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40 SMT Magazine • April 2017 Figure 8 results indicate that the actual vol- ume of voiding decreased with decreasing BLT. Under the 100-gram weight condition, not only did the liquid solder squeeze out, but the voids formed were not able to expand due to the constrained lamellar flow of liquid solder. Furthermore, the voids formed could not move around, and coalescence of voids became neg- ligible. The X-ray images of 100-gram column in Figure 1 show that no large void image can be observed. In summary, the weight on the sandwich dic- tated BLT, which in turn governed the void con- tent. At a very low BLT, the actual void volume was low, although the void area % was not low. With increasing BLT, the void volume increased, but the voiding area % decreased slightly. Since a very thin BLT is not desired based on reliability considerations, the study will be confined to weight no higher than 30 grams from this point on for comparison of SAC305, 63Sn/37Pb, and 57Bi/42Sn/1Ag. Also, with a small sample size prepared for each condition, data scattering was fairly significant. In order to get a meaningful trend, the effect of any giv- en parameter was examined by taking the av- erage value of all data involving other parame- ters. For instance, when examining the effect of oxidation on voiding, all data points were de- rived from the average of all flux quantities re- flow peak temperatures, and weights. 5. Effect of Oxidation and Alloy on Voiding: Figure 9 shows the voiding per- formances of SAC305, 63Sn/37Pb, and 57Bi/42Sn/1Ag on Cu coupons preconditioned with various extents of oxidation. The data for each point was the average of all condi- tions, including flux quantity, reflow tempera- ture, and weight. Voiding of SAC305 was high- er than 57Bi/42Sn/1Ag, which in turn was high- er than 63Sn/37Pb. Initially, an insensitivity of voiding toward oxidation was observed, indi- cating that the oxide formed was easily remov- able. At 60 seconds of oxidation time, the void- ing increased noticeably, reflecting that the ox- ide amount just reached the limit of fluxing ca- pability. At five minutes oxidation time, most VOIDING CONTROL AT HIGH-POWER DIE-ATTACH PREFORM SOLDERING Figure 7: Relationship between BLT and voiding for SAC305. Figure 8: Relationship between voiding volume and BLT. Table 2.