laser marking (original) (raw)

Author: the photonics expert

Definition: a group of methods for labeling materials with lasers

More general term: laser material processing

Category: article belongs to category laser material processing laser material processing

DOI: 10.61835/b2m [Cite the article](encyclopedia%5Fcite.html?article=laser marking&doi=10.61835/b2m): BibTex plain textHTML Link to this page LinkedIn

Laser marking is a common method for labeling various kinds of objects using a laser. The principle of laser marking is that a laser beam somehow modifies the optical appearance of a surface that it hits, or ablates some material. Marking processes are a specific sub-group of laser material processing methods.

Applications of Laser Marking

Laser marking has a huge variety of applications, developed over several decades.

Many industrial products such as machine tools, printed circuit boards (PCBs), integrated circuits, cables, keyboard buttons, credit cards, food packages and bottles need to be equipped with certain labels, such as model and serial numbers, logos, bar codes, “to be use by” dates and the like. Often, the details of those labels need to be different for each single part, so that a simple stamping is not an option.

Often, one must add traceable information for quality control. For example, silicon wafers used for photovoltaic cells or electronics may be equipped with an indication from which boule they were cut and at what position. That way, possible problems can be traced back, so that they can be more quickly identified and solved.

Compared with other marking technologies such as ink jet printing and mechanical marking, laser marking has a number of advantages, such as very high processing speeds, low operation cost (no use of consumables), constant high quality and durability of the results, avoiding contamination, the ability to write tiny features, and very high flexibility in automation. However, it often requires relatively expensive laser marking machinery.

Technical Details

Used Mechanisms

A variety of mechanisms can be used for marking surfaces. These can be subdivided into the following groups:

Examples of laser-induced surface modifications are:

Besides, there are some marking methods which work in the volume of a transparent material. For example, one can produce tiny spots in a glass, using tightly focused laser pulses. Here, one utilizes laser-induced breakdown, often in conjunction with nonlinear self-focusing.

Scanning and Mask Methods

Marking patterns can be formed by suitably moving a laser beam over the workpiece surface. There are fundamentally different methods for that:

Some kind of laser scanner is needed for applying such methods. Often, one employs an f–theta scanning lens, so that the focus positions are all on a plane rather than on a curved surface.

An alternative is to produce a whole label in one shot, using a suitable kind of mask, placed at a suitable location in the beam path such that the mask features are imaged to the workpiece. That latter method, however, requires the fabrication of suitable masks, which implies that the pattern cannot be changed from piece to piece. Also, the masks tend to wear off and need to be regularly replaced. On the other hand, a whole pattern can be produced very rapidly in combination with a sufficiently powerful laser source, provided that the mask is also sufficiently robust. The trend, however, appears to be that scanning methods are more and more used. Their greater flexibility is a main reason for that.

Laser Marking Machines

laser marking station

Figure 1: TruMark laser marking station. The photograph was kindly provided by TRUMPF Laser.

A typical laser marking machine contains a pulsed solid-state laser, a compact beam delivery system and possibly auxiliary items e.g. for removing fumes. They also need to be some means to insert and align the pieces to be marked, or sometimes an automated movement and alignment machinery. In some industrial settings, materials are marked “on the fly” while quickly moving along the marking machine. For improved laser safety, some marking machines are encapsulated and work only while their front door (for supplying workpieces) is closed.

Plastic materials, wood, cardboard, paper, leather and acrylic are often marked with relatively low-power sealed CO2 lasers. Here, one can often use continuous-wave operation.

For metallic surfaces, CO2lasers are less suitable due to the small absorption at their long wavelengths (around 10 μm); laser wavelengths e.g. in the 1-μm region, as can be obtained e.g. with lamp- or diode-pumped Nd:YAG lasers (typically Q-switched) or with fiber lasers, are more appropriate.

Typical laser average powers used for marking are of the order of 10 to 100 W. Some machines with particularly high throughput work with substantially higher powers.

Shorter wavelengths such as 532 nm, such as obtained by frequency doubling of YAG lasers, can be advantageous, but such sources are not always economically competitive. The frequency doubling does not only contribute to the cost of the operators, but also works with the limited efficiency of the order of 50%; the reduced pulse energy then compensates some of the advantage of the shorter wavelength. Nevertheless, for marking of metals like gold, which has too low absorption in the 1-μm spectral region, short laser wavelengths are essential.

Excimer lasers, a kind of pulsed ultraviolet lasers, are needed in special cases, for example for glasses and ceramics.

Demands on Lasers for Marking

Lasers for marking applications must meet a number of demands. Some typical ones are:

Depending on the specific circumstances, different types of lasers can be most suitable for a marking application. For example, Q-switched vanadate lasers can be superior when high pulse repetition rates (> 100 kHz) are important. Fiber lasers, which are in that case actually master oscillator power amplifier (MOPA) systems, are very flexible in terms of pulse repetition rates and interruption of pulse trains, but often emit longer pulses with lower pulse energies and peak power. CO2 lasers can be superior in cases where their long wavelength is suitable – most often, for non-metal materials.

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