PCB Marking and Traceability Requirements Explained
Every board carries some form of marking, whether it is a part number etched into the copper, a date code printed in the legend or a laser etched serial number on a panel. Marking looks like a trivial requirement until a field failure has to be traced to a batch, and at that point the mark on the board is the only thing that links the unit in the customer’s hand to the process records in the factory. Traceability is therefore a property of the marking scheme, and a scheme that was designed for identification alone rarely satisfies it.
Why Boards Are Marked
Marking serves three purposes. It identifies the product and its revision so that the correct assembly program is used, it identifies the batch so that a defect can be contained, and it satisfies the safety and regulatory requirements that apply to the finished equipment.
Those purposes imply different requirements. Identification has to be readable by an operator, traceability has to be machine readable, and the regulatory marks have to follow a defined format. A single marking scheme that satisfies all three is best decided at the design stage rather than added later. Adding a mark to a released design usually means a new fabrication tool, which is far more expensive than including it in the first release.
What Has to Appear on the Board
The minimum set is normally the part number, the revision, the date code and the fabricator’s identification, and many customers add a serial number, a panel position or a customer specific code. The information has to be placed where it will not be covered by a component or removed by the depaneling step.
Space is the usual constraint. A dense board may have no free area for a large block of text, which is why marking is often split between the copper layer, the legend and a data matrix code that carries more information in less space. A two dimensional code can hold the part number, the revision, the date code and a serial number in an area the size of a few characters of text.
Date Codes and Lot Traceability
The date code identifies when the board was made and it is the primary key for containment. When a problem is found in the field or at the customer’s line, the date code narrows the investigation to a defined window of production and to the material lots that were in use at that time.
For traceability to be useful, the date code has to be linked in the factory records to the laminate lot, the prepreg lot, the plating bath history, the press book and the inspection results. A date code that cannot be resolved to those records is only a marking, not a traceability system. The records themselves start with the design release, and our release checklist guide describes the package that has to be agreed before production begins. The test of a traceability system is whether a single board taken at random can be resolved to its material and process records within a reasonable time.
UL and Safety Marks
The UL mark identifies the laminate and the board construction that has been evaluated for flammability and electrical safety. It may be printed on the board, and the associated file number and the material designation have to be recorded so that a substitution is not made without re-evaluation. The laminate designation printed on the drawing and the material actually purchased have to match, and incoming inspection is where that is confirmed.
Other marks appear where the end product is subject to a specific regulation. They should be applied exactly as the standard requires, because a mark that is the wrong size, in the wrong place or in the wrong colour can invalidate a certification and create a liability rather than satisfying it. When a customer specifies a mark, the exact artwork and position should be agreed in writing before the first panel is produced.
Marking Methods
Copper etching is permanent and costs nothing extra because it is produced by the same etching step that forms the circuit, but it can only be read where the copper is not covered by a mask. Legend printing is the most flexible method and can carry text, logos and symbols in any colour.
Ink jet and pad printing are used for variable data such as a date code or a serial number, while laser marking is used where permanence matters and where the surface can tolerate the local heating. Each method has a resolution limit and a contrast limit, and both have to be considered at the design stage. A marking method that cannot resolve the required character height will produce a mark that passes a visual check and fails a scan.
Laser Marking and Readability
Laser marking removes or carbonises a thin layer of mask to expose the material beneath, which gives a permanent mark without ink. The parameters have to be set so that the mark is legible without cutting through the mask to the copper, because a mark that reaches the copper can create a path for corrosion. Depth control is a machine parameter, and the correct setting should be recorded so that it survives a tool change or a service visit.
Readability is the test that matters. A data matrix code has to be verified with a reader at the grade required by the customer, not merely inspected by eye, because a code that looks acceptable to a person can fail the print quality grading that a scanner requires. A verification grade should be quoted in the drawing notes, because the customer and the fabricator otherwise have different ideas of what readable means.
Marking on Assembly and Components
Once components are fitted, marking becomes harder because the free area shrinks and because the assembly may be coated. The mark should be placed on a surface that survives the conformal coating step, or it should be applied after coating if the process allows it. A mark under a coating may still be readable, but the contrast usually drops and the code grade has to be re-verified on the coated surface.
Component marking follows a different set of rules, and it is controlled by the traceability requirement of the assembly rather than by the board itself. Where a serial number is required per unit, the number has to be applied and recorded at a point in the flow where the unit can still be linked to its build records. Applying a serial number after the unit has left the traceable flow defeats the purpose of having one.
Traceability Records Behind the Mark
The mark is only the visible end of a record chain that starts with the incoming material. A complete chain links the customer order to the panel, the panel to the press book, the press book to the material lots, and the material lots to the supplier certificates. Each link is simple on its own, and the chain is only as strong as its weakest link when a recall has to be scoped.
Building that chain requires discipline in the shop rather than sophisticated software. A simple rule that every panel is identified at every step, and that the identification is recorded with the process data, is enough for most traceability requirements. Our fabrication notes guide lists the data that belongs with an order. Where a customer requires full traceability, the record retention period should also be agreed, because the records have to outlive the warranty.
Process Control Points
The controls are the marking content and format, the placement on the board, the method and its parameters, and the verification that the mark is legible and correct. Verification should include a reading of the machine readable code at the required grade. A marking that is applied correctly but never verified is a marking that will eventually be found to be wrong at the customer’s line.
A first article check confirms that the marking is correct before a run starts, and a periodic check through the run catches a worn marking tool or a drifting laser parameter. Our quality documentation describes how these results are classified at gopcb, and marking errors are among the few defects that can make a board unusable even when the circuit itself is perfect. Treating marking as a specification item rather than a decoration is what keeps the record chain intact.
FAQ
What is the difference between a date code and a serial number? A date code identifies a batch and is shared by every board made in that period, while a serial number identifies one unit. A serial number gives finer traceability at a higher cost in process time.
Can laser marking damage the board? It can if the parameters are set too aggressively and the mark cuts through the mask to the copper. The mark should be verified visually and electrically on a sample before a run is released.
Is marking required by a standard? Some marks, such as the UL mark, are required by a certification, while others are a customer requirement. The specification should state which marks are mandatory and where they must be placed.
