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Effect of Area Shape and Area Ratio on Solder Paste Printing Performance
Effect of Area Shape and Area Ratio on Solder Paste Printing Performance
The effect of aperture shape and orientation on solder paste printing performance is analyzed using stencil thicknesses, solder paste types and manufacturers.
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Authored By:
Stefan Harter, Jens Niemann, Jorg Franke
Institute for Factory Automation and Production Systems (FAPS),
Friedrich-Alexander-University Erlangen-Nurnberg (FAU)
Nurnberg, Germany

Jeff Schake, Mark Whitmore
ASM Assembly Systems
Suwanee, GA, USA; Weymouth, UK

Summary
One of the most important design criteria for solder paste printing stencils is area ratio (AR), which is defined by the IPC-7525 as the area of aperture opening to the area of the aperture walls. Accordingly, the use of values for AR of at least 0.66 and above provides good printing performance. It is becoming more frequent in mixed assembly designs, where stencil thickness cannot be reduced to accommodate new fine feature components, that resulting AR levels become critically low and difficult to print. As this motivates our investigation on the printing performance of low AR stencil aperture designs, our research further explores the influence of aperture shape effects that AR itself does not account for. Aperture shapes that vary quite significantly from one another may exhibit unique printability characteristics despite also having equivalent AR values.

In this study, the printing process for miniaturized structures is evaluated using specially designed area ratio test stencils. The design of these stencils includes values for the AR from 0.65 down to 0.45 with rectangular apertures as well as circular apertures for reference. Additionally, the orientation of the rectangular apertures in relation to the printing direction is regarded. To include the effect of area shape, a sufficient number of apertures are gradually adjusted in length and width while still resulting in the same AR. The stencils used are laser-cut stainless steel foils in thicknesses of 80 μm, 100 μm, and 120 μm. Solder paste materials in Type 4 and Type 5 lead-free SAC305 alloy are also evaluated. The investigations contribute to explain the challenges of solder paste printing for highly miniaturized components.

Conclusions
This paper discussed the effect of the aperture shape and orientation on solder paste printing performance. For the experiments three different stencil thicknesses (80 μm, 100 μm and 120 μm), two different solder paste types (Type 4 and 5) and two manufacturers were used.

For apertures with identical area ratios a strong influence is induced by a varying aspect ratio and the orientation of these structures to the printing direction. The results focusing on the transfer efficiency and the standard deviation show, that the shape of a rectangle and an east-west orientation achieve the best results, as illustrated in Figure 10. Furthermore the solder paste type can have an influence, especially when relatively narrow apertures are printed. Based on the boundary conditions of the PCB layout within the printing tests the thicker stencils have the higher aspect ratios and achieve better printing results.

In general the printing results show that also area ratios lower than 0.66 can be quite regularly printed above 75% transfer efficiency, with dependencies on the shape of the apertures. This leads to the idea to extend the current calculation of the area ratio by introduction of the leveling factor NF. Using NF for the calculations, the present assumed design limitations are extended to regard the orientation and shape of the stencil apertures.

Additional research has to be done to find proof for the ideas given in this paper. Different types of solder paste and stencil thicknesses extend the data basis and help to refine the first introduced modeling. This could also lead to nonlinear correlations that have to be included for more precise calculations. New materials as finer grained solder paste types, stencil technologies and stencil coatings shift the limits for obtaining a well controlled stencil printing process. Additionally, new calculations based on fundamental printing tests contribute to understand printing behavior of miniaturized structures to achieve a robust solder paste
printing performance.

Initially Published in the SMTA Proceedings

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