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  3. Tooling holes versus Fiducials - Do I need both?

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Tooling holes versus Fiducials - Do I need both?

John T
John T over 1 year ago

 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.

 

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  • Ulf K
    Ulf K over 1 year ago

    I have always created Fiducials as (electrical) footprints and without any connection already at the schematic level since they then becomes entries in the pick-n-place file. But I have also encountered designs where the fiducials has ben instantiated as mechanical parts thus not having any entries in the pick-n-place file.

    What is the recommended/best practice way to do this?

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  • avant
    avant over 1 year ago

    Tooling holes may be required by the fabricator.

    Fiducials should always be used. Use a solid copper pad relieved of solder mask and covered with solder paste.

    This will help the pick-n-place machine find the fiducials.

    Do not use tooling or any other holes as fiducials. 

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  • Robert Finley
    Robert Finley over 1 year ago in reply to Ulf K

    Fiducials are hard to lose if you start them as components from the schematic.  Old cad tools like P*DS blow them away if they aren't in the netlist.   A fatal error for older Fuji SMT machines was not having a minimum three fiducials on the board, per side, 1mm circle pad with 1mm clearance.  

    I think we only need tooling holes if the board will be tested on an ICT fixture. 

    Tooling holes just for bare-board fabrication are ideally located on far corners of the 12x18" fab panel(fabrication rails).  Often, there isn't a benefit to putting them inside the board.

    Fiducials for Paste stencil alignment are ideally located on the assembly panel rails, outside of the boards, and should be as far apart as possible to improve stencil alignment accuracy.  Same approach as tooling holes for secondary drill/route passes.  

    Fiducials for pick-n-place.  Years ago, we built fiducials into the corners of a BGA footprint.  Fortunately, that's no longer needed.  

    Placement accuracy improves when fine-pitch parts are close to a fiducial target as the SMT machines compensate for any warp or thermal expansion after reading the fiducials.   On the other hand, if you add one too many fiducials,  SMT operators can skip them in the program if they can't justify the time.   Rigid-flex boards shrink/expand with humidity like crazy.  So, it depends.  

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  • John T
    John T over 1 year ago in reply to Ulf K

    Hi Ulf K, I think the idea to have in the schematic is just fine. In that case these will come with reference designators and the norm would be to designate these as FD1, FD2... and so on. From my experience, fiducials were more typically implemented as symbols placed directly in the brd design, not sourced from the bom. The key is that the symbol name should clearly contain the word "fiducial" or "fiducial_variantname". Most production systems will search for this as a keyword in the data and will then propose these for use by the program. 

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  • excellon1
    excellon1 over 1 year ago in reply to avant

    Modern assembly SMT equipment that includes vision assist do not need fiducials. The board is typically scanned and each SMT footprint identified
    as an XY centroid location. Tooling holes may be needed by the fabricator, ask in advance. For a fidicual to be really useful the fiducial should be located at the board origin (0.0), others can be included elsewhere on the board. The reason for the 0,0 location aka board origin is that the pick and place file
    you provide with your board wont require offset programming in the SMT Software.

    I had not used fiducals in years. ASSY houses never asked for them either due to the modern vision equipment they use.

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