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SMT-Dec2017

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30 SMT Magazine • December 2017 laser cutting and laser welding. The step open- ings are cut out of the first stencil. The corre- sponding step areas are cut out of a second sten- cil foil of the desired thickness. The step pieces are placed into the openings of the first sten- cil. Then the pieces are laser welded into place. The thickness of the step area is determined by the thickness of the steel used. The laser weld- ing process is shown below (Figure 2). The micro-machining process is a subtrac- tive process similar to the etching process, but no chemicals are used. The micro-machining process uses a very specialized computer nu- merical controlled (CNC) milling machine to remove very small amounts of material at a time. The micro-machining process is shown below (Figure 3). These three processes for creating step stencils result in different textures within the stepped area. The textures of the step stencils are shown below (Figure 4). Experimental Methodology A step stencil design was created with step- down pockets of varying thicknesses. The base stencil thickness was 4.0 mils (101.6 microns) and the step-down pockets were 3.5 mils (88.9 microns), 3.0 mils (76.2 microns), 2.5 mils (63.5 microns), and 2.0 mils (50.8 microns) thick. Each step area was 1-inch square (25.4 mm) and the step design is shown below (Figure 5). The thicknesses of each step pocket were measured using a FARO arm device. The mea- surements for each step technology were com- pared and contrasted. An aperture pattern was created for the fol- lowing components: 03015 metric, 01005, 0.3 mm BGA, 0.4 mm BGA, and 0.5 mm pitch QFNs. Apertures for each component were cut at varying distances from the step edges; 10, 20, 30, 40, and 50 mils. The intention was to deter- mine how close solder paste could be printed to the step edge for each step stencil technology. Figure 2: The laser welding process to create a step stencil. Figure 3: The micro-machining process to create a step stencil. Figure 4: Textures of step areas for the three step technologies. STEP STENCIL TECHNOLOGIES AND THEIR EFFECT ON THE SMT PRINTING PROCESS

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