Footprint Origin Errors and Pick and Place Coordinates

Pick and place machines do not know where a component is. They know where the origin of the footprint is, and they apply the coordinates in the assembly file to that point. If the origin sits somewhere unexpected, every placement on the board inherits the error, and the mistake is invisible in the CAD tool because the model still looks exactly like the drawing.

Why the Origin Matters

The origin of a footprint is the reference point from which the component’s position is measured. In the library it is usually the centre of the body, but it can also be pin one, the centre of the pad array or a corner of the courtyard, and each convention shifts the coordinate that appears in the assembly file.

As long as the convention is consistent, the assembly file is correct, because the origin in the library and the coordinate in the file refer to the same point. Problems appear when a part from one library with one convention is mixed with parts from another library using a different one, and the mismatch shows up only as a placement offset on the assembled board.

Library Conventions and Their Variations

Different CAD systems and different companies use different conventions, and a library assembled from several sources will contain more than one. A part drawn with the origin at the centre of the body and a part drawn with the origin at pin one will both appear correct in the symbol editor, and the difference becomes visible only when the placement data is used.

Rotation compounds the issue. A footprint whose origin is offset from the body centre rotates about the origin, so the same rotation angle produces a different displacement depending on the convention. A board assembled from a mixed library can therefore have some parts placed accurately and others offset by a fraction of a millimetre.

<img src="https://www.gopcba.com/wp-content/uploads/2020/12/project_image_4.jpg" alt="Footprint origin marker in a CAD library part” />

What Happens When the Origin Is Wrong

An offset origin produces a systematic placement error for that component type. Every instance of the part is displaced by the same amount in the same direction, which is the signature of a library problem rather than a machine calibration issue. The magnitude is usually small enough to be absorbed by the pad, and large enough to affect the joint quality.

Where the error is larger, the consequences are worse. A part placed half a pad to one side can produce an open on one termination and a solder bridge on the other, and on a fine pitch component the displacement may exceed the placement tolerance entirely. The failure appears at assembly, and the investigation starts with the machine rather than the library.

Rotation, Mirroring and Coordinate Systems

Rotation conventions differ between systems as well. The reference direction, the positive rotation sense and the treatment of parts on the bottom side are all conventions, and a mismatch produces a part that is rotated by ninety degrees or mirrored. Mirroring is the more dangerous of the two, because a mirrored part can sit correctly on its pads while the internal connections are reversed.

The bottom side of a board is the usual place for these errors. A footprint is designed for the top side, and when it is placed on the bottom the tool mirrors the geometry, including the pin numbering. Whether the resulting placement is correct depends on the convention the machine expects.

Exporting Assembly Data

The assembly file is a translation of the layout into a format the machine understands. The useful discipline is to export the data and inspect it, rather than to assume that the export is correct because the layout is. A placement coordinate that does not fall on a pad is visible immediately when the data is plotted over the board outline.

Our notes on placement order and pad positioning describe how placement data relates to the sequence in which components are placed, and why a coordinate error can appear as a placement order problem.

Checks Before Releasing the Files

Three checks catch most origin errors. Plot the placement coordinates over the board image and look for any that are not centred on the pad. Compare the component rotation in the file with the rotation in the layout for a sample of parts. And confirm that the library parts used on the bottom side follow the same convention as those on the top.

A fourth check is worth the effort on a new library. Place one component of each type on a deliberately empty area and compare its expected position with the position in the file. The comparison takes a few minutes and validates the whole library convention rather than one part.

Common Library Errors

The most frequent errors are an origin placed at pin one instead of the body centre, a courtyard that does not enclose the component, and a pad geometry that was scaled incorrectly when the part was drawn. Less frequent but more damaging are a mirrored pin numbering and a footprint that was copied from a similar part and never corrected.

Our notes on PCB design quality characteristics describe how library quality is assessed, and why a library that is internally consistent is more valuable than one that is merely large.

Fixing a Library Safely

Correcting an origin in a library part changes the placement data for every board that uses it, which is exactly what is wanted and also exactly what makes the change risky to an existing project. The safe order is to fix the library, regenerate the assembly data, and compare the two versions before releasing anything.

Where a project is already in production, changing the library mid-life is usually a bad idea. Recording the convention in use and keeping the old part until the next revision avoids an unexpected change to a board that is already qualified.

Working With the Assembler

The assembler’s first question about a new product is usually whether the placement data matches the machine’s convention, and the answer is easier to give when the data has been checked before it is sent. Sending the placement file together with a plot showing it overlaid on the board removes most of the back and forth that follows a coordinate issue.

It also helps to state the conventions explicitly: origin location, rotation direction and the treatment of bottom side parts. Our notes on stencil aperture and placement tolerance explain how placement accuracy is budgeted, which is the context in which an origin error becomes a defect rather than a curiosity.

FAQ

Where should a footprint origin be placed? Anywhere, as long as every part in the library uses the same rule and the assembly file follows it. The centre of the body is the most common choice because it makes rotation behave predictably.

Why do some components place correctly and others not? That pattern almost always means a mixed library. Parts drawn with different origin conventions produce different offsets from the same placement file.

Can the assembler correct an origin error? They can offset individual parts, but that is a manual correction applied to every product that uses the part. Fixing the library is faster and removes the risk of the correction being forgotten.