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In the SnBiAg solder joint microstructure, the majority of the solder joint consisted Bi and Sn dendrites (Figure 10f). Small grains of Ag3Sn were visible at grain boundaries, as were Cu6Sn5 particles near the solder pads. It was noticed that Bi dendrite boundary was not well defined after the reflow process.
Figure 10: SEM Images of QFN solder joint reflowed with various alloy solder pastes. a) SAC305; b) SAC0307; c) Material C; d) SnCuNi; e)Material D; f) SnBiAg.
Thermal Cycle Test Results
Results after 3000 Cycles
Table 3 summarizes the thermal cycle test results of different components reflowed using different lead-free alloy solder pastes.
Table 3: Thermal Cycle Test Results.
In general, SAC305 had a better thermal reliability than alternative low Ag alloy materials. The thermal reliability performance depended on the component types and the component design/material. Complete failure was observed for some BGA 196 components, BGA228 components, BGA97 components and resistor 2512 components after 3000 cycles (0°C to 100°C). Major cracks were seen at their solder joint. The cross section images of BGA196 solder joint after thermal cycling test are shown in Figure 11.
Figure 11: Cross section images of BGA196 components after 3000 thermal cycles. a) SAC305; b) SAC0307;c) Material C; d) SnCuNi; e) Material D; f) SnBiAg.
Most cracks happened at the component side of the package although some cracks were also observed at the PCB side. Solder joint reflowed with SnBiAg also had cracks at the interface of SAC305 solder ball and SnBiAg solder paste. The cross sections of 2512 resistors and QFN88 components after thermal cycle test were shown in Figure 12 and Figure 13, respectively. Severe cracking in the solder joint were seen for these components.
Figure 12: Cross section images of R2512 solder joint assembled using different Pb-free alloy solder pastes after 3000 thermal cycles a) SAC305; b) SAC0307; c) Material C; d) SnCuNi; e)Material D; f) SnBiAg.
Figure 13: Cross section images of QFN solder joint assembled using different Pb-free alloy solder pastes after thermal cycle testing. a) SAC305; b) SAC0307; c) Material C; d) SnCuNi; e) Material D; f) SnBiAg.
No failure was observed for some BGA components such as BGA1156 (1.0mm pitch, 35mmx35mm) and BGA64 component (0.8mm pitch, 8mm X 8mm) after thermal cycle testing. Only minor cracks were seen. The cross sections of BGA1156 component are shown in Figure 14. Minor cracking and no failure was also observed for QFP components in this study (Figure 15).
Figure 14: Cross section images of BGA1156 solder joint after thermal cycle testing. a) SAC305; b) SAC0307; c) Material C; d) SnCuNi; e) Material D; f) SnBiAg.
Figure 15: Cross section images of QFP208 solder joint assembled using different Pb-free alloy solder pastes after thermal cycle testing. a) SAC305; b) SAC0307; c) Material C; d) SnCuNi; e) Material D; f) SnBiAg.Page 3 of 5
