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Right Light: Equipment selection is critical when laser marking microparts

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March/April 2013 Volume 6 Issue 2

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By Ulrich Ofer

FOBA Laser Marking + Engraving/Alltec GmbH

+49 38823 55-0

uofer@alltec.org

Lasers offer a flexible, virtually trouble-free way to mark components for identification purposes. As a result, laser marking has gained ground on other parts-marking techniques.

From cell phones to brake discs, computers to camshafts, ID cards to bone screws, lasers are widely used to permanently mark part surfaces. Typical laser markings include alphanumeric signs, logos, machine-readable codes and decorative flourishes.

Lasers impart no stress to the workpiece and can mark a broad range of materials, including metals, ceramics, plastics and glass, as well as flat, curved and irregular surfaces. Laser markings also can be applied rapidly, making the process ideal for mass-production operations. And, many different-size markings can be made.

The remainder of this article discusses lasers used for producing micromarks.

 

Making marks

Laser marking with galvanometer scanner heads (galvos) is a vector-based marking procedure. The galvos control the movement of two mirrors that direct the laser beam to any point in the marking field. The action resembles a pen moving across a piece of paper. Just as the width of a pen mark is determined by the diameter of the pen’s roller ball, the diameter of a beam’s focus determines the width of a laser mark.

The focus of a laser beam depends on two things: its wavelength and the focusing optics. A laser’s focus is a prime consideration when it comes to marking microparts. Unless special, very expensive optics are used, the smallest focus that can be generated is about five times the laser’s wavelength.

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Marking field and focal (f) length: The longer f is, the larger the marking field and the greater the positioning inaccuracy. All images courtesy FOBA Laser Marking + Engraving.

A CO2 laser’s focus is 10 times larger than that of a solid-state laser, such as an Nd:YAG (neodymium-doped yttrium aluminum garnet)/fiber or Nd:YVO4 (vanadate). Accordingly, a focus diameter of roughly 50µm is achievable with a CO2 laser and about 5µm is possible with Nd:YAG/fiber and Nd:YVO4 lasers.

You might think that the rule of thumb would be: Use a CO2 laser for marking larger parts and a solid-state laser for marking smaller parts. The reality, however, is that thousands of CO2 lasers are used to mark electronic components and other small devices.

For a mark to be clear and legible, the height of the smallest character must be at least five times the line width. “Clear” means we can distinguish an “8” from a “B”, for example. For a CO2 laser, this means the smallest recognizable character it could produce would be 0.25mm high.

Characters taller than 0.25mm can be marked quickly and economically with a CO2 laser, which is capable of producing bold, readable letters and numbers with a single stroke. Smaller-focus lasers would need to make two or three strokes to achieve the same legibility, doubling or tripling the processing time. This, along with the outstanding economy and wear life of CO2lasers, is why so many can be found at facilities that manufacture electronic components.

Producing characters smaller than 0.25mm requires a laser with a shorter wavelength than a CO2, like an Nd:YAG. It can reliably produce markings recognizable by automated vision and pattern-recognition systems used for parts inspection and quality-control purposes. These systems, which only accept legibly marked parts, are usually the first and most important readers of laser-marked components.

Another limitation of CO2 markers, compared to Nd:YAG and vanadate lasers, is their relatively small marking field. A small marking field, which results from a small focus and short focal distance, is needed to make micromarks. A small field is acceptable for marking single integrated circuits, for example, but a larger field is better for marking a tray or strip of ICs.

 

Hitting the mark

Every laser-marking application requires balancing the laser’s wavelength, focal distance and marking-field size in order to achieve the desired character height and legibility at production speed. Once this is done, it is time to consider the placement of marks.

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A CO2 marking laser was used to produce single-stroke characters on this integrated circuit.

Several factors must be considered: the mechanical accuracy of the part to be marked, the fixture or cavity where it is placed and the accuracy of the lasermarking system.

The biggest sources of misplaced marks are mechanical inaccuracies that occur during the manufacture of components to be laser-marked and the wearing of fixtures that hold the components. Compensation must be made for these inaccuracies. Acceptable marks cannot be applied if parts are not repeatedly placed below the laser. Even the slightest out-of-position part will result in an incomplete mark and a scrapped workpiece.

Furthermore, mechanical inaccuracies can manifest themselves in different ways from one laser-marking system to the next. That leads to standardization problems that undercut the benefits of mass production.

The second biggest source of incorrectly marked microparts is the laser-marking system itself. The main contributors to this type of inaccuracy are:

  • pointing instability of the laser system;
  • galvo-related (mechanical and/or electronic) problems;
  • nonplanarity of the galvo mirror and galvo fixture; and
  • problems related to the corrective lens that focuses the beam.

Position accuracy, marking field

Absolute positioning accuracy is the difference between system output and the commanded input. The marking field—the area in which a laser can make a mark—is a function of focal length. The longer the focal length, the larger the marking field. But, the larger the marking field, the greater the positioning inaccuracy.

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Lasers can mark regular and irregular surfaces, don’t damage the workpiece and help prevent the manufacture of counterfeit parts.

In mass-production applications involving multiple laser markers, there will be position deviations from one marker to the next, greatly compounding the problem of marking parts accurately. One solution to this conflict is to use a calibrated marking system.

During calibration of a laser-marking system (see opening image, page 45), we test each laser’s output and create a file that shows a grid overlaying the laser’s marking area. An optical-measurement system determines the position of each cross point on the file and compares the measured value of each point to its ideal, theoretical position. Offsets correcting the actual point positions and their ideal positions are stored. Corrections are made only once. The marking system remains accurate as long as hardware components remain unchanged.

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Uncalibrated laser-marking system (top). The red and blue areas depict position inaccuracies within the marking field. The green areas in the graphic at right show the positioning accuracy of a calibrated marking system.

The photographs above show a marking-field pattern with 2,000 measurement points before and after correction.

Besides calibration, cameras can improve the positioning accuracy of laser-marking systems. Two common ways
are to add an internal camera (through the focusing optics) or an external
camera (one mounted close to the marking head).

Both methods have advantages and disadvantages. Internal vision systems are always “correct” in that that they are subject to the same inaccuracies as the laser itself, so the marking results always show correct values. And as long as the part is within the aperture of the vision system, all external inaccuracies are eliminated. The disadvantage of through-the-lens vision systems is the small viewing angle, which is due to the small aperture of the laser mirrors.

The advantage of an external camera is that it works well when the marking area is large. And it can be used in conjunction with a wide range of optics, promoting process optimization. With an external camera, inaccuracies in
large parts, strips or trays can be effectively eliminated with one exception: Internal shifts of the marking position over time are not recognized, and can lead to improper positioning of the mark. Together with the more-difficult mechanical and optical adjustments necessary with an external camera, this method is the second-best way to ensure high-precision marking.

The best approach is to combine internal and external cameras. The results will be outstanding.

Thanks to advancements in laser-marking technologies, including calibration techniques, lenses and power sources, outstanding results can be achieved in the precision marking of microparts. µ

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Ulrich Ofer is vice president of marketing and applications for FOBA Laser Marking + Engraving/Alltec GmbH, Selmsdorf, Germany. Additional information can be found at www.fobalaser.com or by calling FOBA North America, (519) 572-8695. Telephone: +49 38823 55-0. E-mail:  uofer@alltec.org.