LPKF UV – LASER ABLATION / CUTTING SYSTEM FOR THE ELECTRONICS INDUSTRY INTRODUCTION The early laser cutting systems that were used to cut metals provided enough power to melt the material which was then blown away by an assist gas. Typically these were CO2 laser (10.6 micron wavelength, in the far infra-red or IR region) with a relative large beam. Later systems were designed around YAG lasers (1064nm wavelength, near IR), pumped with power hungry flash lamps. These lasers provided smaller beams allowing cutting of greater details (kerf width of <40um). But these lasers typically provided less power and therefore were used to cut only thinner metal sheets, for example stainless steel stencils used to print solder on circuit boards for surface mount technology. LPKF Laser & Electronics, AG has been producing many of these systems for this application. To make further improvements in the solder printing process and other laser based processes for electronic applications LPKF developed a laser system which is able to cut or ablate organic materials, like polyimide, flexible circuits, cover layer, overcoats, solder mask, etc This type of LPKF system uses a YAG laser, the output of which is frequency tripled resulting in a laser beam of 355nm (ultra-violet or UV). A benefit of the shorter wavelength is a significantly smaller beam size which allows cutting of finer details making a kerf less than 20um. In stead of using flash or arc lamps, this laser uses diodes as a light source, which is a much more efficient method and reduces the overall power consumption. Therefore no large cooling system is required to maintain constant temperature of the laser cavity. In the ablation process the material is destroyed into dust with minimal heating of the surrounding material. This process does leave a minimum of molten or charred residue behind depending on the material, which means that when processing e.g. flex circuits no resistive residue (carbonized plastic) is left behind and the quality of ablation/cutting is excellent. LASER ABLATION/CUTTING BASICS The processing speed of laser cutting/ablation and the resulting cut quality depend on both the characteristics of the material being processed and the nature of the laser emission (wavelength, fluence, peak power, pulse width, and pulse rate). It is important to know the absorption characteristic of the material to be cut. Most insulators do absorb radiation in the UV and in the far IR region (compare Fig. 1). Figure 1 – Absorption of metals and insulators Q-switched, frequency-tripled, diode-pumped, Nd:YAG lasers emitting a wavelength of 355 nm are the right choice for applications in the circuit board (especially flex circuits) manufacture and assembly processes, as copper and polyimide can readily be cut or ablated with UV lasers. When employing Q-switching, the lower amplitude tail-end of the laser pulse is removed, yielding shorter optical laser pulses in the range of a few tens of nanoseconds with higher average peak power in the range of a few kilowatts. The process of photon absorption and material removal takes place in the following way (Fig. 2). Figure 2 – Process of Photon Absorption and Ablation. Incident photons from the laser beam are absorbed in a thin layer of the material, up to the optical penetration depth. The thickness of the layer l? where most of the energy is absorbed is determined by the wavelength dependent absorption coefficient ?: It describes the depth, down to where the intensity of the absorbed radiation has decreased to 1/e. The energy is subsequently transformed into heat and transferred to the molecule chains. When the evaporation temperature of the material is exceeded the surface material explosively evaporates and leaves the cutting kerf. The heat also diffuses from the surface layer into the material. The thermal diffusion distance is a function of the material specific diffusion coefficient D as well as the laser-beam dwell time ?L (basically equivalent to the laser pulse length) and describes the distance to where the temperature decreases to 1/e: In any case, to improve cutting quality the thermal influence (dissipation of the local heat) on the material has to be reduced, i.e. this heat affected zone has to be kept as small as possible. The two parameters, optical and thermal penetration depth, allow a good estimation of how much thermal influence is involved in the process of cutting. The smaller both parameters are and the smaller the thermal penetration depth is compared to the optical penetration depth the more the energy is confined to a small volume which increases the thermal gradient and thus the cutting quality. Whereas the optical penetration depth is a material specific constant the thermal penetration depth increases with longer pulse lengths and can be reduced by using short pulse lasers. When the vapor plume leaves the kerf under high pressure its presence through beam scattering can cause additional heating of the sample being cut. Some of the material in the plume precipitates as debris on the sample. Both can reduce the cutting quality. Laser debris can be reduced by e.g. decreasing the ablated volume. To reduce