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By Ranjit Pandher, Cookson Electronics Assembly Materials
Much work has been reported on the melting and freezing behavior of tin/silver/copper (Sn/Ag/Cu SAC) alloys. Knowing melting and freezing points is only part of the information. A slight change in composition may result in a significant difference in solder performance.
A detailed experimental study and thermodynamic calculations were reported by the NIST metallurgy group.1 Figure 1 shows the calculated liquidus surface in the SnAgCu phase diagram as reported in that article. SAC305 and SAC405 are close to the eutectic. There are some differences in the ternary eutectic composition reported by different groups. In this article, the reported eutectic alloy composition is approximately Sn3.7Ag0.9Cu with 217°C eutectic temperature. Low-Ag alloys typically show two distinct melting phases: a low-temperature melting phase corresponding to Sn/Ag eutectic and high-temperature phase corresponding to Sn/Cu eutectic melting around 228°C. See the differential scanning calorimetry (DSC) plots of SAC305, SAC105, and SAC0307 in Figure 2. As the silver level goes down, so does the first, low-temperature peak. Calculated isothermal sections of SAC phase diagram at 223°C, a temperature between the two major peaks seen in low-Ag SAC alloys' DSC, are shown in Figure 3.
Figure 3. The lower left section corresponds to low-Ag and low-Cu alloys and comprises liquid alloys and solid Sn phase. The upper left section corresponds to high-Ag and medium-Cu SAC alloys and consists of liquid phase along with Ag3Sn intermetallic solid phase. The shaded section to the right corresponds to medium-range Ag and high Cu level and consists of liquid and Cu6Sn5 intermetallic. Triangular are in the center, representing all liquid for all practical purposes. In reality true liquidus is usually higher.
As reported by Moon et al1, a tiny peak is seen in the DSC/differential thermal analysis (DTA) plots corresponding to the melting of the primary intermetallic phase high temperature. That temperature will be the true liquidus point. However, since the fraction of this intermetallic phase small, for all practical purposes the liquidus temperature is where rest of the phases have completely melted.
Knowing melting and freezing points is only part of the information. For in-depth understanding of the solder joint formation and its reliability, one has to understand the solidification behavior in more detail. Anderson2 reported a detailed study of the solidification behavior of several near-eutectic SAC alloys and showed significant differences in microstructure of soldered joints even though alloy compositions do not vary greatly. Figure 4a shows the microstructure of Sn3.0Ag0.5Cu; 4b shows Sn3.9Ag0.6Cu; 4c is Sn3.7Ag0.9Cu; and 4d depicts Sn3.6Ag1.0Cu.
Coarse Sn dendrites for SAC305 differ from extremely fine Sn dendrites for Sn3.9Ag0.6Cu. Sn3.7Ag0.9Cu shows a Sn dendrite pattern similar to SnAg eutectic while Sn3.6Ag1.0Cu microstructure does not appear dendritic at all. The large Ag3Sn needle is noticeable in 4b, Sn3.9Ag0.6Cu. Considering most manufacturers' tolerances, this composition may be considered SAC405. A slight change in composition may result in a significant difference in solder performance.
The long, thin Ag3Sn plate is seen in Figure 4b only. This alloy, Sn3.9Ag0.6Cu, is the only alloy shown that is on the Ag-rich side of the eutectic. The probability of Ag3Sn precipitation increases dramatically on that side of the eutectic as one would expect. Henderson et al 3 studied this Ag3Sn platelet formation phenomenon in detail.
Figure 5 shows micrographs of cross-sectioned solder joints formed with two SAC alloys, Sn3.8Ag0.7Cu and Sn2.5Ag0.9Cu. All the solder joints were reflowed up to 240°C peak temperature. Three different cooling rates were studied, 0.2°C/min, 1.2°C/min and 3.0°C/min. It can be clearly seen that a combination of high silver content and slow cooling rate results in the growth of large size Ag3Sn platelets.
Ag3Sn in itself has a higher melting temperature, thus these platelets start precipitating and growing while solder is still in the liquid state. Higher Ag level enhances precipitation while longer cooling time means longer growth time. Sometimes Ag3Sn platelets can grow so large in the liquid stage that when solder shrinks during freezing, the Ag3Sn platelet protrudes outwards, severely deforming the solder interconnection. Figure 6 shows a SAC405 solder sphere reflowed under a profile with a very slow cooling rate. A large Ag3Sn platelet appears at the surface of the sphere. Therefore, even though most OEMs would treat SAC305 and SAC405 alloys near eutectic and tend to use the same reflow profile, these results show that more care must be taken with SAC405. Apart from deforming the solder spheres (or interconnections in general), such large-size Ag3Sn platelets lead to stress build up around them and results in crack initiation in thermally or mechanically stressed solder joints.
REFERENCES:1. Moon, K. W., Boettinger, W.J.; Kattner, U.R.; Biancaniello, F.S.; Handwerker, C.A., "Experimental and thermodynamic assessment of Sn-Ag-Cu solder alloys," Journal of Electronic Materials, Vol 29, 1122-1136 (2000).2. I.E. Anderson, "Development of Sn/Ag/Cu and Sn/Ag/Cu/X alloys for Pb-free electronic solder applications," Journal of Material Science: Mater Electronics, Vol 18, 55-76 (2007). 3. D.W. Henderson, T. Gosselin, A. Sarkhel, S. K. Kang, W. Choi, D. Shih, C. Goldsmith, K. J. Puttlitz, "Ag3Sn plate formation in the solidification of near ternary eutectic Sn/Ag/Cu alloys," J. Mater Res., Vol 17, No. 11, 2775-78 (2002).
Ranjit Pandher may be contacted at Cookson Electronics Assembly Materials, 109 Corporate Blvd., South Plainfield, NJ 07080.