## 3 Results and Discussion  
### Microstructure Evolution of Interfacial IMCs  
Figure 2 shows the cross-sectional SEM images of the interfacial microstructure between Cu lead and Sn-37 Pb solder after reflowing and extreme temperature thermal shock. It could be found that a continuous IMC layer with scallop-type morphology was formed at the interface between Cu lead and Sn-37 Pb solder after reflowing, as can be seen in Figure 2a. The chemical compositions of the IMC layer between Cu lead and Sn-37 Pb solder was identified by EDS analysis, and the corresponding results is presented in Figure 3. It clearly illustrates that the IMC layer was (Cu, Ni)\({}_{6}\)Sn\({}_{5}\) layer. During reflowing, Cu atoms reacted with Sn atoms to form Cu\({}_{6}\)Sn\({}_{5}\) IMCs, Ni atoms diffused from the opposite Ni-P layer to the Cu lead/solder interface and substituted for Cu atoms in Cu\({}_{6}\)Sn\({}_{5}\) IMCs due to the similarity in crystal structure, which finally led to the transformation of binary Cu\({}_{6}\)Sn\({}_{5}\) compounds to ternary (Cu, Ni)\({}_{6}\)Sn\({}_{5}\) compounds when the Ni content in (Cu, Ni)\({}_{6}\)Sn\({}_{5}\) exceeded 2 at.% [12]. The thickness of interfacial (Cu, Ni)\({}_{6}\)Sn\({}_{5}\) IMCs was 1.23 \(\pm\) 0.17 \(\upmu\)m. During extreme temperature thermal shock, the interfacial IMC thickness obviously increased, and the morphology of interfacial IMCs transformed from scallop-type to plane-type. During extreme temperature thermal shock, Cu atoms of the Cu lead diffused through the interfacial IMCs and reacted with Sn atoms from the Sn-37 Pb solder, which resulted in the growth of interfacial IMCs. The shorter diffusion path for Cu atoms contribution to faster growth of IMCs at scallop valleys between the scallop peaks, which resulted in the transformation of IMC morphology from scallop-type to plane-type [13; 14]. As seen in Figure 2d, a very thin layer of Cu\({}_{3}\)Sn IMCs was formed between the Cu lead and (Cu, Ni)\({}_{6}\)Sn\({}_{5}\) IMC layer after 150 cycles. The formation of Cu\({}_{3}\)Sn between the Cu lead and (Cu, Ni)\({}_{6}\)Sn\({}_{5}\) IMC layer can be attributed to the limited diffusion of Ni from the opposite Ni-P layer. The thickness of the Cu\({}_{3}\)Sn IMC layer was 107 \(\pm\) 13 nm. The thickness of the Cu\({}_{3}\)Sn IMC layer changed little with the increase of thermal shock cycles.  
Figure 2: _Cont._  
Figure 4 displays the cross-sectional SEM images of the interfacial microstructure between Sn-37 Pb solder and Ni-P layer after reflowing and extreme temperature thermal shock. During reflowing, the Au layer was completely dissolved into the molten Sn-37 Pb solder due to the high dissolution rate of Au in molten Sn-based solder [15]. Thus, the Ni-P layer was entirely exposed to and reacted with the molten Sn-37 Pb solder. A needle-type IMC layer could be observed at the interface between Sn-37 Pb solder and Ni-P layer. EDS analysis shows that the IMC layer between the Sn-37 Pb solder and Ni-P layer was (Ni, Cu)\({}_{3}\)Sn\({}_{4}\) layer, as shown in Figure 5. The thickness of interfacial (Ni, Cu)\({}_{3}\)Sn\({}_{4}\) IMCs was 0.81 \(\pm\) 0.7 \(\upmu\)m. The thickness of (Ni, Cu)\({}_{3}\)Sn\({}_{4}\) IMC layer gradually increased and the IMC morphology changed from needle-type to scallop-type, and finally to plane-type during extreme temperature thermal shock.  
Figure 3: EDS analysis of the IMC layer between Cu lead and the Sn-37 Pb solder.  
Figure 2: SEM images of the Cu lead/Sn-37 Pb solder interface: (**a**) after reflowing; (**b**) after 50 cycles; (**c**) after 100 cycles; (**d**) after 150 cycles; (**e**) after 200 cycles; (**f**) after 250 cycles; and (**g**) after 300 cycles.  
Figure 4: SEM images of the Sn-37 Pb solder/Ni-P layer interface: (**a**) after reflowing; (**b**) after 50 cycles; (**c**) after 100 cycles; (**d**) after 150 cycles; (**e**) after 200 cycles; (**f**) after 250 cycles; and (**g**) after 300 cycles.  
Figure 5: EDS analysis of the IMC layer between the Sn-37 Pb solder and Ni-P layer.