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50 SMT Magazine • December 2017 cil cleaner was programmed to perform a wet/ vac/dry cycle after every fifth consecutive print. The complete DOE comprised 480 solder paste inspected boards. Effects of the Aperture Shape The main object in this experiment is the investigation whether the aperture form or shape has an influence on the printing results. In the first part, the general result will be taken into consideration with the analysis of all mea- sured data of the transfer efficiency. In further steps the result will be filtered by the criteria of the solder paste type and the stencil thickness. The form comparison is based on adjacent ap- ertures 2 vs. 3 vs. 4 etc. vs. 12. The shape can be analyzed using columns 1 vs. 7 vs. 13. Based on square apertures in columns 7, the orienta- tion is observed comparing 2 vs. 12, 3 vs. 11, etc. up to 6 vs. 8 with the slightest varying ap- erture form. Printing Results With about 8% higher transfer efficien- cies, square apertures perform better than the circular structures. Further analyses of the ba- sic forms indicate no significant difference be- tween square and diamond. In the following, the variations of the rectangular apertures will be discussed. Figure 4 shows the overall view on the transfer efficiency depending on the area ra- tio and the form. It includes the results of all printed boards. The form is visualized by the column number (Figure 2). In general, the figure proves the plausibility and the cor- rectness of the data, because the transfer ef- ficiency rises with increasing area ratios. Fur- thermore, the measured values show a clear tendency that the aperture form and orienta- tion have an influence. If the aperture form is taken into investigation the high aspect ra- tio east-west oriented rectangles exhibit high- est transfer efficiency (column 12). Beginning from the square (column 7) both orientations of rectangles perform better than the square, which seems to be nearly the minimum of ev- ery graph. In addition, the aspect ratio (the difference between the wide and small side of a rectangle), which is rising towards the ends of the graph, shows possible correlation with the transfer efficiency. Besides the aperture aspect ratio, the aper- ture orientation shows evidence to influence the process. The apertures representing columns 2-6 have north-south orientation (i.e., the small side of the rectangle faces the squeegee) while the columns 8-12 are east-west oriented (i.e., the wide side faces the squeegee). Especially the area ratios 0.45 and 0.5 show the clear tenden- cy that the aperture orientation has an influ- ence. The transfer efficiency of the 0.45 area ra- tios raises about 3.5% for north-south orienta- tion and about 8.8% for east-west oriented aper- tures compared to squares. This trend weakens with increasing area ratios but remains recog- nizable throughout all graphs. If the standard deviation is investigated the insights show that the east-west orientation also leads to a slightly higher standard deviation at low area ratios. For this parameter the squares show better results. The standard deviation for the area ratios 0.55 to 0.65 shows no significant differences and no trends can be observed. THE EFFECT OF AREA SHAPE AND AREA RATIO ON SOLDER PASTE PRINTING PERFORMANCE Figure 4: Influence of aperture shape and orientation based on transfer efficiency (a) and variation (b).