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February 2014 • SMT Magazine 21 FEATUrE Figure 12: Effects of stencil alloy and coating on bgA volume repeatability. Figure 14: Effect of microbgA aperture shape on transfer efficiency. Figure 15: Effect of aperture shape on volume repeatability. Figure 13: Effects of stencil alloy, coating and com- ponent orientation on 0201 volume repeatability. experiment #3: microbGa aperture Shape To compare the influence of microBGA ap- erture shapes on print quality, two stencils were produced with identical aperture geometries for all devices except the BGAs. One stencil had specified 10.8 mil circles; the other specified 10.8 mil squares with radiused corners. 1) Print Yields Print yields for both stencils were 70%. They are not depicted graphically. 2) Transfer Efficiency Figure 14 shows the transfer efficiency for both aperture shapes. The square aperture has a higher percentage of paste transfer; it also has a higher volume of paste due to its geometry. The average paste volume deposited from the square aperture is approximately 358mil 3 , whereas the average paste volume deposited by the round apertures was 298mil 3 . The square aperture de- posits an average of 22% more solder paste than the round one. 3) Variation The square aperture design also provided better print volume consistency than the round design, as shown in Figure 15. FINe-TuNING THe STeNCIL MaNuFaCTurING PrOCeSS continues