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7-1 Innovative Sn-Cu Eutectic based Solder with Higer Creep Resistance

   This alloy has already applied for the state invention patent, and the patent application number is 2009100705088.
Compared with the mass of tin-based solders, Sn-Cu eutectic based solder, which contains no silver, has a superior performance-to-price ratio. It has been widely used in the domestic electronic manufacture in the processes of wave-soldering, dip soldering and manual welding.

   However, low oxidation resistance of this alloy severely affected its application in high-reliability electronic products.
The main defect of this brazing filler metal is poor temperature structures, which severely affected its application in high-reliability electronic products. Low oxidation resistance is another disadvantage of this alloy, which increases the residues during the wave-soldering process and raises the cost as well.

   As the problems mentioned above, we conducted thorough study. By the method of microalloy and addition of Co, Ni and P etc.,We successfully improved the properties of Sn-Cu solder, especially the high temperature stability at welding point and the oxidation resistance of brazing filler metal.

   The study shows that, addition of Co and Ni can change the morphology of the compounds on the interface of solder/Copper substrate. Figure 1(a) and (b) show the significant differences in compound's morphology at the interface of the traditional Sn-Cu/Cu and SC-Co-Ni-(p)/Cu. It can be found that traces of Co and Ni have the ability to transform the coarse scalloped compound at the interface into close-grained laminar structure. Although the layer of this product could be thicker in as-welded condition, because of the dense and flat interface, stress concentration is unlikely to happen when the welding point suffers external stresses. Therefore, compared with the scalloped compound, the mechanical properties of the joint has been improved, especially the shock resistance and anti-shake performance.

   Latest study shows that, this laminar compound has a prominent advantage that its high temperature stability is much better than scalloped compound's. In the high temperature aging test, its growth rate is much lower than that of scalloped compound.

   More importantly, it can be found that close-grained laminar compounds effectively inhibit the formation of Cu₃Sn at the side of Cu substrate. Figure 2 shows the microstructure of the interface after heating at 150℃ for 24h,t can be concluded that Cu₃Sn has already generated in the scalloped compound, while there is little Cu₃Sn in the close-grained laminar compound. As we all know, the fomation of Cu₃Sn is due to the different diffusion velocity of Sn and Cu in the interface, as well as the appearance of Kenkidall cavity, which is the main cause of degeneration of mechanical properties, especially the Creep Resistance. At the scalloped interface, Sn and Cu diffuse easily through the scalloped Cu₆Sn₅, so Cu₃Sn can be easily obtained; while there is no such diffusion channel in the laminar interface, so the speed of the formation of Cu₃Sn is reduced greatly.

   Figure 3 shows the results of creep resistance test at the joint of Sn-0.7Cu and SC-Co-Ni-P respectively after high temperature aging at 150℃ for 24h. t is observed that the creep resistant at the joint of this solder alloy is more than doubled.

   At the same time, the wettability of the alloy has also been improved. Figure 4 shows the wettability test results of the two kinds of alloy on the surface of copper. The alloy is of low cost because it contains no Pb and Ag, which are either inhibited or expensive, and its welding technological performances are close to Sn-0.7Cu. It can be applied to the field of packaging of various electronic products, especially those which serve at high temperature for a long period of time.

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