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Laser Marking Enhances Assembly Traceability
December 31, 1969 |Estimated reading time: 5 minutes
Product manufacturing data must encompass all information relevant to the design, verification, components, production and test of each assembly. Company-specific coding and tamper-proof encryption of such information may be used to protect highly proprietary technologies.
By Joyce Laird
Labels, direct inkjet and stencil-based etch marking (dry and chemical) techniques currently are facing difficulties in the application of substantially more data on ever-shrinking printed circuit board (PCB) real estate. Not only must such identifications be performed at production line speeds, they also must be applied to spaces as small as the distances between densely mounted surface mount device (SMD) leads.
The main benefit of using lasers for board marking always has been that it is a fast, fully programmable, noncontact process that is permanent and generally unaffected by neither the steps faced during the assembly process nor those endured in hostile field environments. To be sure, the drawbacks also have always been the higher initial cost of laser marking equipment, limited mark contrast in some applications and controllability of system power relative to delicate SMDs.
New Laser Technology Effects
Two types of lasers generally are used for marking assemblies:
- The CO2 laser, as implied, uses carbon dioxide gas to create the monochromatic light that creates markings at a wavelength of 10,640 nm.
- The Nd:YAG laser uses a doped garnet crystal compound as the medium to create a near infrared beam at the 1,064 nm wavelength.
Figure 1. Using a green wavelength of 532 nm provides 100 percent absorption on the marking surface of this microBGA. Nothing is transmitted through the device that could debilitate the circuit.
The combination of wavelength and laser power plays a role in laser marking in all SMT areas. "Some wavelengths better serve certain applications," said John Derzy of Rofin-Baasel Inc., Boxborough, Mass. "Today, new requirements have led to the development of new lasers and the modification of existing ones for different uses. These include: frequency-doubled YAG or 532 nm 'green' wavelength lasers, solid-state UV wavelengths, split-beam lasers and various diode-pumped lasers that permit the use of different types of crystals and resonator designs to improve performance in specific applications."
Added colleague Patrick Schlather, "A good example would be, if using a 1,064 nm, or standard red, wavelength laser to mark a microBGA, a certain amount of the wavelength is transmitted through that device, which could debilitate the circuit. Using the green wavelength," he continued (Figure 1), "there is 100 percent absorption on the marking surface with nothing transmitted through the device."
Figure 2. New Nd:YAG laser developments have created diode-pumped marking systems offering precise beam-power control and short pulse widths for high contrast and steadily controlled material-penetration depths of less than 25 µm.
New developments in the Nd:YAG laser area basically have eliminated the contrast problem (Figure 2). Specific diode-pumped lasers now offer precise beam-power control and short pulse widths for higher contrast and steadily controlled material-penetration depths of less than 25 µm. This has opened up many applications for lasers, including those for bar-code marking, and others that require fuller contrast levels of readability.
Lasers and Data Matrix Symbology
Data matrix is the hot technology today for all forms of product traceability and is well suited to densely populated PCB assemblies. It is a 2-D symbology that resembles a checkerboard pattern, which permits embedding a great amount of information in microscopic areas (Figure 3). Because data-matrix 2-D symbology only requires a 50 percent or lower contrast ratio for readability, its acceptance has opened the amount of applications in which lasers are used.
Figure 3. An example of a 2-D code and a humanly readable text string on the same board, marked with high contrast using a long-life sealed gas CO2 laser.
"Data-matrix coding also has built-in correction," added Schlather, "so that if a portion of the code is destroyed for some reason, it is still readable."
With the proper lighting, these types of codes can be practically invisible yet just as crisp and pronounced as a jet-black bar code on a white surface to the proper digital decoding reader. A cell can be sized to the equivalent of one laser dot, about 0.005". Similarly, a 60-character message can be embedded in only 1 sq. mm. Because it is a one-to-one aspect ratio, it can be sized up or down corresponding to the available space. Finally, data matrix also permits several forms of encryption and protective devices to be embedded in the data, making this type of marking valuable for proprietary assemblies.
The Importance of Validating Marks
With microscopic codes, it is important to verify and validate markings. Programmed automatic optical inspection (AOI) systems, combined with laser marking, provide 100 percent inspection and validation. Thus, if an error is programmed into the marking algorithm (or something must be added), it should be known at the point where it is the easiest to take action, a capability most laser marking systems have today.
Schlather added that on a few of the higher-end laser marking systems, verification and validation of the symbology are accomplished in conjunction with the marking. The reading device, usually a camera, is integrated directly into the marking device. A code can be marked and read through the same optical train, then validated in real-time in this manner.
"Flexibility is built in, enabling ongoing verification/validation of all laser marks from the smallest to a fairly big mark field with the laser scan head," he continued. "A good example of mark verification and validation would be on a circuit board panel where there might be 20 boards in a 15 x 20" panel and the requirement is to put a separate and unique code on each board. Individual symbologies or 2-D codes can be marked and simultaneously verified and then moved on to the next panel."
What About Cost?
Contrast and controllability no longer are negative issues for laser marking in PCB assembly processing. However, the initial investment for either a stand-alone or in-line laser marking and verification system typically is higher than that for more conventional types of ID systems.
CO2 and many of the newer diode-pumped lasers have closed the dollar gap. But more importantly, when viewed through a cost-of-ownership model, laser markers become substantially better investments as time goes on because there are no major consumables, recurring process costs, or supplies to order, stock or refill. Long-life, sealed-gas CO2 configurations and the diode-pumped, solid-state laser markers require little maintenance. Further, in most PCB processing, laser markers lend themselves to automation and can be operated in an unattended mode with little or no operator interaction.
Conclusion
Field failures can cost more than just a returned product. Reputation, liabilities and future business can depend on the elimination of these types of problems and swift correction if they do occur. Data-matrix coding provides a way to apply and read complete manufacturing data on any board configuration for full lifetime product traceability. The laser appears to be a highly versatile method to apply such symbology. Also, many of the newer specialties such as protective encryption that can be provided by using laser technology offer the full spectrum of control possibilities to the manufacturer.
ACKNOWLEDGEMENT
Author would like to thank Rofin-Baasel Inc. for contributing the graphics featured in the article.
Joyce Laird may be contacted at 13207 Wentworth St., Arleta, CA 91331; (818) 768-1832; E-mail: jlcms@earthlink.net.