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July 2014 • SMT Magazine 81 MEETING FuTuRE STENCIl PRINTING CHAllENGES continues ARTiCLE 7 mil aperture is 0.4375. The transfer efficiency was near the target (100%) and improved as the powder type moved toward type 6. Type 5 ex- hibited the best results; a narrow distribution in volume percent and low standard deviation ex- cept after first pause. It is important to note that the significant drop in transfer efficiency after the 1-hour pause is not consistent with previ- ous experiments; this is attributed to a very low humidity (<10%) and had a dramatic effect on the first print after sitting on the stencil for one hour, for all solder pastes tested. Looking at the first chart in Figure 7, it can be seen that the mean transfer efficiency favors the no-clean flux. The water soluble flux in the Indium6.4 solder paste was more erratic, giving more insufficient, as well as excessive, solder paste deposits. The second chart reveals that most of this inconsistency occurs with the 6 mil aperture. This result is consistent with previous experience. The rosin/resin bases found in most no-clean fluxes increase the material's tacky properties (adhesive and cohesive attractions) of the solder pastes. When releasing from the stencil aperture, adhesion of the solder paste to the PCB pad helps pull the paste from the sten- cil aperture as the PCB is lowered from the sten- cil. This is especially true for the smaller pads, as there is very little surface area for the paste deposit to cling to. The cohesive forces or at- traction of the flux-coated powder particles to one another are also increased. These cohesive forces help the paste release from the stencil walls and retain shape of the stencil aperture after printing. Variability Gauge Variability Chart for Volume (%) Figure 8 shows stencil printing results for square vs. circular apertures. The results for 7 and 8mil square apertures are designated S07 and S08 respectively, and those for 7, 8, and 9 mil circular apertures are C07, C08, and C09 respectively. The 7 mil square aperture exhibits acceptable results with less than10% standard deviation and good volume percent aperture filling. There were, however, a couple of outli- ers after the one-hour pause. The 7 mil circular aperture exhibits erratic behavior as shown by its high standard deviation. The 8 mil circular is comparable to the 7 mil square with much smaller standard deviation and good volume percent aperture filling. Consistent with past experience for the same area ratio, the square aperture with radius corners produced deposits with more volume of solder paste—21% more in this test. Variability Gauge Variability Chart for Volume (%) Perhaps one of the more dramatic changes in transfer efficiency and standard deviation is noted for the same aperture size and shape, but with pads that differ in solder mask. Sol- der mask defined pads exhibit far better results than the non-solder mask defined or copper defined. This result is also consistent with pri- figure 7: Mean of volume percent by flux and flux/aperture interaction.