Keeping PCB Fiducials Visible Through the Assembly Sequence

A fiducial only helps a machine when it can be recognized at the stage where it is needed. A target visible on the bare board may later be hidden by a connector, fixture, label, or coating. Review fiducials against the complete assembly sequence, including the second side and any downstream processes that use vision alignment.

Know which coordinate frame is being located

Eurocircuits describes copper fiducials used by printers and placement machines to determine panel position and orientation, and distinguishes panel-level references from references on individual boards. Its stated target count is a requirement of its own assembly process; confirm the corresponding requirement with the assembler selected for your job. Eurocircuits: assembly fiducials.

Mark which targets establish the panel frame and which establish a board or critical component region. Ensure the placement and stencil data use a consistent coordinate interpretation. A panel drawing should identify the same targets that the machine program expects. Do not rely on a tooling hole or an arbitrary exposed pad as a replacement without process approval.

Check the actual fabricated target

• Verify copper shape and size against the assembler’s vision requirements.
• Inspect the solder-mask opening and the clear surrounding region.
• Keep legend, copper patterns, and labels from confusing the target image.
• Confirm the intended finish and surface condition.
• Review target placement near board edges and fixture clamps.

Inspect the manufacturing output rather than only the CAD screenshot. A target may look correct in the copper editor while a mask or paste layer changes its final appearance. The release review should also catch accidental paste apertures on fiducials that are intended to remain free of solder deposits.

Walk through each process step

Start with bare-board printing, then component placement, first reflow, second-side handling, through-hole insertion, and any selective soldering or coating. At each step, check whether the required marks are still visible. For instance, a target placed under a through-hole connector may work for SMT placement but become unusable after that connector is installed.

Resolve recognition problems with evidence

If a machine struggles to locate a target, preserve the camera image and identify whether the issue is geometry, contamination, lighting, program selection, or obstruction. Moving every target may be unnecessary. Once the arrangement is proven, retain it in the panel and board documentation. Reassess visibility when a fixture, label, component body, or assembly sequence changes. This small review prevents a registration feature from becoming a production interruption that is discovered only after the boards arrive.