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58 SMT Magazine • February 2014 bIG IDeaS ON MINIaTurISaTION continues fer efficiency) between using type 4 and type 4.5 solder paste. With low area ratios, the net gain of using type 4.5 solder paste was an extra 2–3% in transfer efficiency. Please bear in mind though, that only one solder paste formulation was tested and other type 4/4.5 solder pastes might behave differently. Whilst transfer efficiency data is a good reference point for the effectiveness of a pro- cess, a solder joint ultimately requires a certain amount of solder for a good connection; there- fore actual volume is a more critical and useful measurement. Table 4 provides the average volume of sol- der paste printed for both circular and square apertures for each of the conditions tested: with and without ProActiv, and with type 4 and type 4.5 solder paste. Again, each data point repre- sents the average of 1080 measurements made over a 30-board print run. The trends seen in the transfer efficiency analysis (with respect to ProActiv vs. standard squeegee printing and type 4 vs. type 4.5 solder pastes) are still prevalent in the volume data. However, the volume data serves to highlight the difference in printed volume between circu- lar and square apertures. A square aperture of a given size has a vol- ume that is 21.5% greater than its circular coun- terpart. Generally, for apertures with area ratios between 0.44 and 1.00, this trend was observed in all print tests irrespective of ProActiv/squee- gee and type 4/type 4.5 comparisons. With apertures having area ratios above ~1.00, then the volume increase in paste deposited with a square rose to 29%. This implies that the fill- ing and release dynamics are different with ap- ertures over a certain area ratio, although the exact mechanism was not investigated further. With apertures having area ratios below 0.40, the differences in actual volume printed became even more significant. The bar charts in Figure 6 detail the paste volume printed (for all experimental conditions) with circular and square apertures having critical area ratios based around the knee of the paste transfer ef- ficiency curve. At the extreme, with aperture area ratios of 0.34, virtually no solder paste was printed with a standard squeegee process, either through a circular or square aperture. By using FEATUrE Table 3: Average paste transfer efficiency 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.