick-mvErick MV wrote 03/25/2022 at 17:41 • 13 min read • Like

DESCRIPTION

Fiducials are little target registration marks that are printed on PCBs, they are placed on the top copper layer (and bottom if you're doing 2-layers) and allow the vision system of the pick and place to recognize where the PCB is at. They are not placed on the solder mask layer or silkscreen because they are not as precisely aligned to the parts as the copper itself.

In this article we will see how fiducial marks can be correctly designed and placed on a printed circuit board, so that the SMT process occurs without possible errors in the assembly of surface mount components on the PCB.


image 1 & 2 shows a Raspberry Pi with its fiducual marks on both side of the PCB.

INTRODUCTION

The purpose of fiducial markers is to provide reference points for imaging systems. Unfortunately, many beginner designers don’t have in-depth knowledge about fiducial titles. For this reason, we’ve prepared this comprehensive guide on fiducial PCB markers. Keep reading to learn more.

In PCB design, a fiducial mark is a round shape of copper that serves as a reference point for Pick & Place mounting machines. PCB fiducial markings help machines recognize the orientation of PCBs and their surface mount components with very small pitch packages such as Square Flat (QFP), Ball Grid Array (BGA) or Flat Square without pins (QFN).

There are two types of fiducial marks commonly found on printed circuit board designs: global and local. Global fiducial marks are a copper reference that is placed on the edge of the PCB and allows the machine to determine the orientation of the board relative to the X-Y axis. Placement machines also use the fiducial mark to compensate for any distortion when clamping the PCB.

Local fiducial marks are copper markers that are placed on the outside corner of a square potted, surface mount component. Assembly machines use them to precisely locate the footprint of the component and reduce the number of errors in its placement. This is especially important for components with a large square package and fine pitch.


ARE LOCAL FIDUCIALS NEEDED YET?

As pick-and-place machines become more precise, some assembly shops are doing away with local fiducials. Why take up real estate with non-functional markings when it's not absolutely necessary? Of course, it's never that black and white. Deciding whether local fiducials are necessary depends on a few different factors. Here they are.  

1. Board size

More than anything, the decision of whether to use local fiducials depends on board size. If the board is small enough, globals can act as locals, as the global fiducials can situate a relatively smaller surface area. But as board sizes increase (anything over 8 x 8, give or take), you’ll need help from local fids to achieve perfect placement.

2. Component Specific

Components with lower pitches and pin counts may not require local fiducials for placement. But the finer a component's pitch and the higher its pin count, the more necessary it'll be to use local fiducials for added precision. Generally speaking, any component with a pitch under 1.0mm will need local fiducials. BGA's with pitches under 0.5mm - 0.8mm generally require local fiducials. 

3. The Factory Machines Capabilities

The use of local fiducials largely depends on the type of machines. It’s always important to discuss pick-and-place machine design and specs before deciding whether it’s necessary to include local fiducials. And lastly, even if local fiducials aren’t absolutely necessary to place a fine pitch component, having them designed into the board could come in handy for repair and rework, not to mention peace of mind.


DIFFERENCES BETWEEN: TOOLING HOLES, FIDUCIALS, AND RAILS.

"A fiducial is not hole on the pcb, is not a 1mm copper dot, it must be designed as pick and place machines rules"

HOW TO DESIGN FIDUCIAL MARKERS FOR JLCPCB SMT ASSEMBLY SERVICE

In this case we are going to study how to design fiducial markers on  EasyEDA and Altium CAD Software for designing PCB circuits, and how to place a correct order at JLCPCB on the of the best and cheapest PCBA manufacturers on the world.

GOOD PRACTICES ON PLACING FIDUCIAL MARKERS ON A PCB

If you want to get the most out of automounting, you need to use fiducial marks correctly. There are a number of important guidelines when placing fiducial marks in a design:

- Fiducial markings are made by laying a non-perforated layer of copper in a circular fashion. Fiducial marks must not have a solder mask.

