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February 2014 • SMT Magazine 57 bIG IDeaS ON MINIaTurISaTION continues dimensions used to manufacture the stencil. Both the top and bottom side of stencil aper- tures were measured (to take into account any taper), and the average of the two measure- ments were taken for volumetric calculations. Typically all apertures were undercut by 10– 15 microns. For larger aperture dimensions this is not necessarily a major issue. For example, a 550 circular aperture has a volume of 23.76 nanoliters. The corresponding cut aperture was measured at 540 microns in diameter, with a volume of 22.94 nanoliters, 3.42% less than with its intended Gerber dimensions. For smaller aperture sizes, though, such a differential can have a significant impact on the volume of the aperture and its area ratio. This in turn can affect transfer efficiency and the final volume of paste deposited. With the nominal 250-micron apertures, the percent of difference in measured volume compared to Gerber di- mensions was approximately 10%. The differ- ential increased to 17% with apertures based on a target size of 150 microns. This highlights the increased importance of stencil design and stencil manufacturing accu- racy when considering ultra fine pitch stencils (with aperture area ratios below 0.5). It can be the difference between a successful process and a failure! The benefits of using ProActiv ultrasonic squeegees have been reported by these authors before 13,14,15 . In this study similar trends were observed. Table 3 provides the average paste transfer efficiency for both circular and square apertures for each of the conditions tested— with/without ProActiv, and with type 4 and type 4.5 solder paste. Each data point represents the average of 1080 measurements made over the 30 board print run. For aperture area ratios below 0.5, the use of ProActiv affords an increase in solder paste trans- fer efficiency over a standard squeegee process. Effectively, the knee of the transfer efficiency curve is kicked out, resulting in the opportunity to work with aperture area ratios down to 0.4, whilst still maintaining paste transfer efficiency above 60%. With regards to solder paste particle size, the data indicates only small differences (in trans- FEATUrE Table 2: Stencil aperture measurements and volume calculations for circular and square apertures compared to gerber theoretical values. Notes: Gerber stencil thickness used in calculations = 100 µm. Measured stencil thickness used in calculations = 101.6 µm. 1 nanoliter = 1,000,000 cubic microns.