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Reproducible copper weldingCombining IR and green light is key
ROFIN-LASAG AG: Christoph Ruettimann, Richard Bartlome, Noémie Dury
A prerequisite for laser material proces-sing of copper is direct interaction bet-ween the focused laser radiation and
the copper surface that requires part of the radiation to be absorbed at the surface. As FIGURE 1 shows, for a polished copper sur-face at room temperature, less than 5% of the laser radiation is absorbed (at 1 µm laser wavelength). Despite this, there are a range of measures that can be taken to allow copper to be welded successfully and efficiently, par-ticularly with solid-state or fiber lasers.
The first of these measures is coating the surface with more absorbent metals, such as nickel or chromium, and mechanical prepara-tion of the surface, for example, roughening or painting, also increases the absorption. However, both of these measures increase production costs as they require additional fabrication steps. Chemical impurities in the copper can also influence absorption beha-vior and make it more difficult to achieve a reproducible welding process. As can be seen from the absorption curve, shorter laser wavelengths would be an advantage. A diode
GreenMix laser in action
laser (0.8 to 1 µm, infrared) or a frequency-doubled Nd:YAG laser (532 nm, green), for example, can deliver these wavelengths, alt-hough neither of these lasers has yet reached the stage where copper is industrially viable. Heating at the point of incidence of the laser radiation is faster or slower depending on the level of absorption, and the risks of overhea-ting and splatter formation become unpredic-table. The problem of poor reproducibility is even more pronounced for spot welding than for seam welding. Each individual laser pulse impinging on the cold copper material expe-riences different initial conditions. However, because the absorption depends not only on the wavelength and the surface characteris-tics, but also on the material temperature (ab-sorption improves significantly at increased temperatures) and the intensity of the laser radiation, there is additional scope for impro-ving the welding process.
Overlaying two wavelengthsThe poor reproducibility experienced in cop-per welding can be significantly improved by overlaying a laser pulse with a wavelength of
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532 nm onto one with a wavelength of 1 µm. If the two laser beams are focused using the same chromatically compen-sated lens, it generally results in the copper surface heating up from the center to the edge of the weld spot. This is due to the better ab-sorption of the green wavelength, thus raising the temperature of the copper and therefore raising the absorption of both wavelengths. With increased absorptivity, variations in the surface characteristics have a less significant impact. For example, if a laser beam is made up of 15% green and 85% IR and the different initial absorptions of the two wavelengths are taken into account, approximately the same quan-tities of energy from the green and IR radia-tion will be absorbed (at room temperature). FIGURE 2 shows pure IR welding (approx. 1 MW/cm2), pure green welding (approx. 1 MW/cm2), and welding with the combinati-on of both wavelengths on a 300-µm-thick copper strip. The theoretical spot size is ap-proximately 200 µm for both wavelengths. In the case of a pure IR pulse, no fusing of the copper surface is observed, while with a pure green pulse, some fusing can be seen. Over-laying the two pulses results in a significant enlargement of the weld pool.
Process detailsChoosing an appropria-te IR pulse shape allows the relative green compo-nent, i.e., the conversion efficiency, to be adjusted. Figure 3 shows a typical IR pulse shape and the resulting green conversi-on. The laser pulse can be split into three pha-ses, which are described in more detail below. At the beginning of the laser
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Wavelength [microns]
532 nm 1064 nm
pulse (0.5 to 1 ms), the pulse intensity is de-liberately set to ensure that the highest pos-sible proportion of IR radiation is frequency-doubled. The combination of IR and green radiation heats the copper surface until the absorption is sufficiently high for IR, and the copper surface begins to melt. Typical values for the pulse power during the melting phase are between 0.5 and 2.0 kW, depending on the thickness of the copper strips. As soon as the copper begins to melt, the pulse power is sharply reduced. Although this means that very little green is then converted, the absorp-tion of IR is high enough in the melting pha-se. From this point onwards, the welding is continued with IR only (negligible green com-ponent). The duration of the welding phase is determined by the extent and depth of the
Green IR + GreenIR
Fig. 2: Comparison of IR, green an a combination of both wavelength.
Fig. 1: Wavelength dependency of copper absorptivity.
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weld spot. At the end of the pulse, the power is reduced using a defined time constant to positively influence the cooling phase of the welding and thus its metallurgical and me-chanical properties (for example, hardness).
