In the grand scheme of PCB design, deciding on the fiducial marker placement may seem like more of an afterthought. You may be surprised to find out these little marks actually play a pivotal role in good manufacturing. While great fiducial marks provide consistency and efficiency, poor fiducial marks can mean extra time and money sunk into reprogramming and rework. To set up both yourself and your manufacturer for consistent accuracy and quality, follow these 5 guidelines.
- Understand How Fiducial Marks Work
- Pick the Right Spot
- Pick the Right Number
- Optimize the Design
- Avoid Certain Finishes
First, make sure you understand what a fiducial mark actually does. Understanding the “how” will help you more easily answer your “why”. And as long as you’re here, we’ve got you covered! According to Miriam-Webster, the word “fiducial” means:
“Taken as standard of reference”
A fiducial mark on a PCB is the standard of reference used by automated machinery such as Surface Mount Technology (SMT) and Automated Optical Inspection (AOI). The mark consists of a lone copper pad distanced from any other visible landmark. Without fiducial marks, the machinery will either place components incorrectly or refuse to operate altogether. However, by reading various fiducial mark locations placed across the PCB, automated equipment can determine the exact location to place or scan components.
Take note, most machinery does not technically read what you place on the PCB. Rather, it recognizes the reflection of the fiducial pad. Bear this fact in mind and the next steps will make more sense.
Where should you place the fiducial marks for the best outcome? The answer will vary depending on your overall PCB panel design, but there are some general rules you can use to nail it every time. We recommend adding at least panel and global fiducial marks to your design. That way, the manufacturer can select the best choice based on their machinery.
Panel fiducial marks are placed on the panel rails. Use these marks to allow for a quicker manufacturing process.
Automated equipment will use these marks to position the panel as a whole. As this method ignores the individual PCBs on a panel, panel fiducial marks are best used for designs with moderate-to-large components.
For the best results, place fiducial marks as far from each other as possible – typically the rail corners.
Global fiducial marks are placed on each individual board. Use these to increase the accuracy of the manufacturing process.
In comparison to panel marks, global marks are a much more dependable method of accurate triangulation. During production, machinery will calculate each PCB location separately. The one downfall is this: On panels with numerous PCBs, reading each global fiducial mark can add a good deal of processing time.
As with panel marks, global marks are most effective at a distance. Try to place them near the corners of the PCB.
In contrast to previous marks, local fiducial marks are usually considered additional rather than mandatory. Place these marks around specific components too small or complex to entrust to standard accuracy. Component examples include fine-pitch QFPs, QFNs and BGAs.
For best accuracy, place these fiducial marks at opposing corners of the corresponding component.
In most cases, we recommend using three panel fiducials, three global fiducials per PCB, and two local fiducials per component. We’ll show you some suggestions on orientation, but first let’s talk about the reasoning behind the quantity.
1 Fiducial: With only one fiducial mark available, the scanning software cannot determine the proper rotation for the PCB. A machine is effectively unable to run a PCB with only one fiducial mark.
2 Fiducials: With two fiducial marks available, machines can operate correctly. However, two risks are at play here.
- A two-mark setup offers good but usually not great location tracking. If you are working with fine pitch components, they may not be as accurate as you would like.
- Opposing fiducials might allow for operator error. Should the PCB be inserted upside down, the machine may still see the fiducials and continue on its merry way. This slip-up causes at best wasted time, and at worst a disastrous component pile-up or permanent PCB and equipment damage. (The chart below shows how to negate this risk.)
3 Fiducials: Three is the optimal number of fiducial marks for a proper PCB run. Including a third fiducial mark adds an extra point for triangulation, increasing the accuracy overall. It also removes any possibility of a mistakenly rotated board getting past the cameras.
4 Fiducials: While it may seem like adding four points can only further increase accuracy, there is rarely much more to be gained by this point. The major downfall here is that the fourth fiducial mark can reintroduce the danger of processing an upside-down panel. Take extra care when going this route.
2 marks is considered the standard for component accuracy.
This is important: Avoid placing fiducials against the edge of the panel. Placement machinery often uses clamps to lock the PCB in place during population. If the clamps cover the fiducials, you’ve got a serious problem. Try to center fiducial marks at a minimum distance of 3mm from the edge (we recommend 5mm to negate any risk.)
Here are some examples of fiducial marker placement do’s and don’ts:
Once you’ve selected the locations for your fiducial marks, it’s time to decide how they will appear. While some manufacturing equipment is programmed to recognize various shapes like diamonds, squares or hourglasses, not all machines will handle it. For this reason, we strongly urge you to stick with the universally accepted circular fiducial mark.
The size of the mark, on the other hand, is a bit more flexible. The average range for a fiducial pad is 1-3mm in diameter. This size allows the camera and equipment software to quickly recognize the pad as a fiducial mark.
The fiducial pad needs to be finished with the metal used on the rest of the board. (Remember, the pad is made to reflect light.) For this reason, do not cover the pad with solder mask, silkscreen or any other material.
Proper clearance around the fiducial pad is vital. Place an open area around the pad (no copper, solder mask, silkscreen, etc.) Having that space allows the camera to pick up on the mark without visual interference.
The diameter of the open space should at least double that of the pad size. So, for a 2mm pad, you would want at least a 4mm clearance area around it. The shape of the clearance area is less vital; circular and square areas are two popular designs.
Now here’s a stumbling block most people don’t think about. Did you know a poor quality finish can disrupt the production of your assembly? Let’s go back one more time to the concept of fiducial reflection.
Fiducial pads need to be flat to reflect a uniform image. The copper marks are plated with whichever metal finish you select. Processes like electroplating and immersion are reliable in their uniformity, whereas Hot Air Level Solder tends to be a little more variable. At Meyer, we’ve seen great quality HASL finishes and not-so-great HASL finishes.
If the finish has any variations in thickness, it fails to reflect correctly. While not insurmountable, it does force the production operators to take extra time recovering the marks. Depending on how rough the problem is, this could mean going through each panel using a pencil eraser to diffuse the reflection, editing software programming to compensate, or re-soldering the fiducials altogether. In short, it takes costly time to fix.
If you must use a HASL finish for your PCB, try to find a board fabrication company with good reviews on quality. The higher the quality, the flatter the finish usually comes out. Also discuss the process with your manufacturer – they may be able to refer you to a good fabricator or offer insight on their equipment’s fiducial-reading capabilities.
Fiducial marker placement may seem trivial in comparison to most aspects of a PCB, but those fiducials are the beacons that guide automated assembly. Using the tactics we’ve discussed, you’ll be able to pave the way for excellent accuracy and a streamlined operation.
Have you encountered pitfalls or learned fiducial design techniques not mentioned here? Help your community succeed by sharing in the comments below!