# Tin Whisker Growth Inhibition in RE-Doped Sn-Zn Soldered Joints
## 3 Results and Discussion  
In the previous research of solders doped with rare earth, the excess of rare earth addition would cause the formation of mass RE-phase. Therefore, the oxidation of these RE-phase was considered as the main reason which induced the tin whisker growth in the solder matrix [12; 13; 14]. However, there are various situations of tin whisker growth in soldered joints as shown in Figure 1. It can be concluded that the tin whisker sprouted out from RE-phase near the interface layer in Figure 1a was similar to the whisker growth in the solder matrix, moreover, Figure 1b,c demonstrated the whisker growth without the participation of RE-phase in Nd-doped soldered joint. The EDS results in Figure 1d,e have shown that the compositions of the microstructure near the tin whisker root were Cu-Zn-Sn intermetallic compounds (IMC), which indicated that the tin whisker directly sprouted out from the twisted interfacial intermetallic compound layer. The tin whisker growth near the interface of soldered joint is greatly affected by the formation and conversion of interfacial IMC layer [15]. The diffusion between atoms caused stress to accumulate during the aging process while the stress further accelerated the diffusion reaction. Moreover, the volume change caused by the growth of IMC layer was blocked by the solder matrix, which also caused stress to accumulate in the corresponding grain boundaries. When the stress reached a certain level, voids, and fractures appear in the IMC layer which aggravate the stress concentration and finally induced tin whisker growth. Considering about the reliability of the soldered joint, besides the formation of RE-phase which may induce the tin whisker growth in RE-doped soldered joints, the growth of IMC layer should also be well controlled. The compositions of IMC layer in Sn-Zn series soldered joint could be determined as Cu\({}_{5}\)Zn\({}_{8}\) after soldering [16]. The thickness of IMC layer increased with aging time, however, an IMC layer which was too thick would deteriorate the mechanical properties of the soldered joint since the IMC is hard and brittle [17]. It was reported that Sn-Zn series solder possess favourable mechanical properties compared with Cu\({}_{6}\)Sn\({}_{5}\)/Cu\({}_{3}\)Sn IMC layer in Sn-Ag-Cu and Sn-Cu series solder [18]. Figure 2 showed the morphology of IMC layer in Sn-Zn-Ga-Nd solder at different aging time, the results indicated that in Figure 1(a),b the thickness of IMC layer increased with aging time, while in Figure 1(c),d the compositions of the IMC layer transformed into a mixture of Cu-Sn-Zn compounds. Meanwhile, the morphology of IMC layer was greatly affected by the discrepant atom distribution and diffusion rate. At the early aging process, the diffusion between Zn and Cu atoms is the leading factor of diffusion reaction, thus the thickness of Cu\({}_{5}\)Zn\({}_{8}\) layer is increased with aging time. After an extended period, kirkendall voids appears in the Cu\({}_{5}\)Zn\({}_{8}\) layer as a result of the consumption of Zn atoms. Moreover, the leading factor of diffusion reaction change to the diffusion between Cu and Sn atoms owing to the depletion of Zn atoms near the interface, thus the IMC layer transform into Cu-Sn-Zn compounds as Figure 1(e) shows. Besides, the IMC layer in Sn-9 Zn-0.5 Ga-1 Nd soldered joint couldn't be maintained uniformly and showed a stratification pattern. From the EDS results in the IMC layer as shown in Figure 1(f), it can be inferred that the compositions of the IMC layer was approximate Cu\({}_{5}\)Zn\({}_{8}\)/Cu\({}_{6}\)Sn\({}_{5}\) near the solder side, while the compositions of the IMC layer was approximate Cu\({}_{6}\)Sn\({}_{5}\)/Cu\({}_{3}\)Sn near the Cu substrate side. In addition, there were cracks of Cu\({}_{5}\)Zn\({}_{8}\) IMC found near the solder side during the aging process as shown in Figure 2(a). The formation of these cracks in solder matrix were caused by the discrepant distribution between Cu and Zn atoms.