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May 2015 • SMT Magazine 35 SOLDER PaSTE PRINTING: QUaLITy aSSURaNCE mETHODOLOGy continues to be met in order to get an acceptable process result: 1. Aperture adjusted according to the component and to the pad 2. Aperture adjusted according to paste 3. Aspect ratio larger than 1.5 4. Area ratio larger than 0.66 All of the above conditions generally need to be fulfilled to get both good solder paste re- lease and a correct soldering result. The aspect ratio is the relationship of the width of the aperture divided by the thickness of the stencil. If it is smaller than 1.5 the trac- tion force from the stencil aperture walls will be too large and the paste will not release com- pletely. Consequently, there will be less paste on the board land than intended and the solder result will not be acceptable. The same kind of consequence will also occur if the area ratio of the aperture is too small. It is possible to calcu- late the area and aspect ratio by utilizing the following formulas: Feature Table 1: aperture sizes, area ratio and pitch between the pads within the test pattern.