Are Fiducial Marker Placements on PCBs Still Necessary with Modern Manufacturing Capabilities?
About 10 years ago, I stopped watching horror movies. In my younger days I really enjoyed being scared silly, but when I started my engineering career I became more interested in action and sci-fi genres. This is likely because I was getting my fair share of horror stories at work when simple mistakes resulted in catastrophic post-production nightmares.
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When I started my electronics design career, through-hole components were extremely popular and surface mounted components were a rare sight. When (Quad Flat Package) QFP packages of microcontrollers (MCU) became popular, I had no choice but to migrate from the old plastic leaded chip carrier (PLCC) footprint. This is because PLCC requires an additional socket while QFP can be mounted on the PCB directly. As far as I could tell, it was only a matter of time before chip manufacturers stopped producing MCU in PLCC packages in favor of QFP or similar packages.
When my PCB assembly suppliers dropped me an email stating that they were unable to machine assemble the MCU on the 200 production boards I ordered, my nightmare began. Being accustomed to PLCC sockets, which are through hole components, it didn’t occur to me to provide fiducial markers on the PCB. Failing to do so meant all the QFP packaged MCUs with tiny pitches had to be manually assembled.
This resulted in a higher percentage of boards being rejected and countless hours spent fixing faults from imperfect manual soldering. Since then, I make a point of always using fiducial markers in my designs, even if my suppliers tell me that they have upgraded their machines to work without the markers.
What Is a Fiducial Marker and How Does It Help in Manufacturing?
In PCB design, a fiducial marker is a rounded shape of copper that acts as a reference point for pick and place assembly machines. Fiducial pcb markers help machines recognize the orientation of the PCB and its surface mount components with packages that have tiny pitches like Quad Flat Package (QFP), Ball Grid Arrays (BGAs) or Quad Flat No-Lead (QFN).
There are two types of fiducial markers commonly found in printed circuit board designs: global fiducial markers and local fiducial markers. Global fiducial markers are a copper reference placed on the edge of the PCB that allows the machine to determine the orientation of the board with respect to the X-Y axis. Placement machines also use the fiducial marker to compensate for any skew when the PCB is clamped.
Local fiducial markers are copper markers that are placed outside of the corner of a quad packaged surface mount component. It is used by assembly machines to precisely locate the footprint of a component and reduces errors in component placement. This is especially important when you have fine pitched and large quad packaged components in your design.
Always check with your manufacturer for fiducial marker requirements.
Are Fiducial Markers Necessary with Modern Manufacturing Technology?
I have always designed my printed circuit boards with both global and local fiducial markers. However, when I came across an article that explained the possibility about omitting local fiducials, I was intrigued. It made sense to remove fiducial markers on smaller PCBs to maximize space for signal traces.
As a result of advancements in manufacturing technology, local fiducial markers can be omitted under certain conditions. On smaller boards, modern assembly machines can place SMT components using only global fiducial points. Fiducial markers may also be omitted for components that have a larger pitch. For example, surface mount components with pitches of 1.0 mm and above can be placed accurately by the latest machines.
That being said, it is important to discuss the extent of your manufacturer’s machine’s capabilities before removing local fiducial markers in your design. I’ve learned the hard way that not all manufacturers are equipped with machines that are powered by the latest technology. On the other hand, global fiducial markers should never be omitted from your designs. Even if you are working with some of the most advanced manufacturing capabilities.
Best Practices for Using Fiducial Markers in PCB Design
If you want to get the best out of machine assembly, you need to get your fiducial markers right. There are few important guidelines when it comes to placing fiducial marker in your design.
The fiducial marker is made by placing a non-drilled copper layer in a circular shape. The fiducial marker must be free from solder mask.
The optimum size of a fiducial marker should be between 1 and 3 mm. A clearance area similar to the diameter of the marker must be maintained.
For global fiducials, 3 markers are placed on the edge of the boards for the best accuracy. In cases where there is insufficient space, at least 1 global fiducial marker is required.
The fiducial marker must maintain a distance of 0.3 inches to the edge of the board, excluding the clearance area of the fiducial marker.
For local fiducial, place at least two fiducial markers diagonally on the outside edge of the surface-mounted component.
When the board is larger, any angular mis-alignment during manufacturing will be smaller. This is because a small angular deviation will be easier to detect when the distance between the fiducials is larger.
A Note on PCB Fiducial Size
PCB fiducial size is generally 1 to 3 mm, but the correct size you need depends on the assembly machines your manufacturer uses. Some manufacturers recommend adding 3 fiducials at the corners of the board as this gives 2 angular alignment measurements and allows the pick-and-place machine to infer the correct orientation. Some manufacturers will state a specific size, which also depends on the assembly equipment being used by your manufacturer. In general, the diameter of the solder mask opening should be double the diameter of the bare copper for the fiducial, although some manufacturers prefer the solder mask opening be triple the fiducial diameter. In addition, the PCB fiducial size on the same board (both global and local) should be consistent and should not vary by more than ~25 microns.
If you're assembling a 2-layer board, the top and bottom layer fiducials should sit on top of each other. This may be surprising; one would think the layout of fiducials should be mirror images of each other, but I have never seen a manufacturer state this in their guidelines. The top and bottom layer PCB fiducial size should be the same, including the solder mask opening.
Two common PCB fiducial size and solder mask opening recommendations.
Local fiducials tend to be as small as 1 mm with a 2 mm solder mask opening, although pay attention to the D-3D rule shown in the above image as your manufacturer may prefer this larger solder mask opening for your PCB fiducial size. The local PCB fiducial size is usually not much larger than 1 mm in order to allow for trace routing and to leave room for other components. For small components, such as a 0201 resistor or chip-size BGA, the assembly machine will be accurate enough that a local fiducial is not necessary, and the machine will know exactly where your components need to sit.
There's no harm in checking if your PCB fiducial size and solder mask opening are correct before sending your design off to your manufacturer. Fiducials are classified as mechanicals in your board and are not connected to anything, so it is a simple matter to modify the footprint for a custom fiducial and place a new fiducial if needed. Some manufacturers will modify your PCB fiducial size for you if they are not properly sized.
Using professional PCB design software, like Altium’s CircuitStudio®, a fiducial marker can be placed by inserting a pad, changing the hole size to zero, and setting the correct value for the PCB fiducial size.
Need more tips in placing fiducial markers on your design? Contact an expert at Altium.
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