the thermal losses in the cutting area and therefore heating within the adjacent material, low pulse repetition rates, high cutting speeds or a combination thereof can be used. This will result in a longer cooling phase between the pulses and a shorter interaction time between the laser and the material. Even multiple passes with a lower power setting and a delay time between each pass to allow cooling of the material can help to reduce the heat affected zone. Fig. 2 shows how temperature increases in the border area of the cutting kerf after multiple laser pulses. Figure 3 – Temperature in cutting kerf after multiple pulses. Ablation is often used to remove layers of the material, so in order to cut through a sample, multiple passes might be necessary. This also increases control over the amount of heat being generated by each pass, and thereby the quality (cleanliness) of the cut. The ablation process can be used for metals too, but with today’s equipment the processing time is significantly longer compared to the previous systems, making this process only useful for very thin metal sheets and as mentioned before, for organic based material. To accomplish ablation in metals it is necessary to increase the peak power density, which can be done by increasing the power, reducing the beam size or for the same power reducing the laser pulse width. A significant factor in the calculation of the theoretical minimum beam size is the laser wavelength and therefore ablation can be accomplished more easily with shorter wavelengths. Even though in laboratory settings any of these improvements have been demonstrated with very short wavelengths and pulse widths down to femto seconds, at this time for practical and economic reasons it is necessary to choose and make an acceptable compromise. APPLICATIONS Such a newly developed system has been installed at A-Laser in Beaverton, Oregon, a division of FCT Assembly, Inc. where this system is used to cut polyimide stencils. Good paste release from a stencil depends on the ratio of the area of the opening in the stencil, representing the extraction force, to the wall area of the opening, representing the friction force which keeps the paste in the stencil. But as Kapton provides very little friction, paste release is excellent. Figure 4-cutting speeds vs. material thickness (KAPTON) As mentioned before the speed of cutting depends on the thickness of material. Fig. 4 shows the cutting speed vs. thickness of polyimide (Kapton). Besides cutting stencils this same laser system is used to cut components out of polyimides (e.g. for flex circuits and/or coverlayers), mylars, adhesives, prepregs (for circuit boards), thin metals up to 75 um (0.003”) and is used to ablate solder mask, circuit board layers and other materials. This system is particularly useful when there are tolerances or shape details that can not be manufactured with traditional milling or CO2 lasing processes. Most of these parts can be cut with exceptional size tolerances of less than 6um. The positional accuracy is also exceptional at 25um or better over a 600×600mm working area. As these varying materials have different ablation properties, it is necessary to develop specific settings (tools) for each. Even different thicknesses require different tools. And with higher cut quality requirements cutting speed has to be adapted. However the software that is part of the system makes evaluating and changing settings a relatively easy task. Figure 5 shows the cutting of cover layers (polyimide or Kapton). The kerf width is 30 micron and the cutting speed was 95mm/sec. Figure 5- cutting of polyimide (KAPTON) Note especially the superior quality of cutting with this UV laser system. In figure 6 the cutting quality is compared with the cutting quality achieved with a pulsed CO2 system. At left is shown the CO2 quality with a mark speed of 3400mm/sec and at right the UV-laser (355nm), with a speed of 95mm/sec. Figure 6- quality achieved with CO2 versus UV-laser LASER SYSTEM To allow very fast and precise movement of the table linear motors are being used. For movements in a small area (approximately 50×50 mm) the laser beam is being deflected by a precision galvo system. This also increases the overall operating speed of the system, as the galvo components represent very little mass. While maintaining full accuracy, speeds of over 400mm/sec can be used. In order for the system to maintain its accuracy it is necessary to operate it in a constant temperature room. A glass scale and built-in calibration procedures allow for high precision at all times. Other than power (three phase, 400Vac) it only needs high pressure air (>8bar) to operate. As this is a diode pumped system with long life diodes, the maintenance has been reduced significantly. An exhaust system with high quality filtration is part of the system as is the built in small chiller to maintain constant temperature of the laser itself. Authors: Ahne Oosterhof, (Founder, A-Laser, Beaverton, OR.) Dr. Hüske (Innovation Manager, LPKF Laser & Electronics, GmbH, Garbsen, Germany.) Dr. Meier (Senior Technical Consultant, LPKF Laser & Electronics, GmbH, Garbsen, Germany.)