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July 2016 • SMT Magazine 97 cut and it can also include a rest period between repetitions. The cooling time in this example was 100 ms. To compare examples of cutting through the different materials, bare FR4, FR4 with cop- per and a full-cut FR4 were investigated, with the results showing in Figure 12. Because different efforts are needed to cut through different materials, different cutting times result and for the more difficult situations higher temperatures are being measured. Still the temperatures at a distance of about 0.1 mm remain quite acceptable. Quality vs. Time As mentioned, the laser beam does ap- ply some heat to the workpiece. In order to minimize the impact of the heat the beam is scanned multiple times over the cutting path to distribute and minimize heat build-up. For this reason, the beam control system allows adjustment of the movement speed and beam power, but it is also possible to insert rest peri- ods between cutting paths. These rest periods are more important when the cutting path is short and the beam would be back in the same location more quickly. When board layout and component place- ment are done well away from the sides of the individual boards, the cleanliness of the side- walls is of less concern and the laser parameters can be selected for maximum cutting speed, meaning higher beam power, faster beam speed and shorter rest periods between cutting paths. On the other hand, when the cleanliness of the sidewall is critical, more care has to be taken in the selection of the machine settings. Figure 13 presents a visual difference between these two strategies. Surface Analysis To determine the spectrum of chemical components left on the cut surface the energy dispersive X-ray (EDX) analysis method was performed. For reference a routed sidewall was carefully polished and cleaned to show the nor- mal composition of a board. Spectral lines from four chemical components are displayed in false color for the polished sidewall (Figure 14) and Figure 12: Comparing temperatures in different materials. Figure 13: Visual difference between high quality/ low speed and fast speed/lower quality. Figure 14: Polished and cleaned sidewall. MATERIAL EFFECTS OF LASER ENERGY