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84 SMT007 MAGAZINE I JULY 2022 Later, both Sue and Andy felt the first class was easy and Sue suggested they get some ice cream then discuss what they learned in Chuck Tower's stencil printing class. "I thought the stencil design discussion was quite straightforward," Sue began. Andy's heart sunk a bit. Everything was straightforward to Sue. But this was one of his strengths. "Well, for one thing, the 'Five Ball Rule' was clear," Andy began. "At least five 'balls' of the solder paste to fit within the width of the sten- cil aperture is easy to understand." "I found out we typically use solder pastes with Type 3 or Type 4 solder powder 1 so we can determine the range of solder ball sizes and the largest solder ball from the Solder Powder Type Chart," Sue commented. "And the aspect ratio, the width of the aper- ture divided by the height, being greater than 1.5, is now clear to me, but the area ratio..." Andy said. "I agree, it's a little more complex. It is the area of the aperture opening divided by the area of the sidewalls. To make the solder paste stick to the printed wiring board pads more than the side walls of the aperture, the ratio must be greater than 0.66," Sue explained. "You know, I think I finally get it," Andy said. "But can we derive what the area ratio is?" challenged Sue. Andy proceeded to draw a circular aperture. "Well, we know the area of the opening is pi times the radius squared," he said. Figure 1: The Five Ball Rule is important in stencil design. Figure 2: The Solder Powder Size Chart is helpful to determine if a stencil aperture obeys the Five Ball Rule.