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February 2014 • SMT Magazine 59 bIG IDeaS ON MINIaTurISaTION continues FEATUrE ProActiv with type 4 solder paste, a printed vol- ume of 0.61 nanoliters was achieved with a cir- cular aperture design and 1.01 nanoliters with a square aperture. By using type 4.5 solder paste the maximum volume printed was increased to 1.11 nanoliters. This data highlights how ProActiv, together with considered choice of square or circular apertures and type 4/4.5 sol- der paste, can push the lower limits of the print- ing process. Whilst not as extreme, but nonetheless just as significant, the same trends can be seen in the data with aperture area ratios of 0.40 and 0.46. With an area ratio of 0.40 the volume of paste printed ranged from 0.57 to 1.81 nano- liters, depending on process condition. When working towards a specific volume, for exam- ple 1.8 nanoliters (which could become typical for an ultra-fine-pitch component), then the significance of aperture design and process choices becomes apparent. Careful selection will have to be exercised to ensure a process can be delivered. The transfer efficiency and volume data dis- cussed thus far is helpful in understanding the potential scope for future surface mount assem- bly. But for the full picture, it is also essential to understand the true capabilities and repeatabil- ity of a process associated to any material and process choice. In this respect, the scatter and standard de- viation of the experimental data collected was also considered. Focusing in again on aperture area ratios around the knee of the transfer ef- ficiency curve, Figure 7 plots every single data point collected for apertures with area ratios of 0.34, 0.40 and 0.46. Simplistically, this gives the engineer a great view of what is happening in a process. The outliers represent potential board level defects and the spread of the data gives an indication of how "in control" the process is. The charts clearly show the benefits of us- ing ProActiv with low, challenging area ratio apertures. This is exemplified by the data for the 162 μm (0.40AR) aperture. With a standard squeegee process the scatter of data was large (and low) indicating that the process conditions were not capable. By using ProActiv the data set was significantly tightened up with acceptable Table 4: Average volume deposited for various circular and square aperture area ratios; with/ without ProActiv (PA) and with type 4 (T4) and type 4.5 (T4.5) solder paste. Notes: Values have been rounded to 2 decimal places. 1 nanoliter = 1,000,000 cubic microns.