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42 SMT Magazine • April 2017 of the samples showed significant non-wet- ting. Since the image analyzer could not pro- cess those images properly for voiding, this data is not presented in the graph. 6. Effect of Flux Quantity and Alloy on Voiding: Figure 10 shows the two opposite trends of flux quantity that were observed. For SAC305 and 63Sn/37Pb, the voiding decreased with increasing flux quantity. This can be at- tributed to a better wetting caused by more flux [2,3,4,5] . For 57Bi/42Sn/1Ag, the flux quantity ef- fect showed an opposite trend. The higher void- ing caused by the higher flux quantity could be explained by the flux burn-off rate outweighed the wetting improvement rate. Since the flux used in this study was developed mainly for the SAC alloy system, it is reasonable to expect that the flux was much less effective at a low solder- ing temperature. 7. Effect of Reflow Temperature and Alloy on Voiding: Figure 11 shows that, de- pending on the alloy type, a totally opposite trend could occur. For the low-temperature al- loy 57Bi/42Sn/1Ag, with and increasing peak temperature, the voiding first increased, then decreased. The initial increase trend can be at- tributed to the low activity of flux, and the va- porization factor dominated the voiding behav- ior. At this temperature range, the flux activi- ty was low and the more volatile constituents came out. The subsequent decrease trend at 170°C to 180°C peak temperature can be attributed to the flux wetting factor, which increased con- siderably from 170°C to 180°C. This flux wet- ting dominating behavior was also observed on 63Sn/37Pb when the reflow temperature in- creased from 205°C to 215°C. However, when the reflow temperature further increased from 215°C to 225°C, the voiding increased again. This was attributed to the dominating flux out- gassing phenomenon, presumably through VOIDING CONTROL AT HIGH-POWER DIE-ATTACH PREFORM SOLDERING Figure 10: Effect of flux quantity and alloy on voiding. Figure 11: Effect of reflow peak temperature and alloy on voiding. Figure 9: Effect of oxidation and alloy on voiding.