PCB Labeling And Traceability Requirements

A finished board carries more than copper and solder mask. It carries a date code, a part number, a revision, a flammability marking, and increasingly a two dimensional code that links it to a production record. That information is what makes a board traceable, and traceability is what makes it possible to answer a question months later about which panel a board came from and what process it saw.

This article explains what has to be marked, which marking methods are used, how the requirements differ between lot level and serialised traceability, and where the marking should be placed so that it survives assembly.

What Has To Be Marked

The flammability marking is the one that is mandatory in most markets, and it identifies the laminate rather than the board. It includes the manufacturer of the material or the board, the material designation, the flame class such as 94V-0, and a file number that refers to the certification. A board that carries an unapproved material, or that has no marking at all, cannot be shown to meet the requirement if a customer asks.

The remaining markings are driven by the customer and by the industry. A date code in a four digit week and year format identifies the production period, a part number and a revision identify the artwork, and a country of origin marking is required in some markets. Where the product contains lead free solder or complies with material restrictions, a symbol or a code is usually added, and where the assembly is sensitive to static or to moisture, a warning marking may be applied at the assembly stage rather than on the bare board.

Marking Methods

Silkscreen printing is the traditional method. It is inexpensive and it covers the whole panel in one step, but it has limited resolution, it can smear in a fine detail, and the ink can fade or lift during a hot assembly process. It is well suited to a part number, a revision, and a logo that do not change between panels.

Inkjet printing adds variable data. It can print a serial number, a data matrix code, or a lot number that changes from panel to panel, and it can do so without a screen for each change, which makes it the usual method for traceability. Laser marking produces the most durable result by changing the surface of the material or of the mask, which survives reflow, washing, and coating, and it can be applied to the solder mask, to a metal stiffener, or to the surface finish. Marking directly on the finish requires care, because a laser that penetrates the plating can damage the solderability of the area around it, so the parameter set has to be developed for that surface rather than assumed.

Labels are also used, particularly for serial numbers applied at the assembly stage, but they are the least permanent option. A label can fall off, it can be placed over a pad or a test point, and its adhesive can leave a residue that interferes with coating. Where a label is required, its position and its temperature rating should be defined on the drawing.

Lot Level And Serialised Traceability

Lot level traceability identifies the panel or the production lot, which is enough when a problem would affect everything built in the same period. A date code plus a panel number on the rail gives that level of resolution, and the records that go with it are the fabrication date, the material batch, and the test result for the lot.

Serialised traceability identifies each individual board, which is required where a failure has to be resolved to a single unit, and it is common in medical, automotive, and aerospace work. The serial number is applied as a data matrix code so that it can be read automatically at each station, and the code is linked to the records of every process that touched the board, from fabrication through assembly and test. The relationship between the panel and the individual boards is part of the scheme: a code on the rail has to map to a code on each board, so that a panel level fault can be resolved to the units that were affected.

Placement Rules

A marking is only useful if it can be read at the end of the process. It should not be placed where a connector, a shield, a heatsink, or a stiffener will cover it, and it should not be placed where a component will be mounted directly over it on the other side. The marking should remain visible after coating, which means either marking a surface that is masked during coating or accepting that the coating material has to be legible over, which is rarely the case.

For a machine readable code the requirements are stricter. A data matrix needs a quiet zone around it, a minimum cell size, and enough contrast for the reader, which usually means a dark mark on a light background or the reverse with a defined difference. The code should be placed away from the board edge, away from tooling holes, and away from areas that will be routed or scored, because a partially removed code is worse than no code. Where the board is small, the code is often placed on the panel rail and the mapping to the boards is held in the traceability system rather than printed on each unit.

The Records Behind The Marking

The marking is only the entry point to a record. The code has to resolve to the fabrication date, the laminate batch, the panel identification, the fabrication test result, the assembly lot, the inspection result, and the test data for that unit. The record has to be retrievable for the retention period that the industry and the customer require, which is measured in years and sometimes in decades for medical and aerospace products.

Building that record is a data management task rather than a marking task, and it is the part that is usually underestimated. A code that is applied but not linked to anything is a decoration, and a traceability system that cannot produce the record when it is asked for is worse than one that was never claimed, because the requirement has been accepted without being met.

Common Problems

The most common problem is a marking that disappears during fabrication, because the fabricator needed the space for a coupon or because the legend was placed on a layer that is not printed on every panel. The second is a distinction that is too fine to read: a small font, a low contrast ink, or a code with an inadequate cell size passes a visual check at the bench and fails at the scanner.

The third is a mismatch between the bare board and the assembly. Where the assembly code is applied over the board code, or where the assembly house assigns its own lot number, the link between them has to be recorded deliberately, otherwise a failure that is traced to an assembly lot cannot be resolved to a board supplier. Reviewing a first article with a scanner, rather than with an eye, is the cheapest way to find all three problems. The fabrication side of the same documentation is described under PCB design and fabrication, the marking position as part of the outline under board outline and mounting design, and the wider framework under PCB design quality characteristics.

FAQ

Is a date code enough for traceability? It identifies the production period and is enough for lot level tracing, but it cannot resolve a failure to an individual unit. Serialisation is required where that resolution is needed.

Does laser marking damage the board? It does not damage the laminate, but a laser applied to a surface finish can affect solderability if the parameters are not developed for that surface. Marking on the solder mask or on a metal stiffener avoids the question.

Can a serial number be applied as a label? It can, and it is common at the assembly stage, but a label is less durable than a printed or laser marked code and its position has to be chosen so that it does not cover a pad or a test point.