- The optimal size of a fiducial mark is between 1 and 3 mm. A clearance similar to the diameter of the mark should be observed.

- To achieve maximum accuracy with global fiducials, 3 marks should be placed on the edge of the plate. In cases where there is not

enough space, at least 1 global fiducial mark is required.

- The fiducial mark should observe a distance of 0.3 inches (7.5 mm) from the edge of the plate, excluding the free area of the fiducial  mark.

- For local fiducials, place at least two diagonal fiducial marks on the outside edge of the surface mount component.

- When the plate is larger, any angular misalignment during manufacturing will be less. This is because any small angular deviation will be easier to detect when the distance between fiducials is larger.

Using EasyEDA Software to place Fiducial Markers on PCB Design.

LCEDA / EasyEDA:

In the part search form, find a fiducial mark and place it on the PCB directly:

ALTIUM DESIGNER:

The reference point is actually a solder pad.

To convert it in a fiducial. double-clicking on the pad and set like this:

VALID PROCESS OF PROCESS EDGE AND IMPOSITION REFERENCE FOR JLCPCB

INVALID EXAMPLES OF REFERENCE PLACEMENT POINT ON A PCB

DATUM POINT (MARK POINT) PLACABLE AREA

Any marks placed within this area will not be recognized and will not be used.

As we can see in the following figure, the mark point edge should be minimum 3.35mm far from the board edge:

Global fiducial mark Placement

The global fiducial markers (the small plate fiducial markers) are used to locate all circuit elements on a board. 

The number 1,2,3,4 in the figure below is typically placed on the edge of the process

Small plate reference: Digit 1.1, 1.2 in the figure below;  2.1,2.2; 3.1,3.2; 4.1,4.2; 5.1,5.2; 6.1,6.2; 7.1,7.2; 8.1,8.2; 9.1,9.2; and 10.1,10.2;

HOW  TO PLACE GLOBAL FIDUCIAL MARKS

PCB typePlacement instructionsMinimum number of placementsPlacement requirements
Single boardYou can just have one of the imposition or panel baselines
It is recommended to place 4 Minimum of 3 positions
-lWire the 4 points in a shape that must be of an equal edge ladder.
-Distribute as far as possible in the longest diagonal position.
The same inPosition- Recommendation: Both imposition and panel baselines exists.
- If there are no small plate basis guidelines, the fork plate situation cannot be handled.  Can only be considered as single board production.
- If no imposition baseline exists, 4 small plate baselines are selected for use as the imposition baseline.



It is recommended to place 4 imposition baselines If you can place the small plate fiducial, place a minimum of 2.
- The 4 imposition reference points are wired in a shape that must be an equal-edge ladder.
- Distribute as far as possible in the longest diagonal position.
- The small plate datum is distributed as far as possible in the longest diagonal position of the single plate.
Different Inposition


You can just have one of the imposition or panel baselines. In fact, the small plate benchmark is not usable, is considered as a whole, and cannot handle the situation of the fork plate.




It is recommended to place 4 imposition baselines
- The 4 imposition reference points are wired in a shape that must be an equal-edge ladder.
- Distribute as far as possible in the longest diagonal position.
- The small plate datum is distributed as far as possible in the longest diagonal position of the single plate.
Double-Side Inposition



You can just have one of the imposition or panel baselines.  Considering the small plate as a whole, it cannot handle the fork opening situation.





It is recommended to place 4 imposition baselines
- The 4 imposition reference points are wired in a shape that must be an equal-edge ladder.
- Distribute as far as possible in the longest diagonal position.
- The small plate datum is distributed as far as possible in the longest diagonal position of the single plate.
The greatest effect on the production line is to prevent staplyout

LOCAL FIDUCIAL MARK

As pick-and-place machines become more precise, some assembly shops are doing away with local fiducials. Why take up real estate with non-functional markings when it's not absolutely necessary? Of course, it's never that black and white. Deciding whether local fiducials are necessary depends on a few different factors.

