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Figure 8: Surface resistance measurement.
If so desired, a simple cleaning process can be added and will remove the remaining particles. This can be done by wiping with a smooth dry or wet tissue, using compressed air or brushes.
Thermal Effects
Even though UV laser can be called “cold” lasers, there still is some heat being generated. Its impact is very dependent on the settings of the laser system. The laser beam inserts some heat into the material being cut and heat is being removed by dispersion into the material, radiation into the environment and convection into the air using forced air flow over the material.
Figure 9: Simulation of heat accumulation.
The heat equation is a parabolic partial differential equation that describes the distribution of temperature in a given region over time.
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).
Figure 10: Linear temperature sensors (circled) on the test board.
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.
Figure 11: Cutting in one material type, measuring at different distances.
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 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 copper and a full-cut FR4 were investigated, with the results showing in Figure 12.
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