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42 The PCB Magazine • November 2016 ADVANCED UV LASERS FOR FAST, HIGH-PRECISION PCB MANUFACTURING A commonly used material in flex PCB manu- facturing is the copper/polyimide/copper lami- nate. The foil thicknesses within the laminates have shrunk over time, with copper and poly- imide layers currently down to below 10 and 13 µm, respectively, and likely to trend thinner still. Common flex PCB laser processes include profile cutting and both blind and through via drilling. Compared to ABF resin-on-copper via drilling, flex PCB via drilling has the additional require- ment that two very different materials—copper and polyimide—must be processed, ideally with the same laser source. As it turns out, CO2 lasers, with their far infrared (~10 µm) wavelengths, are not suitable because the long wavelength light is strongly reflected by the copper. Hence, UV DPSS lasers are used heavily in flex PCB manufacturing. Using a high-power 30 W UV laser, process- es for drilling both blind and through vias have been developed and characterized for drilling throughput. The flex PCB laminate consisted of 1 mil thick polyimide laminated on both sides with ½-mil copper foil. Since the materials are fairly thin, very small vias of 25 µm diameter or below can be percussion drilled using a very small focus spot. The small spot size and strong coupling of the UV light to both the copper and the poly - imide allows for processing with relatively low energy levels, which means the laser can be oper- ated at very high PRFs, thereby achieving high drilling rates. If larger vias are required, a larger focus spot could be used (with more energy per pulse from the laser) and for yet larger vias, high- speed beam scanning optics can be employed to rapidly move the tightly focused beam in a cir- cular pattern—a process technique known as tre- panning—while the laser is ablating the material. Typically, the small-circle trepanning process is limited by the speed of the scanning optics, and sometimes operating the laser at very high PRFs can result in undesired heat affects. In such cases, a laser with a lower PRF (and therefore lower av- erage power) is preferred in order to match the speed of the beam scanning equipment, thereby ensuring best quality. For percussion drilling (Figure 5 a, b), throughputs can be very high because there are no moving parts required to drill, and the laser can be operated at a very high PRF since the focus spot is small and therefore energy requirements are reduced. The percussion drilled blind vias in Figure 5 were drilled with a laser-capable drill rate of about 9,000 vias/sec, while the drill rate for the through vias was more than 5,500 vias/sec. Both drill processes used a higher laser PRF of 300 kHz. The trepan drilled via in Figure 5c involved a 2-axis scanning galvanometer processing head that deflected the beam with small circular mo - tion—an approach that is inherently slower com- pared to percussion ablation. In this case, three repeat scans at 200 mm/s were used, which re- sulted in an effective drill rate of >250 vias/sec. In this case, the laser PRF was much lower at 60 kHz in order to match the speed of the beam scanning equipment. The 3D optical profilometer data plot in Figure 5d shows the high-quality, much de - sired, very low edge burring that can be achieved with careful process optimization. In this case, the 2−4 µm edge burr is not much larger than the native roughness of the copper foil. Coverlay Patterning In flex PCB manufacturing, coverlay pattern- ing is an important process for cutting various Figure 5: Optical photomicrographs of blind (a) and through (b) vias in Cu/PI/Cu laminate as well as trepanned via (c), and surface topology of trepanned via (d) generated with pulsed UV lasers.