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52 SMT Magazine • February 2014 parison has also been made between type 4 and type 4.5 solder paste as well. The data presented here will help provide guidelines for stencil aperture designs and strat- egies for ultra-fine pitch components such as 0.3CSPs. Introduction 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? 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) fundamen- tally dictates what can and cannot be achieved in a print process. For next-generation compo- nents and assembly processes these established rules need to be broken! New stencil printing techniques are becom- ing available which address some of these chal- lenges. Active squeegees have been shown to push area ratio limits to new boundaries, per- mitting printing for next-generation 0.3CSP technology. Results also indicate there are po- tential yield benefits for today's leading-edge components as well. An increasingly important part of the over- all equation that is often overlooked is stencil aperture shape and design. With shrinking area ratios, every cubic micron of solder paste that can be printed is becoming critical. For a given aperture area ratio a square aperture design pro- vides the opportunity to deposit 21.5% more than its circular counterpart. When working with sub-0.5 area ratio apertures this becomes very significant. The work reported here represents the start of a series of experiments to help further under- stand the significance of square vs circular aper- ture formats, together with the impact of other material factors, and to ultimately provide de- sign guidelines for ultra-fine pitch printing. Stencil Printing rules As illustrated in Figure 1, the area ratio cal- culation in its simplest form is a ratio between the aperture opening area and aperture wall surface; therefore the factors that make up an aperture, stencil thickness, aperture diameter, width and length all influence the resultant area ratio. With these two examples, we can ob- serve the effects. A stencil of 100 microns and a circular aperture of 240 microns would result in an area ratio of 0.6; by changing the diameter to 200, the area ratio would now be calculated FEATUrE bIG IDeaS ON MINIaTurISaTION continues Figure 1: Area ratio.

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