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April 2017 • SMT Magazine 43 VOIDING CONTROL AT HIGH-POWER DIE-ATTACH PREFORM SOLDERING combined vaporization and thermal decompo- sition. The dominating outgassing behavior con- tinued on the SAC305 system. Here, the void- ing increased continuously with the increasing reflow temperature. The alternating dominant voiding mecha- nisms can be elucidated by examining the TGA data of the flux, as shown in Figure 12. At a tem- perature below 100°C (zone I), no volatiles came out. At 100-210°C (zone II), moderate weight loss occurred. Between 210°C and 300°C (zone III), rapid weight loss was observed. At temper- atures higher than 300°C (zone IV), the weight loss rate slowed down again. A cross-comparison of Figure 11 and 12 shows that the voiding increasing trend at tem- peratures above 210°C can be easily attributed to the rapid weight loss rate with the increasing temperature. Below 210°C, the weight loss rate is moderate. The turning point on the void - ing rate at 170°C can be explained by the flux being activated around 170°C. Below 170°C, the activity was very minute, and voiding in- creased by the flux vaporization rate was great- er than voiding reduced by improved wetting. Once the flux was activated, the flux activity increased with increasing temperature [6] , and the voiding decreased with increasing flux ac- tivity [7, 8] . The flux was activated above 170°C, thus the voiding reduced by improved wetting outweighed the voiding increased by increased outgassing. Discussion 1. Solderability Factor The voiding of SAC305 in the weight study was higher than that in other factors of the study. This was due to the difference in solder- ability of different sources of Cu coupons em- ployed for those studies. Figure 12: TGA thermograph of flux used in this study.