Reliability Study of Low Silver Alloy Solder Pastes

Estimated reading time: 17 minutes

Microstructure of various alloys after thermal cycle testing was analysed with SEM equipment. Within SAC305 solder joints, blocky Cu6Sn5 particles formed and Ag3Sn particles had begun to elongate (Figure 16). Some inter-granular cracking was visible at the boundaries between Sn dendrites as well as between dendrites and intermetallic species. Cracking along the intermetallic layers was also observed.

Figure 16: Ag3Sn plate formation in QFP solder joint assembled using SAC305 paste after thermal cycle testing.

Using SAC0307 solder, the number and size of the Ag3Sn grains was reduced. Unlike SAC305, thermal cycling did not result in the formation of large Ag3Sn plates (Figure 17). However, dislocations along grain boundaries were visible, and more extensive than for SAC305, indicating that the coarsening of Sn dendrites does increase their tendency to crack.

Figure 17: Microstructure of QFN88 solder joint assembled using SAC0307 paste after thermal cycle testing.

Material C behaved in a similar fashion to SAC0307, showing extensive cracking along grain boundaries in almost all samples. The BGA solder joints showed a reduction in the growth rate of Ag3Sn particles compared to SAC0307. In the lower half of the Material C BGA solder joints, larger particles of Cu6Sn5 were observed (Figure 18).

Figure 18: Microstructure of QFN88 solder joint assembled using Material C solder paste after thermal cycle test.

The layer of Cu3Sn adjacent to the PCB pad was thinnest for solder joints composed of SnCuNi solder, suggesting that the insertion of Ni into the intermetallic layers reduced the rate of diffusion within the crystal structure (Figure 19).

Figure 19: Microstructure of QFN88 solder joint assembled using SnCuNi solder paste after thermal cycle test.

Relatively little coarsening of the microstructure of Material D solder joints occurred after thermal cycle testing. Sn dendrites and Cu6Sn5 were visible (Figure 20). There are several possible explanations for how little coarsening was observed. Firstly, Co has previously been shown to substitute for Cu in Cu6Sn5 grains, introducing a substitutional defect which inhibits coarsening [6]. The presence of a low concentration of Bi is known to have a Zener pinning effect on Sn grains, which increases the energy barrier to grain growth [7]. Cracking of the BGAs largely occurred at the bottom of the solder joint, as opposed to at the top. Under strain, the more brittle solution-hardened Sn matrix formed by Material D was more likely to crack than the more ductile Sn matrix at the top of the solder joint, which had a composition close to that of SAC305.

Figure 20: Microstructure of QFN88 solder joint assembled using Material D solder paste after thermal cycle test.

Figure 21 shows the typical microstructure of SnBiAg after thermal cycling. By far the most growth in the intermetallic layers occurred in the SnBiAg solder joints, some of which were more than doubled in thickness during thermal cycle testing. The thickness of the intermetallic layers became comparable to those formed by the other solder alloys, suggesting that the thin layers observed prior to thermal cycle test were at least in part a result of the low reflow temperature. The concentration of Sn in the solder appeared to have little effect on the intermetallic layer thickness. A Cu3Sn layer formed, similar to in thickness to that of the SnAgCu alloys. Coarsening of the Sn and Bi dendrites occurred. Both Ag3Sn and Cu6Sn5 particles increased in size. Cu6Sn5 particles, found close to the Cu solder pads, were also more frequent, indicating further dissolution of Cu from the solder pads into the bulk of the solder occurred during thermal cycle testing.

Figure 21: Microstructure of QFN88 solder joint assembled using SnBiAg solder paste after thermal cycling.

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