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54 SMT Magazine • December 2017 sitive to changes in aspect ratio. In general, the rectangles with the east-west orientation typi- cally achieve the highest transfer efficiency, es- pecially at low area ratios. In comparison to the other stencil thick- nesses the course of the graphs show some de- viation especially concerning the effect of the orientation. One reason might be the difference on the absolute size of the aperture and the dif- ferent shapes of the aperture depending on the stencil thickness. Thereby the 120 µm stencils occupies the biggest apertures with distinctive differences in their shapes. The course of the standard deviation has all the same remarkable points in common. The trends in Figure 9 show a decrease of the stan- dard deviation starting at the square and follow- ing the east-west orientation before they under- go a rise in columns 11 and 12 for area ratios of 0.55 and above. Area ratios show an ongoing decrease also for apertures with lowest values of the standard deviation in column 11 and 12. In contrast the vertical apertures in north-south orientation have a higher standard deviation than a square. Overall the course of the graphs for the 120 µm stencil thickness is not consistent and no gen- eral trend can be observed. Discussion The results of the experiments comparing the aperture form show the influence on the stencil printing process especially for small area ratios below 0.6. The upcoming discussion in- troduces some ideas for the integration of the results in present standard rules. The first insight of the experiments shows that a rectangle shape leads to better results than the square. Combined with the analy- sis of the influence of the stencil thickness it is known that the effect depends on the size of the aperture. All stencils have the same area ratio, which also means that the absolute size of each aperture is not identical. Combining the aspect ratio with the transfer efficiency, a higher as- pect ratio might lead to higher transfer efficien- cy. Transferring this point to the orientation of the aperture evidence of printing improvement under test conditions can be found where the wide side of the aperture faces the squeegee. Continuing with the aperture orientation analysis it was found out that there are differ- ences between the performance of the east-west and the north-south directions. This leads to the idea to extend the common calculation of the area ratio. The analyzed data has shown the orientation and form of the aperture influenc- es the printing results, particularly at small area ratios where unexpectedly high average paste transfer occurred. To include this thesis in the area ratio calculation a levelling factor (short: NF) will be introduced. Under the present condition it raises the real area ratio to a higher level by including the form and orientation of the aperture. There- by a new area ratio is calculated, which could serve as the new decisive area ratio. To devel- op the levelling factor the slope of the average improvement of the transfer efficiency was cal- culated. For this experiment a nearly linear co- herence can be determined. The idea is to use the determined slope, which was calculated on a transfer efficiency base and transfer it to calcu- late the area ratio. The slope will be multiplied with the difference of the length of the smaller Figure 9: Influence of the aperture shape and the orientation for the 120 µm stencil. THE EFFECT OF AREA SHAPE AND AREA RATIO ON SOLDER PASTE PRINTING PERFORMANCE