PCB Serialisation and Panel Level Traceability Marking

Serialisation is only useful if the mark survives every process the board goes through and can be read at the point where the information is needed. That means the marking method, the position, the symbology and the reading equipment are one design decision rather than four separate ones. This article covers where the mark goes, how it is made, and how panel level mapping connects a board to the panel it came from.

What Serialisation Is For

A serial number connects a physical board to a record, and the record carries the material lots, the process settings and the test results of that unit. Without the link, a field failure can only be investigated at the level of the product rather than the unit.

The value of the link depends on what is recorded against it. A serial number that only identifies the date of manufacture answers one question, while one that also identifies the drill, the lamination cycle and the test programme answers most of the questions an investigator will ask.

Symbology and Data Content

A two dimensional code is used where a large amount of data has to fit in a small area, and a data matrix is the common choice because it is readable at low contrast. A linear barcode has more tolerance to print quality but carries far less data.

The content should be the minimum that identifies the unit uniquely, with everything else held in a database. Encoding a full bill of materials into the mark makes the code large and fragile, and it also makes the mark unreadable as soon as one module fails.

Where the Mark Goes

The mark is placed on a surface that is not covered by a component, is not handled repeatedly and will not be removed by a later process. The top side away from the board edge is usual, and it should be clear of the fiducials, the gold fingers and the test points.

On a panel, the mark position has to survive depaneling, which means it is placed inside the board outline and clear of the break lines and the rails. Where the board is small, the mark goes on the underside rather than in an area that will be populated.

The available area is checked on the finished layout rather than on the artwork, because mask and copper both change the contrast the reader sees.

Laser Marking and Its Parameters

A laser marks the mask, the copper or the laminate depending on the material and the parameters, and each produces a different contrast and a different durability. Marking the mask removes material and exposes the surface below, which reads well but removes protection.

The parameters that matter are power, pulse frequency, marking speed and the number of passes, and they are set to produce a readable mark without cutting into the copper beneath. Where the mark is applied to a plated surface, the parameters are different from those used on the mask, so the two surfaces are qualified separately.

Read Rate and Verification

Read rate is the proportion of marks the reader can decode on the first attempt, and it is the number that determines whether serialisation is practical on a production line. A code that reads at 70 percent stops the line more often than a code that reads at 99 percent, whatever the difference in data content.

Verification uses a graded reader with defined lighting rather than the production scanner, because a scanner is tuned to succeed while a verifier measures the print quality against a standard. The two measurements describe different things and both are useful.

Verification is done on the production surface and after the processes that follow marking, since a mark that grades well when it is made can lose contrast after coating or cleaning.

Panel Level Mapping

Panel mapping records the position of each serial number within the panel, so that a board can be traced back to its position and to the material that went into the panel as a whole. The map is produced at the marking step and carried through assembly.

The map is what makes a partial panel useful. Where several boards from one panel fail for the same reason, the map shows whether they were adjacent, which points to a fabrication or handling cause rather than an assembly one.

Durability Through the Process

The mark has to survive cleaning, coating, reflow and any mechanical handling, and each of those can reduce contrast. Cleaning can remove residue from the mark or fill it, coating can cover it, and reflow can change the surface around it.

The practical approach is to verify the mark after the last process that could affect it, using the same reader that will be used in service. Where the mark fails at that point, the marking parameters or the position are changed rather than a second attempt being added later in the flow.

Data Handling and Privacy

A serial number that is stored in a database has to be retrievable years later, which means the database is part of the product rather than part of the production line. Where the code carries data directly, it has to remain readable without the database.

Customer requirements sometimes specify that no identifying mark may be visible, which is met by marking the underside or by using a code that carries no human readable text. The requirement is recorded on the drawing so that the mark is designed rather than added.

Documentation

The drawing should state the symbology, the data content, the position with its keep-out, the marking method, the durability requirement and the read rate to be demonstrated. Without those items, the marking is produced to the shop’s default and the traceability depends on luck. Related acceptance practice is described in our board quality guide.

The marking step sits in the flow after the processes that change the surface and before the processes that could damage the mark. Coating over a mark is acceptable where the coating is transparent and the read rate is verified after it has been applied.

Where a board is marked before reflow, the mark sees the full thermal excursion, so the parameters are qualified for that rather than on a room temperature sample. A mark that reads well on a bare panel can lose contrast as the mask around it cures further.

Readers are installed at the points where the serial number is consumed rather than only at the end of the line, because a mark that fails at final test has already passed stations where the information was needed. Each station has its own illumination and working distance.

Handling after marking matters as well, because boards stacked face to face rub the mark against the panel above. Packing is arranged so that the marked surface is not loaded, which usually means stacking boards on their reverse faces.

The database is part of the design: records need a retention period that matches the product life and an export format that survives a change of system. A serial number whose records vanish when the production system is replaced has delivered traceability only for as long as that system lived.

FAQ

Which symbology should be used? A data matrix for most boards, because it carries enough data in a small area and reads at low contrast.

Can the serial number be added after assembly? It can, but the marking process has to be qualified on an assembled board, and the area available is usually smaller than on a bare board.

Why map the panel? Because it converts a set of individual failures into a pattern that identifies whether the cause was fabrication, handling or assembly.