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.
A variety of mechanisms can be used for marking surfaces. These can be subdivided into the following groups:
- Laser engraving: one can use a laser to ablate a surface to some extent, often with a depth below 100 μm, but sometimes more. Compared with other methods (see below), the processing depth normally needs to be higher to obtain a sufficiently clear visual appearance.
- Removing surface layers: the appearance of a surface can be changed by ablating a previously present surface layer, which can e.g. be an oxide layer (e.g. on anodized aluminum) or a paint coating.
- Surface modification: there are many ways of modifying a surface such that its visual appearance changes.
Examples of laser-induced surface modifications are:
- A metal surface can get a colored appearance by the formation of the thin oxide layer caused by heating the surface for a short while.
- There are industrial laser additives which can be activated or bleached by laser radiation.
- Many polymers (plastics) become black and the laser radiation due to carbonization (slight burning); similar effects occur on paper, cardboard and wood.
- Many polymers expand upon laser heating and then become a lighter appearance.
- In other cases, surface structures or small bubbles are formed.
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:
- Best quality is achieved with vector methods, where the laser beam, e.g., draws the contours of letters and digits.
- For graphical patterns, it is sometimes more appropriate to systematically scan the full area while disabling the laser at parts of the area.
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
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.
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.
Demands on Lasers for Marking
Lasers for marking applications must meet a number of demands. Some typical ones are:
- The laser wavelength should be such that sufficient absorption is achieved in the marked material. Note that strongly increased absorption during processing (anomalous absorption) can normally not be utilized in laser marking, since the applied intensities are quite moderate.
- A certain optical peak intensity or fluence must be reached on the workpiece. Marking processes often exhibit a certain threshold, below which no satisfactory result can be achieved, even with multiple pulses. (That threshold value is strongly dependent on the physical mechanism used.) On thus requires some suitable combination of peak power or pulse energy and beam radius at the focus, and the pulse duration also has some influence. Tight focusing is also required for achieving high resolution, and together with a reasonable working distance this means that a sufficiently high beam quality is needed. In some cases, the limited effective Rayleigh length of the used beam makes it necessary to use positioning in three dimensions, making sure that the focus is on the workpiece.
- For fast processing, the laser needs to offer a high enough pulse repetition rate, which together with the pulse energy implies a certain average power. (In certain regimes, the repetition rate of a Q-switched laser is limited by the possible average power, in other regimes by the laser dynamics or the achievable pulse duration.)
- In some cases, the Q-switched pulse train must be switched off for certain time intervals in order to move the laser head to another position where the marking is restarted. That switching often introduces the problem that the first pulse has a higher energy, which can spoil the marking quality. Some lasers are equipped with means for first pulse suppression to avoid such problems.
- The laser setup should be compact and should not require complicated cooling arrangements. Air cooling is preferable and often possible, since the handled average powers are modest (e.g. some tens of watts).
- The cost of ownership must be moderate – not only in terms of installation cost, but also concerning lifetime and maintenance.
- As industrial environments can be harsh, a robust laser design is essential for reliable operation. For example, the laser should tolerate some level of mechanical vibrations and of back-reflections of laser light.
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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