Reliability Study of Low Silver Alloy Solder Pastes

Estimated reading time: 17 minutes

Figure 4: High-temperature Pb-free reflow profile.

Figure 5: Low-temperature Pb-free reflow profile.

The thermal cycle testing was performed in an air-to-air thermal cycle chamber, from 0 to 100 °C with 10-minute dwell time at each peak temperature, and a temperature ramp rate of approximately 10 °C per minute. The chamber profile of temperature versus elapsed time is shown in Figure 6.

Figure 6: Thermal Cycle Temperature Profile – 0°C to 100°C.

The thermal cycle test was terminated after 3000 cycles. The resistance of the all components were measured before and after thermal cycle test for failure. Cross sections were performed for the samples before and after the thermal cycle testing.

Results and Discussions

Microstructure Analysis

Intermetallic Layer Thickness

The intermetallic layer thickness was measured for the solder joints assembled with various lead-free alloy solder pastes. The intermetallic layer thickness at the PCB side after the reflow process was between 2 μm to 2.5 μm for SAC 305 material and alternative lead-free high melting temperature alloys. The intermetallic layer thickness at the PCB side after the reflow process for SnBiAg was less than 1 μm. The thin layer formed when using this alloy was due to the low-temperature reflow profile used. It was noticed that during thermal cycle testing the IMC layer of SnBiAg grew significantly to around 2μm, which was similar to the intermetallic layer thickness of other tested lead-free alloys. There was no significant change in the IMC thickness for SAC 305 and alternative low/no silver high temperature alloys after thermal cycle testing. The thicknesses of the intermetallic layer at the PCB side after reflow and after the thermal cycle test were shown in Figure 7.

Figure 7: Intermetallic layer thickness measurements at the PCB side after reflow and after thermal cycle testing.

The intermetallic layer thickness at the component side after the reflow process was around 1.5 μm to 2 μm. It was slightly thinner than the IMC thickness at the PCB side. Similarly, the IMC thickness at the component side for alternative low silver high temperature alloys after thermal cycle testing was not changed significantly. The IMC thickness of low temperature SnBiAg was thin (~1.2 μm) after the reflow process. This thickness increased to around 1.7 μm after the thermal cycle testing. The thickness of the intermetallic layer at the component side was shown in Figure 8.

Figure 8: Intermetallic layer thickness at component interface after reflow and after thermal testing.

Microstructure Analysis after Reflow Process

The cross-sections of all components were performed after the reflow process. In general, good solder joints were observed for alternative alloys. Incomplete mixing was seen for the most BGA components reflowed with the SnBiAg solder paste. The inhomogeneous solder joint was observed due to the low temperature profile used for SnBiAg solder paste. Head in pillow (HiP) solder joint was also seen for the large components such as BGA1156 reflowed with SnBiAg solder paste. The cross section images of BGA solder joint reflowed using alternative alloy solder pastes are shown in Figure 9.

Figure 9: Cross Section Images of BGA 196 components after Reflow Process.

The solder joint microstructure of the alloys were further analysed using a scanning electronic microscope (SEM). Cu6Sn5 intermetallic layer was the common feature for all alloy solder joints. For SAC305 solder joint, Sn IMC were mostly seen in the solder joint, along with Ag3Sn and Cu6Sn5 intermetallic species (Figure 10a). Areas of both the Sn/Ag3Sn binary eutectic and the Sn/Ag3Sn/Cu6Sn5 ternary eutectic were visible at grain boundaries of the Sn dendrites. The presence of the Sn/Ag3Sn binary eutectic suggesting that Cu6Sn5 was the last phase to solidify. In samples reflow with SAC0307 solder paste, the reduction in Ag content of SAC0307 solder paste resulted in a reduction in the number of Ag3Sn particles in the solder joint microstructure, accompanied by the formation of larger Sn dendrites (Figure 10b). Large areas of Ag3Sn/Sn binary eutectic were not visible. The microstructure of Material C solder joint after reflow showed that Cu6Sn5 particles in the bulk of the solder appeared smaller in Material C than in SAC0307 (Figure 10c). However, compared to SAC0307, the doped alloy did not show any statistically significant difference in the composition or thickness of the intermetallic layers. The BGA solder joints of both alloys were also similar, closely resembling those of the all-SAC305 assembly, though in both cases there was a slight reduction in the size of the Ag3Sn particles.

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