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18 SMT Magazine • February 2014 its must fall within their specified ranges for the print to be considered a pass. As little as one de- posit out-of-spec will cause the print to be a fail. The print yields are show in Figure 3. With the exception of parameter set 3, the treated stencils yielded 10–20% better than the untreat- ed ones. Additionally, parameter set 2 produced the highest yields. The treated stencil in param- eter set 3 yielded 0% due to a miscut aperture (Figure 4). 2) Transfer Efficiency Transfer efficiencies (TE) are the ratio of the volume of the measured deposit to the volume of the stencil aperture and are expressed as a percent, or, more simply put, the percentage of solder paste that releases from the aperture. The aperture volumes used in the calculations are computed based on the average measured ap- erture dimension and stencil thickness, not on their specifications. The data from parameter set 3 were not in- cluded in transfer efficiency or repeatability comparisons due to the miscut aperture. Most of the stencils transferred about 80% for BGAs (Figure 5) and 100% for 0201s. Fig- ure 6 shows the TE results for 0201s orient- ed at 0 degrees; similar results were found at 90° orientation (not shown). No significant difference in transfer efficiency was noted with the different cutting parameters, with the exception of the POR sample. The treated stencil that was cut at the POR parameters appears to have 100% TE for the BGAs and 120% for the 0201s. Years of baseline data in- dicate mean TEs of approximately 80% and 100% for the two device types, respectively. At the BGA's 0.65 area ratio, 100% TE is not realistic; neither is 120% for the 0201s. There- fore, special causes of the anomalous data were investigated. Positional inaccuracy, the most likely pos- sible cause of excess solder volumes, was in- FINe-TuNING THe STeNCIL MaNuFaCTurING PrOCeSS continues FEATUrE Figure 3: Effect of cutting parameters and nano- coating treatment on print yields. Figure 5: Effect of cutting parameters and coating on transfer efficiency. The higher the TE, the better. Figure 4: Miscut stencil aperture on stencil 4.