Tooling holes versus Fiducials - Do I need both?

 In technical discussions this week, a question arose in relation to space saving: "Can I opt to use fiducials over tooling holes? Why do I need both?"

It is a valid question; both fiducials and tooling holes are designed into our PCBs to assist assembly equipment to locate the footprints within that PCB.

Fiducials are filled copper circles (preferred shape) on the PCB. Typical diameters are from 0.9mm up to 1.2mm. They are completely open in soldermask to surface finish; they need to shine. Fiducials are used by automated camera equipment to calibrate the location of features on the PCB. They are used in processes such as component placement and automatic optical inspection. Fiducials should be placed in pairs, spaced diagonally apart at the opposite extremes (x and y) of the design. The larger the diagonal distance between these two fiducials, the larger the bounding box which they represent. Greater diagonal fiducial spacing generates higher positional accuracy. A third fiducial can be placed for additional accuracy and linearity. However, two fiducials can be sufficient and the third may be ignored in industrial production routines due to processing-time constraints.

It is advisable to choose a fiducial size that is distinct from the surrounding PCB features. For example, typical via or testpoint diameters should be different from the fiducial diameter to avoid any potential misrecognition. A fiducial typically has a larger soldermask opening than standard pads. It is important that any soldermask layer mis-registration should never impede the fiducial exposure. Typically, it is recommended to maintain a large keepout around a fiducial (for example, with two times the diameter) to prevent optical interference. The same goes for proximity from the PCB edge where it can be obscured by conveyor equipment or clamps.

Tooling holes are also used to locate features on the PCB. These are often used in production equipment that does not employ the use of expensive camera equipment. Examples of these are plentiful, including pinning or connector-assembly equipment, In-Circuit Test (ICT) fixtures, depaneling equipment, or mechanical housing assembly jigs. Tooling holes are a faster and less expensive approach for production. Camera equipment is an expensive add-on to any machine and also takes extra time to process information. Tooling holes allow us to physically locate and go.

Typical tooling hole diameters are >=3mm in industrial applications. These diameters are governed by the equipment pin thickness, which need to be mechanically robust. So what's the difference? Why not just use one or the other? As an example to explain, in automated equipment, a PCB or panel travels along a conveyor; the PCB is stopped inside the machine by a collision with a retractable stopper. At this stage, the position of the PCB is only roughly known. The PCB is able to move slightly across the width of the conveyor, as the PCB must travel freely. After the collision with the stopper, the PCB has some positional play in both the x any y direction, and some slight rotation.

A camera looks for the fiducial at the expected xy co-ordinate according to the ideal PCB data. It then recognizes that the feature it is looking for is delta x and y out of position, and adjusts the entire assembly program accordingly. But in a dense layout for some equipment, there may be a risk to recognize incorrect features as the fiducial. In this case, the entire program offsets incorrectly and fails. The PCB can be too far out of position to be correlated accurately. Here, a tooling pin is useful to physically push/pull the PCB into a more correct position.   

So, why not just use tooling holes and not fiducials? The primary drawback of tooling holes is the tolerance. A fiducial is created at the same time as the remaining features on the copper surface – the very same features we are trying to locate. The tolerance for distance from fiducial-center to smd pad-

center is almost negligible. On the other hand, hole-center-to-pad-center tolerances can be in the order of 100um or greater, depending on the quality of your supplier or random outliers.

Non-plated drill holes are typically performed at the end of the fabrication process. However, if you truly need the tightest tolerance possible between hole to pad, then talk to your PCB supplier about "first-stage drilling". These are holes that the suppliers drill for themselves, typically in waste areas of the PCB, to help align layers during their fabrication process. Therefore, the copper-pad-to-hole-center tolerance is much better. The drawback is that these holes must be covered temporarily during several fabrication steps at a cost. It is possible for you to specify certain holes to be “drilled during first-stage drilling” on your master PCB drawing. This allows you to improve your tooling hole positional tolerance. Therefore, talk to your PCB supplier about first-stage drilling.

However, it is still generally not feasible for the tooling hole to reach the same level of pad-to-pad-center tolerance of fiducials in the copper layer. Therefore, we should relate tooling holes to low-level accuracy and fiducials to finer decimal places. Tooling holes have the much-needed practical use of physically adjusting the PCB into position. This is especially important for positioning on manually loaded jigs. However, for fine accuracy, fiducials are essential to calibrate to those critical measurements.

If you have any questions or comments about this topic, please share in the comments section below.