Stencil Aperture Design for Next Generation Ultra Fine Pitch Printing
The work reported here represents the start of a series of experiments to help further understand the significance of square vs circular aperture formats.
Production Floor
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Authored By:
Mark Whitmore, Jeff Schake & Clive Ashmore
DEK Printing Machines Ltd
11 Albany Road, Weymouth
Dorset, DT4 9TH, UK
Summary
Miniaturisation is pushing the stencil printing process. As features become smaller, solder paste transfer efficiency is becoming more critical.
In latest research work, actual paste deposit volumes and transfer efficiency have been monitored and compared for both square and round apertures with area ratio's ranging from 0.20 thru to 1.35. This covers apertures sizes of between 100 and 550 microns in a nominal 100 micron thick stencil foil. In addition, the effect of ultrasonically activated squeegees has been assessed as part of the same experiment. A further comparison has also been made between type 4 and type 4.5 solder paste aswell.
The data presented here will help provide guidelines for stencil aperture designs and strategies for ultra-fine pitch components such as 0.3CSP's.
Conclusions
The next generation of ultra fine pitch components will place extreme demands on the stencil printing process. The requirement for printing solder paste through stencil apertures with area ratios below 0.5 will become common place. The data presented here indicates that with judicial choice of stencil design and materials it will be possible for designers to work with aperture area ratios down to 0.4.
To optimise a process it is becoming increasingly important that an engineer has a good understanding of stencil aperture design specification, material properties and process options/aids available to him. The interactions between all of these facets is becoming more complex and critical to the successful implementation of a process.
Initially Published in the SMTA Proceedings
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Comments
In this paper you claim that the area of a square is 21.5% larger than an equal sized circle in several places. This is incorrect. The ratio of the area of a square with side d and circle with diameter d is simply: As=d^2, Ac=pi(r^2)=(pi/4)(d^2), therefore As/Ac=4/pi=1.273. The square's area is 27.3% larger than the circle of the same size.
Dennis Cote, Harding Instruments
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