A local reference point is recommended if the pin spacing is less than or equal to 0.4mm and the PCB size is longer and wider than 100mm.

Dimension requirements for JLCPCB Pick and Place Machines:

There are currently two types of devices supported. 

- Solid circle.

- Holes. (This type is only applicable when there are no dense pin chips, and can only be used when all components have pin spacing greater than or equal to 0.8mm)

Solid circle

The best datum marker is a solid circle. The optimal diameter of the fiducial markers is 1.0mm. 

There should be a blank area around the reference point, without any other circuit elements or markings in the area. The minimum size of the blank area should be twice the radius of the reference point.

JLC also supports this dimension when reviewing the order.

JLCPCB preffers D-2D fiducial PCB design, also you could use D-3D rule in your fiducial design.

Holes

This type is only applicable when there are no dense pin chips, and can only be used when all components have pin spacing greater than or equal to 0.8mm.

This type is also a pad for an insert type device that does not have any other circuit elements or markings in the pad area. The minimum size of the pad area should be twice the reference point radius.  As shown below:

When reviewing an order, if a solid round benchmark is not found and the component meets the requirements, an attempt is made to find a baseline for the hole type.

Hole diameter allowed: 0.8 - 1.5 mm

Note on fiducial size for PCB layout


The fiducial size for PCBs is generally 1 to 3mm, but the optimal size depends on the mounting machines used by the manufacturer. Some manufacturers recommend adding 3 fiducials at the corners of the plate as this provides 2 angular alignment measurements and allows the pick and place machine to deduce the correct orientation. 

Some manufacturers indicate a specific size, which also depends on the mounting equipment they use. As a general rule, the diameter of the solder mask opening should be twice the diameter of the pure copper for the fiducial mark, although some manufacturers prefer the solder mask opening to be three times the diameter of the fiducial. In addition, the fiducial size in the same plate (both global and local) must be homogeneous and not vary by more than ~25 microns.

When mounting 2-layer plates, the fiducial marks of the upper and lower layer must be one on top of the other. This may seem surprising: you'd think fiducial provisions should be mirror images of one another, but I've never seen a manufacturer mention this in their guidelines. The fiducial size for PCB of the upper and lower layers must be the same, including the opening of the solder mask.

Local fiducials are typically as small as 1mm in size, with a 2mm solder mask opening; however, attention should be paid to the D-3D rule shown in the image above, as the manufacturer might prefer this larger solder mask opening relative to the fiducial size of a PCB. The local fiducial size is generally not much larger than 1mm to allow for track routing and to make room for other components. For small components, such as a 0201 resistor or a chip-sized BGA, the assembly machine will be precise enough that a local fiducial is not needed, and it will know exactly where to place the components.

There is nothing wrong with checking if the fiducial and soldermask opening are the correct size before sending the design to the fabricator. Fiducial marks are classified as mechanical elements of the board and are not connected to anything, so it is easy to modify the footprint of a custom fiducial and place a new fiducial if necessary. Some manufacturers will modify the fiducial size if it is not suitable.

You can easily design and place your fiducial marks, pads, polygons, and any other copper structures for your PCB when you use the world-leading PCB design and layout capabilities of Altium Designer®. Users can take advantage of a simple, integrated platform with circuit design and PCB layout features to create printed circuit boards for manufacturing. When you're done with your design, and want to send the files to your fabricator, the Altium 365 platform makes it easy to collaborate and share your projects.

Source: https://resources.altium.com/es/p/are-fiducial-marker-placements-pcbs-still-necessary-modern-manufacturing-capabilities

USEFUL FILES IN DESIGNING FIDUCIAL MARKS FROM JLCPCB:

Adding Tooling Holes in a PCB

How to Order PCBs from JLCPCB

Discounts for you to order your PCBs with JLCPCB

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