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34 SMT Magazine • May 2015 patterns have also been added around the board edges to evaluate how the printer performs in these areas. The test board has been designed to be able to handle prints of 300, 400 and 500 mm width. The central area of the test board has a pattern that can be utilized to evaluate a possible step-up area on the stencil. The stencil has a thickness of 127 µm and was made from fine-grain steel. Tests have been performed with both etched and laser cut stencil versions. Within each star arm, the blocks include squares with rounded corners, circles and ob- long apertures (Figure 4). The precise size of the apertures as well as on the PCB is given in Table 1 together with the area ratio and pitch. Each individual size is repeated five times before the next size block starts. Each type of aperture (e.g., square, circle, etc.) is rotated 180° in order to evaluate the size at different positions within the machine. Before continuing with the discussion of the reference boards and stencils, it is worth revisit- ing some of the basic rules for designing stencil apertures and how it relates to the pad area. The volume of solder paste that is optimal for a pad is mainly dependent on the type of component that is utilized. The deposited paste volume is theoretically given by the size of the aperture and the thickness of the stencil. As the board and stencil separates from each other during the cycle there will be a collection of different forces acting on the solder paste. Solder paste will either be transferred to the board pad or stick to the stencil aperture walls. Whatever happens is closely related to the following im- portant factors: • Board-stencil separation speed • Aperture area and aspect ratios • Aperture side wall geometry • Aperture side wall finish The board-stencil separation speed is set as a machine parameter and is not directly related to the stencil design. The other three, however, are directly related to the material, manufacturing methods and the design of the stencil apertures. In general, when designing a stencil aper- ture there is a number of conditions that have SOLDER PaSTE PRINTING: QUaLITy aSSURaNCE mETHODOLOGy continues Feature figure 3: Test pattern on a printed circuit board and its equivalent on the stencil. Figure 4: pattern within an arm of the star.