Laser Marking on PCB Assemblies: Date Codes and UID

Assembled boards are increasingly marked by laser rather than by ink, because a laser can write a unique identifier on every unit without a stencil or a print cycle. The change brings new questions about contrast, substrate damage, and how the mark survives conformal coating and cleaning. This article covers what laser marking can and cannot do, and how to specify it so that inspection and traceability records stay reliable.

Why Marking Matters on Assembled Boards

A mark serves three separate purposes. It identifies the product and revision so that a technician can match a board to its documentation, it records a date or lot so that a quality team can trace a batch back through production, and it carries a unique serial number for warranty and failure analysis. Printed legend can do the first job, but a stencil cannot economically write a unique number on each unit.

Regulatory and contractual drivers reinforce the practice. Automotive, medical, and aerospace customers commonly require unit-level identification, and contract manufacturers need a mark that can be read by an automated camera at the end of the line. Where a product is small or the board area is crowded, a compact machine-readable code is often the only option that fits.

Laser Marking versus Printed Legend

Screen printed legend uses epoxy ink cured onto the surface, and it produces a high-contrast, inexpensive mark when the design is fixed. Changing the text requires a new stencil or a new print program, and a unique identifier per unit is impractical. Ink adhesion also depends on the surface energy of the solder mask, so a change of mask supplier can quietly change the durability of the mark.

Laser marking removes the ink process entirely. A fibre or ultraviolet laser changes the surface of the solder mask or the underlying copper, and the mark is generated directly from a database so that every unit can carry its own code. There is no consumable, no curing step, and no stencil change, but the process has to be controlled carefully because the same energy that produces a legible mark can damage the laminate underneath.

Contrast on Solder Mask and Bare Metal

Contrast depends on the material being marked. White and light coloured solder masks usually mark dark, because the laser chars or ablates the pigment, while black masks are often marked by foaming the resin, which produces a light mark. Green and blue masks can be ambiguous, so a sample should always be marked and measured rather than assumed.

Marking directly on copper or on a gold finger gives good contrast but risks exposing metal that will later corrode or oxidise. Marking through an opening in the mask is common for a logo, but it should never be used for a code that must remain readable after coating. The practical rule is to mark the solder mask, keep the energy just high enough for the required contrast, and verify readability after every subsequent process step.

Date Codes and Lot Identification

A date code links a unit to the material lots and process conditions used on the day it was built. Common formats encode the year and week, sometimes with a shift or line identifier, and they are normally placed where they remain visible after assembly. Because the code is short, it can be applied by ink or laser, and many factories use the same format for both.

The code is only useful if it is recorded. The traceability records that a factory keeps should tie the date code and, where present, the serial number to the solder paste lot, the reflow profile, the operator, and the test result. Where a field failure returns without a readable mark, the lot cannot be narrowed down, which turns a small containment into a broad recall.

Unique Identifiers and Data Matrix Codes

Machine-readable codes make unit-level tracking practical. A data matrix code of three to five millimetres can hold a serial number, and a vision system at the end of the line reads it in a fraction of a second. Placement matters: the code needs a quiet zone around it, a background with enough contrast, and a location that is not covered by a shield can, a connector, or a label applied later.

Reading conditions should be tested, not assumed. Curved or flexible boards distort the code, conformal coating adds reflections, and a matte black mask can reduce contrast below what a camera needs. Reputable practice is to mark a sample, coat it, run it through the line, and then attempt a read with the same camera and lighting that production will use.

Damage Risk: What a Laser Can Do to a Board

A laser that is set too high will damage more than the surface. Above the mask, the beam can carbonise the resin, expose glass weave, or cut into copper, and a conductive path of carbonised material across two traces is a reliability hazard. Ultraviolet lasers with short pulses reduce the heat affected zone, but even they must be set with a power and frequency study before a job is released.

Damage also hides. A mark that looks acceptable can leave microcracks in the mask or a change in the surface energy that affects conformal coating adhesion. Where a coated board is required, the coating should be applied over a marked sample and inspected for dewetting. The energy margin between a legible mark and a damaged surface is often narrow, so periodic verification of the marking recipe belongs in the process control plan.

Specifying a Marking Standard

The specification should state the content, the symbology, the location, the size, and the acceptance criteria. State whether the mark must survive cleaning and coating, define the minimum contrast or grade for a machine-readable code, and set a pass or fail criterion for the sample that is approved. Where a customer supplies a marking standard, quote it on the drawing rather than paraphrasing it.

For prototype and small batch work, the practical advice is to keep the mark simple and place it where it can be inspected. A board that carries a readable revision, a date code, and a compact serial number covers almost every requirement, and it can be reworked or re-marked if a build has to be corrected.

Choosing a Marking Location

Location decides whether the mark survives to the end of the product life. Avoid areas that will be covered by a shield can, a connector body, a battery holder, or a label applied at final assembly, and keep the code clear of the panel rails and any routing path used during depaneling. On a crowded board, reserving a marking field during layout is as important as reserving space for a test point.

Keep the mark away from high-stress regions as well. A laser mark on a flexing area of a thin board changes the surface locally, and repeated bending can start a crack at that boundary. Where a board will be depaneled by routing or by scoring, place the code several millimetres from the break line so that debris from the process does not blur the contrast. The same panel and prototype requirements that govern tooling strips apply here.

FAQ

Can laser marking replace the printed legend entirely? For text and codes it can, and it removes a process step. Many designs still use printed legend for large reference designators because printing is faster over a large area, and use the laser only for the variable data.

Will the mark survive conformal coating? It survives if the coating wets the marked surface properly. Mark a sample, coat it, and check readability, because some laser settings change surface energy enough to cause dewetting or a loss of contrast.

How do I choose between a date code and a serial number? Use a date code when lot-level containment is enough and board area is scarce. Move to a unique serial number when the customer requires unit-level traceability or when field returns must be traced to a single build.