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February 2014 • SMT Magazine 61 FEATUrE as a percent of the actual volume printed. It is generally accepted that if the percent stan- dard deviation is maintained below 10%, then a process is in control. As can be seen from the charts, this becomes a more discerning measure with decreasing area ratio. Generally, each experimental run held a 5% standard deviation with apertures hav- ing area ratio's down to 0.53, indicative of a stable process to that point. Below 0.53, then subtle differences were noted. Figure 9 homes in on the percent standard deviation data for aperture area ratios below 0.6. Comparing all experimental runs within the same chart, the curves fall into two distinct groups. The curves to the left were the result of using ProActiv. It can be seen that ProActiv enabled process per - cent of standard deviation to be kept below 10% with apertures having an area ratio down to 0.4. Under one specific condition, ProActiv with a square aperture design and type 4.5 solder paste, the process was capable down to an aperture area ratio of 0.34. In contrast the standard squeegee processes bottomed out with area ratios of approximately 0.46. In comparing aperture shape, the data in - dicated that lower area ratios designs can be used with square apertures (compared to cir- cular apertures) and still maintain a process with a percent standard deviation under 10%. From the data presented here, it is appar- ent that there are many interacting circum- stances to consider when designing a process, and these are assuming greater importance as we push into the realms of sub-0.5 area ra- tios. Ultimately, each component requires a defined amount of solder paste for good joint assembly so this will dictate requirements. However, the mechanism by which this is de- livered, as shown from the experimental data here, can vary depending upon aperture de- sign, solder paste material used and printing process utilised. Summary The next generation of ultra-fine-pitch com- ponents 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 bIG IDeaS ON MINIaTurISaTION continues Figure 8: Standard deviation charts.