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70 SMT Magazine • September 2014 cil to get side walls that are smooth enough for proper solder transfer, and therefore volume. 2. Poor aperture design with area ratios that are too small for the stencil technology chosen Area ratio is a good tool to predict solder paste transfer. The area ratio is the area of the aperture opening divided by the area of the in- side aperture wall. In the solder paste printing process, the squeegee blade delivers solder paste into the stencil aperture as it travels across the stencil surface. When the board separates from the stencil, the solder paste in the stencil en- counters a competing process: solder paste will either transfer to the pad on the PCB or it will stick to the inside aperture walls. The smaller the area ratio, the more difficult it is to achieve complete paste release. However, with the in- troduction of electroformed stencils, it was discovered that the smoother walls associated with this stencil technology provided better paste transfer, and good paste transfer could be achieved at lower area ratios. In fact, good paste transfer has been achieved with area ratios as low as .50 with electroformed stencils. If you're not getting enough paste transfer to give you the solder paste volume you need and well-formed bricks, check to see that the aperture size you are using is appropriate for the stencil fabrica- tion method and verify the area ratio suitability for the stencil type being used. 3. Insufficient aperture paste fill caused by squeegee speed, squeegee pressure, a poor squeegee blade, inadequate paste roll, or poor or outdated solder paste The quality, condition, and use parameters for squeegee blades play a huge role in suc- cessful paste transfer. A quality blade in top condition is a must. Additionally, the quality and condition of the paste cannot be underes- timated when investigating potential problem areas. The ShorT SCooP figure 2: Area ratio explained. ImprOvING SteNCIL prINtING reSULtS continues

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