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94 SMT Magazine • July 2016 minimizing any deposits on top of the panel. When the cutting inside the 50 x 50 mm area is finished, the table is moved to the next square until the project is finished. Location precision From the original design data (e.g., Gerber file) the laser system can use panel fiducials to locate where the cut is intended to go. The ta- ble movement, in conjunction with the galvo movements, is computer controlled and allows the beam to be located within 25 µm of where it is supposed to be. However, when singulating boards or flex circuits, the precision of the panel image is typically less and therefore it often be- comes necessary to use additional fiducials for smaller portions of the panel. It is even possible to use recognizable sections of the board pat- tern for more precise board edge location re- quirements. Residue on Board Surface Even though an airflow passes across the area being cut (Figure 6), not all of the material expelled from the kerf is caught. Some remain- ing particles are powdered epoxy and glass par- ticles. None of these are measured to be larger than 20 µm and they averaged around 10 µm. (For reference see the circled area in Figure 7.) Their size and quantity should not raise any concerns. But to determine if the redeposited mate- rial can cause any problems, a test board was designed made of FR4 material, 800 µm thick (Figure 7). The test board had four patterns with sets of two groups of interdigitized fingers. Each pair of these fingers was connected to the edge of the board for easy measurement of the sur- face insulation resistance (SIR). As part of the test, a slot was cut in close proximity to the fingers. After cutting the slot, these test boards were subjected to a climate test (40°C, RH=93%, no condensation) for 170 hours and the SIR was measured. In all measurements, the values ex- ceeded 10E11 ohm—indicating that the SIR is not negatively impacted (Figure 8). 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 be- MATERIAL EFFECTS OF LASER ENERGY Figure 6: Airflow and exhaust. Figure 8: Surface resistance measurement. Figure 7: Surface after laser cutting.