## 3 Results and Discussion
### Mechanical Properties and Fractography  
To evaluate the effect of interfacial microstructure evolution on the mechanical property of Sn-37 Pb solder joints of QFP assemblies, pull test was carried out. Figure 8 shows the variation of the pull strength with extreme temperature thermal shock. For each condition, at least eight leads of QFPs were pulled to obtain the average value of the pull strength, which resulted in the error in the pull strength. The pull strength of the as-reflowed solder joint was 65.13 MPa. The pull strength declined almost linearly with thermal shock cycles. The pull strength was reduced about 31.54% after 300 cycles. [PERSON] et al. [23] proposed that the failure of solder joints could be defined as more than 50% decline in the solder joint strength. The reduction ratio of the pull strength was lower than 50% after 300 cycles. This indicates that the fatigue life of the solder joint was higher than 300 cycles. To verify the fracture behavior of Sn-37 Pb solder joints, the fracture surfaces were examined using SEM and EDS. Figure 9 shows SEM images of the fracture surfaces of Sn-37 Pb solder joints. The fracture surface of Sn-37 Pb solder joints after reflowing showed ductile dimple feature (Figure 9a), indicating that the fracture occurred within the solder matrix. The fracture morphologies of solder joints after 50 cycles and 100 cycles are similar to that of solder joints after reflowing, as can be seen in Figure 9b,c. When the interfacial IMCs both at the lead side and the Ni-P layer side were thin, the pull strength of the interfacial IMCs was higher than that of the solder matrix. As a result, fracture tended to occur within the solder matrix, and the pull strength of the solder joints was determined by the \begin{table}
\begin{tabular}{c c c} \hline
**Interfacial IMCs** & **Time Exponent \(n\)** & **Correlation Factor \(R^{2}\)** \\ \hline Interfacial IMCs at the lead side & 0.57 & 0.98 \\ Interfacial IMCs at the Ni-P layer side & 0.34 & 0.98 \\ \hline \end{tabular}
\end{table} 
Table 1: Time exponent of the interfacial IMCs. Figure 7: Ln plot of the growth of interfacial IMCs during extreme temperature thermal shock. solder matrix. CTE mismatch of different constituents in the assembly resulted in cyclic stress and strain in the assemblies, especially in the solder joints during extreme temperature thermal shock [24]. The accumulation of stress and strain induced thermal fatigue and degraded the mechanical properties of the solder matrix, and thus the pull strength of the solder joints declined with extreme temperature thermal shock, as shown in Figure 8.  
After 150 cycles, the fracture surface consisted of two different fracture morphologies: dimple and cleavage pattern, and revealing a mixed ductile-brittle fracture (Figure 9d,e). EDS analysis results indicated that the phases with cleavage pattern on the fracture surface were (Cu, Ni)\({}_{6}\)Sn\({}_{5}\) IMCs, as shown in Figure 10a. It is indicated that the crack occurred partially within the solder matrix and partially through the (Cu, Ni)\({}_{6}\)Sn\({}_{5}\) IMC layer. [PERSON] et al. [25] have pointed out that the formation of Ni\({}_{3}\)Sn\({}_{4}\) by solid-state reaction between Ni and Sn caused a 10.7% volume shrinkage, and the formation of Cu\({}_{6}\)Sn\({}_{5}\) by solid-state reaction between Cu and Sn resulted in a 5% volume shrinkage. The growth of interfacial IMCs both at the lead side and at the Ni-P layer side during extreme temperature thermal shock led to volume shrinkage and thus caused stress concentration near the interface. Thick IMC layer was prone to crack initiation due to its brittle nature. The interfacial IMCs grew thicker during extreme temperature thermal shock, and the thickness of (Cu, Ni)\({}_{6}\)Sn\({}_{5}\) IMC layer at the lead side was much higher than that of (Ni, Cu)\({}_{3}\)Sn\({}_{4}\) IMC layer at the Ni-P layer side.