Fiducial Teaching: Vision Alignment and Board Stretch Compensation
A placement machine does not know where the board is until it looks at a fiducial, and the measurement it makes there is applied to every component it places afterwards. The fiducial design, the teaching of the threshold and the compensation for board movement together decide whether the placement lands where the design intended.
What the Vision System Measures
The camera finds the fiducial, calculates its centre and compares that position against the position the programme expects. The difference is an offset in X, in Y and in rotation, and the machine applies it to every placement in the programme from that point on.
Most machines use two or three fiducials so that a scale error can be separated from a translation. With one mark, a board that has grown during lamination appears simply to be offset, and the error is worst at the far corner of the panel. The number of marks is therefore chosen from the accuracy needed at the furthest component rather than from a house rule, and a fine pitch layout is planned with that in mind.
Fiducial Shape, Size and Contrast
A fiducial is normally a filled circle or square of 1 mm to 2 mm, with a clear area of at least twice its width around it and no copper or trace inside that area. The mask opening is made 2 mm or more larger than the mark so that the mask edge never crosses the feature the camera is measuring.
Contrast is what the vision system actually needs, and a fiducial on a matt laminate with a matt finish is easier to see than one on a glossy surface that reflects the camera’s own light. A mark that is visible to the eye can still be invisible to the camera for that reason. The clearance has to survive fabrication as well, because a plated or an imaged feature that intrudes into it changes the shape the camera sees and can pull the measured centre away from the true one.
Placement and Number of Marks
Three marks placed diagonally across the board allow both translation and scale to be measured, and two marks allow translation and rotation only. On a panel, the marks are placed on the panel itself rather than on individual boards, unless the boards are separated and placed individually.
Where the array is large, local fiducials are added near fine pitch devices so that the machine can correct for the local distortion of the panel. The local marks are placed on the board outline rather than inside a component keep-out, and their positions are copied into the placement programme.
Teaching the Programme
Teaching sets the search window, the brightness threshold and the shape tolerance the machine will accept. A window that is too small fails to find a mark on a board that is at the edge of its tolerance, while one that is too large can lock onto a via or a test point that happens to look similar.
Teaching is done on a production board rather than on a bare panel, because the solder mask colour and the finish change the contrast. The settings are then saved with the programme and re-verified whenever the board finish or the mask changes. Teaching is repeated when the machine lamps are replaced as well, because the brightness at a given threshold changes with the source, and a programme taught on an aged lamp may not find the mark under a new one.
Board Stretch and Scale Compensation
Laminates move during fabrication, and the movement is not the same in both axes, which is why the machine measures scale rather than assuming a nominal size. A typical scale error for a large panel is 0.05 to 0.1 percent, which is 0.5 mm across a 500 mm panel and far more than a placement tolerance.
The machine can correct a linear scale in X and in Y, but it cannot correct a non-linear distortion, such as one region of the panel having moved more than another. That is the case the local fiducials exist to handle, and it is also the case where a placement accuracy study has to be done across the whole panel rather than at one corner. Where the distortion is repeatable, a table of local offsets can be given to the machine, but it is specific to the panel and to the lamination cycle that produced it, so it is a compensation rather than a cure.
Local Fiducials at Fine Pitch
A local fiducial is a smaller mark, often 0.5 mm, placed beside a fine pitch device so the machine can measure the local position immediately before placing that part. Using it removes the error that accumulates between the global marks and the device.
The local marks have to be clear of paste, because paste on a fiducial changes the contrast and can make the mark unfindable. Where the stencil aperture layout leaves room, the local mark is placed outside the print area; where it cannot be, the mark is protected by a mask dam.
Errors That Appear as Placement Offset
The common errors are a fiducial covered by mask, a mark with insufficient contrast, paste printed over a mark and a search window that has locked onto the wrong feature. Each produces a placement error that looks like a machine calibration problem and is not one.
A mark that is the wrong shape, such as a circle with a notch where a trace enters the clearance, will also fail the shape tolerance intermittently, which produces an error that comes and goes. A mark that is found on one panel and missed on the next usually has a fabrication cause rather than a programming cause, and the pattern across the lot is the evidence that separates them. Our PCB fiducials notes describe the geometry that avoids those cases.
Verification and Measurement
The result is verified by placing components onto a bare or paste-free board and measuring their positions, either on the machine’s own measurement function or under a microscope against a glass plate. The measurement is made at several points across the panel so that the scale correction can be seen working.
A placement offset that is the same everywhere is a translation, one that grows across the board is a scale error and one that changes part way across is a distortion. Separating the three is what tells the engineer whether to re-teach the programme or to change the panel.
Documentation and Records
The record should carry the fiducial positions and sizes, the teaching settings, the measured scale correction and the verification measurement. Where the panel design changes, the marks move and the programme is re-taught rather than adjusted by eye.
Our fabrication notes describe how the fiducial specification is written so that the shop keeps the clearance and the mask opening the vision system needs, which is a fabrication detail that decides an assembly result. A fiducial is one of the few features on a board that exists only for the assembly process, so its specification is written by the assembly engineer and verified on the bare panel before the first build.
FAQ
How many fiducials does a board need? Two are enough for translation and rotation, and three allow scale to be measured as well. On a large panel the third mark is worth having, because the scale error is usually the largest part of the total.
Can a fiducial be placed under a component? It can be placed anywhere that stays visible during placement, and it cannot be covered by a component that is already fitted. Local fiducials are therefore placed beside the device rather than under it.
Why does the machine sometimes fail to find a mark that is clearly visible? Usually because of reflection from a glossy finish or because the mark lacks contrast under the machine’s own light. The threshold is re-taught on the production surface rather than adjusted until the failure stops.
