What is the effect of silver (Ag) alloy in solder paste? What is the differences to expect for solder paste with SAC305 and SAC4

1)Though in many cases the difference between SAC305 and SAC405 was small,and not always statistically significant, it was observed that SAC405 generally outperformed SAC305 joints. Under 0°C to 100°C thermal cycling,no statistically significant difference between SAC305 and SAC405 was observed at the N1% failure level. Under harsh environment testing, SAC405 joints were significantly better than SAC305 joints,lasting 1.7X longer at the N1% level.Mixed joints based on SAC405 showed very few failures,and had a much lower extent of cracking than mixed joints based on SAC305. Due to the low level of failures in some of the mixed assembly cells, it was not possible to determine if they were statistically different, although the difference appears marked.It is important to note that in each case, the failure mode for SAC305 and SAC405 joints was the same

2) In general high-silver SnAgCu alloys perform poorly in high-strain rate situations(drop shock) but exhibit better thermal fatigue resistance than lower or no Ag alternatives.

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Several articles in a recent SMTnet Express Newsletter [https://smtnet.com/express/index.cfm?fuseaction=archives&issue=20210909] addressed topics of your question.

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"For the lead free soldering of electronic assemblies, the inclusion of silver in the alloy results in better wetting compared to non-silver containing alloys and also allows for a broader process window. Silver also improves joint reliability compared to non-silver lead-free and tin-lead alloys."

Found it in an article here in the library "https://smtnet.com/library/files/upload/Effect%20of%20silver%20in%20common%20leadfree%20alloyspaperEng042309.pdf"

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