Correlations Between the Microstructure and Fatigue Life of Near-Eutectic Sn-Ag-Cu Pb-Free Solders

Relationships between the microstructure of near-eutectic Sn-Ag-Cu Pb-free solder joints and room-temperature fatigue lifetimes were studied. Correlations between the lifetimes of single Sn grained, SAC205 solder joints with the orientation of the Sn grain, and with differences in Ag(3)Sn and Cu(6)Sn(5) precipitate microstructures were sought. Correlations between the number of Sn grains and fatigue life were observed. Surprisingly, it was found that Ag(3)Sn precipitates were highly segregated from Cu(6)Sn(5) precipitates on a length scale of approximately 20 mu m. Furthermore, large (factor of two) variations of the Sn dendrite arm size were observed within given samples. Such variations in values of dendrite arm size within a single sample were much larger than observed variations of this parameter between individual samples. Few significant differences were observed in the average size of precipitates in different samples. Although effects of average precipitate microstructure on lifetimes were not clearly delineated, one sample showed an anomalously high number of the smallest size (30 nm to 50 nm) Ag(3)Sn precipitates, and this sample also exhibited a much longer lifetime than all the other samples. Thus, some evidence was presented that samples of particular orientations and precipitate microstructures can exhibit anomalous fatigue lifetimes.