Comparison with pure IR weldingFigure 4 shows a comparison between pure IR welding and welding with IR and green laser light. The pulse duration was ~1 ms, and no pulse shaping was used (rectangular pulses).
The weld spots in the upper half of the figure were made using IR pulses with a 0.8 kW pulse po-wer, while the weld spots in the lower half were created with 1 kW IR and 0.1 kW green. As ex-pected, the reproducibility of the pure IR welding is very poor, and significant fluctuations in diame-ter and even missing weld spots can be observed. The reprodu-cibility of the welds with IR and green light is considerably bet-ter. No defects can be identified. The insert in FIGURE 4 shows a series of weld spots created using the pulse form described in FIGURE 3. The weld spots achieved using this method look almost identical. A more precise analysis reveals that the varia-
tion in the weld spot diameter is well below 10%, and 100% full-penetration welding was achieved from the copper ribbon to the base material. This figure meets the strict specifi-cations of medical applications, for example.
Fig. 3: Typical pulse shape for copper welding with resulting green conversion.
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«Weld phase»Constant power >99% IRDetermines weld spot size
«Solidification phase»Decreasing power
Fig. 4: Comparison of (top) pure IR welding and (bottom) IR plus green welding. Insert: weld spots after optimization of the pulse shape.
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Efficiency considerationsIn a further investigation, the melting threshold intensities of pure IR, pure green, and a mix-ture of both types of radiation were deter-mined and compared. TABLE 1 summarizes the measured melting threshold intensities. Although the melting intensity for pure green radiation is the lowest, the frequency con-version represents a significant limitation on process efficiency. However, if a mixture is used instead of pure IR radiation, the melting threshold is practically halved for identical la-ser efficiency and thus represents an effecti-ve increase in process efficiency.
summarizes the technical specificatuons of the GreenMix Laser.
Various industrial applications can be addressed with the GreenMix laser. Currently the working range of the laser is restricted to component dimensions below 300 µm. FIGU-RE 5 summarizes this laser›s working range for different joint configurations. R&D work is ongoing to push the limits of the working ran-
Table 1: Melting threshold of cu for different wavelength
WAVELENGTH MELTING THRESHOLD
INTENSITY OF CU (MW/CM2)
IR (1064 nm) 10Green (532 nm) 288% IR + 12% Green 4.6
Table 2: Technical specifications
TYPE LASAG SLS GX 1500Average power, max.
15 W
Pulse power, max. 2.0 kWPulse width 0.3 - 20 msRepetition rate 2 - 30 HzTransport fiber dia-meter
100 or 200 µm
Numerical aperture of fiber
0.11
Other features • Pulse Power con-trol
• High-resolution Pulse shaping
• Pulse-on-demand Power burst
• Universal Inter-faces
• Simple integration in production envi-ronments
Fig. 5: Summary of welding depth with GreenMix laser.
Industrial implementation and application examplesThe Lasag SLS GX 1500 laser is equipped with the patented GreenMix technology. Un-like other industrial green laser concepts, this solution uses only one laser source, which naturally decreases investment costs of the system. Both wavelengths are transported onto the copper surface using optical fibers and a dedicated processing head. TABLE 2
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ge to component sizes of 0.5 mm and above.Some classical real-world applications of this laser are: ribbon bonding for medical applica-tions (e.g., pacemakers); wire bonding (e.g., lead frames on heat sinks) and wire separati-on for the electronics industry and mobile de-vices; and contacting of solar modules (e.g., flexible cells) for the photovoltaics industry.
Dr. Christoph Ruettimann ([email protected]) is the head of research and develop-ment, Dr. Richard Bartlome is project manager for optics and optomechanics, and Noémie Dury is the head of laser application at Rofin-Lasag in Thun, Switzerland.
SummaryThis article describes methods that signifi-cantly improved the reproducibility of laser welding of copper materials. The combinati-on of IR and green light using intelligent pulse shaping not only results in improved repro-ducibility of the welds, it also brings about a considerable increase in process efficiency.
09.04.2013, Industrial Laser Solutions www.industrial-lasers.com
ROFIN-LASAG AGC.F.L. Lohnerstrasse 24P.O. Box 17CH-3602 ThunSwitzerland
Phone: +41 33 227 45 45Fax: +41 33 227 45 [email protected]
