Pick And Place Coordinate File Errors And How To Avoid Them
A board can be electrically perfect and still stop a production line because the pick and place coordinate file is wrong. The machine does not read the schematic or the netlist; it reads a table of positions, and if a position is offset, the part goes down in the wrong place. The failures are predictable, and most of them trace back to three problems: missing coordinates, coordinates that sit off the pad centre, and a mismatch between the placement table and the bill of materials.
Understanding what the file contains, and what the upstream design data does not contain, removes most of the back and forth between the designer and the assembly house. The sections below cover each failure mode in the order that it usually appears during a release.
What A Coordinate File Has To Contain
A usable pick and place coordinate file lists one row per component, and each row carries five fields. The reference designator identifies the part, such as R1 or U3. The X and Y coordinates give the centre of the component in the units and origin that the machine expects. The layer field states whether the part sits on the top or the bottom side, and the rotation angle states how the part is turned about its centre.
When any of those fields is missing or inconsistent, the machine either skips the part or places it incorrectly. Units are a frequent source of trouble: a table exported in mils and imported as millimetres puts every part in the wrong place by a factor of about 25. The header should state the unit explicitly, and a reviewer should confirm it rather than assume it.
Gerber Data Does Not Carry Component Positions
Gerber is a plotting format. It describes the shapes that will be drawn on each layer, and it has no concept of a component, a reference designator, or a centre point. A pad is a rectangle in the data, not the landing site of a part. This is why an assembly house cannot derive a placement table from Gerber data alone, even though the pads are all present in the file.
The position information lives in the design database, so it has to be exported deliberately, from the same source as the board outline and the netlist. Exporting from a different revision is a common mistake, and it produces a file that is internally consistent and still wrong, because the pads it refers to no longer exist.
Origin, Rotation Angle And Layer Conventions
A whole board can be shifted by a single origin error. If the designer sets the origin at the lower left corner of the panel and the machine expects it at the lower left of the board, every part moves by the difference. The symptom is distinctive: all coordinates are wrong by the same amount and in the same direction, which points at the origin rather than at the individual parts.
The rotation angle is the second global convention. The same physical orientation can be described as 90 degrees or 270 degrees depending on whether the angle is measured clockwise or anticlockwise, and on whether the zero direction points along the X axis or the Y axis. Mirroring for the bottom side adds a third variation. These conventions should be agreed in writing with the assembly house before the first order, because they are cheap to fix at that stage and expensive to fix later.
The BOM Mismatch That Stops A Line
A BOM mismatch appears when a component listed in the bill of materials has no matching row in the placement table, or the reverse. The check exists because the two files are exported separately and can drift apart. Some mismatches are harmless. A through hole part, for example, is hand soldered and does not need a machine coordinate, so its absence from the placement file is expected.
A missing coordinate for a surface mount part is not harmless. The machine has no instruction for that part, so it is simply not placed, and the omission may not be noticed until the board reaches test. Comparison tools that highlight the designators present in one file and absent from the other turn this from a manual review into a short list of exceptions that a person can resolve in minutes.
Correcting Off Centre Coordinates
Coordinates that sit off the pad centre are usually an export artifact. Older layout tools placed the origin of a footprint at the first pad rather than at the geometric centre, and that convention survives into the exported table. The result is a part that is placed with a small but consistent offset, which reduces the solder fillet on one side and increases it on the other.
Modern assembly software can snap an off centre coordinate to the geometric centre of the pad pair, and this correction is reliable when the offset is uniform. When a large fraction of the parts are affected, however, the honest answer is to re-export from a corrected library rather than to patch each row, because a library that is wrong in one place is usually wrong in others as well.
A Short Checklist Before Release
Confirm the units and the origin, and state both in the file header. Confirm that the layer field matches the side that the part is actually on, and check the rotation convention with one known part as a reference. Compare the designator list against the bill of materials and resolve every exception, including the ones that turn out to be through hole parts.
Finally, verify that the placement table, the Gerber data and the bill of materials all come from the same design revision. Records of the revision should travel with the files. A mismatch between revisions is the one error that survives every other check, because the files are individually correct and the assembly is still wrong.
Process Control and Verification
On a design of this kind, Gerber data is the item that decides how the rest of the board is arranged. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.
Related reading: our fabrication notes, board quality and design release notes cover the same ground.
FAQ
Can the assembly house generate the coordinate file for me? Some can, if the original design database is provided along with the Gerber output. From Gerber alone it is not possible, because the format carries no component information.
Why do some components appear in the BOM but not in the placement table? The usual reason is that they are through hole parts that are hand soldered. Confirm this before treating the difference as an error, and check any surface mount part that is missing a coordinate.
What unit should the coordinate file use? Either millimetres or mils is acceptable if the header states it, but the choice should match what the machine expects. Converting between the two after export is a reliable way to introduce a scaling error.
