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Stencil Printing Process Guidelines for 0.3mm Pitch Chip Scale Packages
Keywords: advanced stencil printing, ultra fine pitch printing, 0.3CSP technology
The SMT print process is now very mature and well understood. However as consumers continually push for new electronic products, with increased functionality and smaller form factor, the boundaries of the whole assembly process are continually being challenged. Miniaturisation raises a number of issues for the stencil printing process. How small can we print? What are the tightest pitches? Can we print small deposits next to large for high mix technology assemblies? How closely can we place components for high density products? …And then on top of this, how can we satisfy some of the cost pressures through the whole supply chain and improve yield in the production process! Today we are operating close to the limits of the stencil printing process. The area ratio rule (the relationship between stencil aperture opening and aperture surface area) fundamentally dictates what can and cannot be achieved in a print process. For next generation components and assembly processes these established rules need to be broken! New stencil printing techniques are becoming available which address some of these challenges. Active squeegees have been shown to push area ratio limits to new boundaries, permitting printing for next generation 0.3CSP technology. Results also indicate there are potential yield benefits for today’s leading edge components aswell. An increasingly important part of the overall equation that is often overlooked is stencil aperture design. With shrinking area ratio’s, every cubic micron of solder paste that can be printed is becoming critical. For a given aperture area ratio a square aperture design provides the opportunity to deposit 21.5% more than its circular counterpart. When working with sub 0.5 area ratio apertures then this becomes very significant. 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 0.10 thru to to 1.37. This covers apertures sizes of between 100 and 550 microns in a 100micron 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. This data has then been used to look at specific details for a 0.3CSP test vehicle. The latest results will be presented and used to provide recommendations for stencil aperture designs and strategies for ultra-fine pitch components with specific reference to 0.3CSP’s. This work builds on information first presented at IMAPS 2011, apaper which was voted best in session at that time.
Mark Whitmore, Future Technologies Manager
DEK Printing Machines Ltd
Weymouth, Dorset
United Kingdom

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