| Summary: | 博士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 101 === Microminiaturization of electronic products may lead solder joints to fusion failure during the electrification. Additionally, developing current-resistant lead-free solders is imperative for lead-free issues. In order to investigate how solders act in high-current densty occasions, Sn-based test specimens were used for this research, and it mainly investigated the relationship between electrification-induced microstructural change and electrification-fusion mechanism for pure Sn, Sn-based eutectic alloys (Sn-9Zn, Sn-3.5Ag, Sn-0.7Cu, Sn-3Ag-0.5Cu and Sn-37Pb) and Sn-xZn alloys (x = 7, 9, 20, 30, 40, 50, 60, 70, 80, 90, 100 wt.%).
It is difficult to confirm that whether the electrification-induced initial fusion trace of pure Sn emerges from grain boundary or grain interior. Due to the non-uniform electrical and thermal distribution, fusion traces of pure Sn are interconnected to form larger network-like fusion paths and tend to intersect the grain boundaries in a high-angle manner. Further fusion fracture will happen by sideward spreading of the fusion paths. On the other hand, the heating-induced fusion phenomenon of pure Sn also belongs to network-like fusion paths.
The commomality of electrification-induced fusion behavior in Sn-based eutectic alloys: The fusion region initially emerges from the eutectic phase, extends to the primary phase (-Sn), and then the mutual fusion regions will further interconnect and cause fusion fracture. However, volume fraction of eutectic phase and eutectic temperature (Teutectic) seems to have less effect on critical fusion current density (CFCD) than the fusion latent heat per unit of eutectic phase volume (△H+→L), electrical conductivity, and heat dissipation (or thermal consuctivity). Additionally, the electrical, thermal properties of Sn-9Zn are mostly better than those of the other Sn-based eutectic alloys, and those of Sn-37Pb are the worst. On the other hand, the fusion phenomenon does not occur on primary Zn-rich phases. The result of electrification-fusion test shows the descending CFCD order of Sn-based eutectic alloys is Sn-9Zn 〉 Sn-3.5Ag 〉 Sn-0.7Cu 〉 Sn-3Ag-0.5Cu 〉 Sn-37Pb. Thus, Sn-9Zn has the potential to be applied in hign-current density occasions.
A large area of network-like fusion paths in pure Sn can not be observed in the Sn-based eutectic alloys. Besides, the individual phases of Sn-based eutectic alloys have different electrical, thermal properties and volume fractions, making the fusion connecting form more various but the fusion regions have poorer connectivity than pure Sn. Therefore, most of Sn-based eutectic alloys (except for Sn-37Pb) have higher CFCD value than pure Sn.
The linear regression statistic of electrification-fusion test in Sn-xZn alloys shows CFCD values have good relationship among electrical conductivity, latent heat of eutectic region contained in per unit solder volume (△Hf) and volume fraction of eutectic phase. Fusion phenomenon does not take place in Primary Zn-rich phases during the electrification. Moreover, the dominant influence on electrification-fusion characteristics in Sn-xZn alloys mostly is Sn/Zn eutectic phase. When Zn content increases from 7 wt.% (hypoeutectic) to 9 wt.% (eutectic), dendritic primary -Sn gradually decreases and primary Zn-rich phase crystallizes. Since primary -Sn is also a fusion phase, making Sn-9Zn has higher CFCD than Sn-7Zn. When Zn content increases from 9 wt.% to 30, 70 wt.% (hypereutectic) and 100 wt.% (pure Zn), in order to fuse higher Zn content of Sn-xZn alloys, the fusion-free Zn-rich phase may presumably hinder the connecting path for Sn/Zn eutectic fusion phase from extending further. Thus, the CFCD of Sn-xZn alloys increases with increasing Zn content.
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