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February 2014 • SMT Magazine 19 vestigated first. The positional accuracy was found to be within 0.5 mil in both X and Y directions, so it was ruled out as a root cause. The investigation then turned to the bottom of the stencil, where numerous topographical features were observed (Figure 7). Small bits of metal fused to the bottom of the stencil appear to have separated the stencil from the PCB, preventing good gasketing. The origin of these features is unknown. Closer inspec - tion of stencil 4 indicates that similar features may be a contributor to the miscut aperture (Figure 4). 3) Volume Repeatability Print volume repeatability is measured by dividing the standard deviation of the print vol- ume readings by the mean of the readings, and is also known as the coefficient of variation. It is expressed here as a percentage. The effect of the cutting parameters and coatings on volume repeatability is shown in Figure 8. Parameter set 2, which offered highest yields in this test, also produced the high- est variation, which is undesirable. His- torical data indicates CVs of approximately 10%, which is the benchmark for the BGA device. Interestingly, the CV for stencil 1, the one with the metal projections on the bottom side, was the lowest of the test and slightly lower than the benchmark. The CVs for the 0201s were nearly identical to those FINe-TuNING THe STeNCIL MaNuFaCTurING PrOCeSS continues of the BGAs and are not shown. Regardless of cutting parameters, stencils treated with nanocoating consistently provided better print volume repeatability than those with- out. FEATUrE Figure 6: Effect of cutting parameters and coating on transfer efficiency of 0201s oriented at 0°. Figure 7: Topographical features found on the PCb side of stencil 1. Figure 8: Effect of cutting parameters and coating on print volume variation. The lower the CV, the better.