SMT007 Magazine

SMT-July2016

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July 2016 • SMT Magazine 93 Infrared lasers can be called "hot" lasers as they heat and burn a path in the material to be cut. With UV lasers it is possible to ablate the material. A short high-energy pulse enters the top layer of the material and evaporates and ex- plosively removes a layer of the material. By go- ing over the same path several times, ultimately a cut is obtained through the material. As very little heat is produced by the UV beam, there is very little or no burned material on the edges of the cut depending on how the laser is being used (Figure 3). Depending on the wavelength of the light, some materials reflect it and some are com- pletely transparent. For the ablation method to work, the laser beam has to penetrate into the material to be cut. Figure 4 shows how various circuit board constituents react with different wavelengths. To be able to ablate all of them the UV laser is a good choice. UV lasers (wavelength ~350 nm) have become economically attractive only for the past 10 years. Shorter wavelength and excellent optics al- low for a very small beam size, often around 15 to 25 µm. This allows cutting a very narrow kerf in the panel resulting in minimal waste between boards, especially as the mechanics of the sys- tem allow very precise beam location. The ex- ample in Figure 5 shows part of a panel with very small boards. When the routing process was used the number of boards per panel was approximately 125, and after re-layout of the panel to use laser cutting the number of boards increased almost three-fold. This resulted in a very significant economic advantage. In the laser system used in this example, a panel is placed on a perforated surface with downdraft, or mechanically mounted on the high precision X-Y movable table to prevent the panel from moving during the cutting opera- tion. For boards with components on both sides a special support pallet is required. To cut all the paths on a panel the area is divided in blocks of 50 x 50 mm in which the laser beam is moving using precision computer controlled mirrors mounted on galvanometers. The beam movement speed within this area is well controlled and can be as high as 1000 mm/s. While cutting, airflow passes across the panel as shown in Figure 6 to remove debris and Figure 3: Infrared cut versus ultra violet cut. Figure 4: Reaction of various circuit board constituents with different wavelengths. Figure 5: Example for a panel with very small circuits. MATERIAL EFFECTS OF LASER ENERGY

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