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96 SMT Magazine • July 2016 ing removed by dispersion into the material, radi- ation into the environment and convection into the air using forced air flow over the material. The heat equation is a parabolic partial dif- ferential equation that describes the distribu- tion of temperature in a given region over time. Equation 1 The resulting graph (Figure 9) shows the gradual increase in temperature for multiple passes with the laser beam along a cutting path. Ultimately a balance will be reached between applying heat and dispersing, radiating and convection of heat away from the cut area. In order to determine what actually occurs in the circuit board material near the kerf cut by the laser, linear temperature sensors were placed on a test board (Figure 10). In this test, the tabs were cut, some of which are bare FR4, some are FR4 with copper and some are FR4 without the routed slots and the nearby temperature rise was measured. The tabs where the sensors were placed were cut with the cutting path at different distances from the sensor. Even when cutting within 0.1 mm from the sensor, the temperature reached only 100°C, well below any temperature the board is normally being exposed to during the soldering process. The cutting parameters for this example were: P = 12.4W, v=244 mrn/s, rep = 30, CT = l00 ms, full-cut FR4 (thickness 400-450 µm). Cooling time (CT) is the time it takes for the beam to return to the same location. During this time other sections of the outline are being Figure 9: Simulation of heat accumulation. Figure 11: Cutting in one material type, measuring at different distances. Figure 10: Linear temperature sensors (circled) on the test board. MATERIAL EFFECTS OF LASER ENERGY