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Effects of Sb and Zn Addition on Impact Resistance Improvement of Sn-Bi Solder Joints
Keywords: Sn-Bi-Sb-Zn solder, drop impact resistance, intermetallic compound layer
Recently, low temperature mounting methods have become increasingly important to conserve energy and then to protect the global environment. Therefore, this research field has attracted much attention, and various R&D activities are being conducted worldwide to develop new materials and processing methods. The Sn-Bi eutectic solder that has a low melting point of 139C is considered to be a promising material to realize low temperature soldering, in particular, in electronic packagings such as Multi-Chip Modules, sensor modules, and LCDs. Although Sn-Bi eutectic solder has a poorer ductility than Sn-Ag-Cu solder, we already reported that solder ductility and the drop impact resistance of the BGA packaging can be improved dramatically by adding Sb to Sn-Bi eutectic solder. However, poor impact reliability upon high temperature aging is still a problem of this solder. This impact resistance degradation is believed to be due to the formation of a brittle Bi rich-layer near the joint interface with copper electrodes, during the growth of Cu-Sn layer. On the other hand, in this study, it was found that Sn-Bi eutectic solder with a little amount of Sb and Zn can suppress the reduction of the joint interfacial strength with a copper electrode, because Cu-Zn reaction layer would form near the joint interface, thus preventing Bi segregation. Therefore, it is expected to improve the drop impact resistance even after high temperature aging. The test samples were obtained by assembling BGA package using reflow process at 180C. After the reflow process, and then soaking at 125C for 250 hours, drop test was performed with a maximum distortion of 0.2% until package failure occurs or up to the maximum of 40 drops. The resistances of daisy-chained assemblies were measured during the drop test, and the failure criterion was set be 1000 ohms or more. After the reflow process, the drop impact resistance of the Sn-Bi-Sb-Zn solder was found to be ten times of those of Sn-Bi and Sn-Bi-Ag, and three times of that of Sn-Bi-Sb solder. It was found that addition of Sb to Sn-Bi solder minimizes the grain size of the eutectic microstructure, and improves the solder ductility significantly. In addition, the hardness of Sn-Bi-Sb-Zn solder in the intermetallic compound layer was found to be lower than that of Sn-Bi-Sb solder. Therefore, it is effective to stress relaxation at around the joint interface, thereby suppressing the crack formation during the drop impact test. After aging at 125C, a drop impact resistance increase of less than 10% was observed with Sn-Bi-Sb-Zn solder. In contrast, in packages with Sn-Bi, Sn-Bi-Ag, and Sn-Bi-Sb solders, package failures occurred within two drops. Moreover, small cracks were observed near the joint interface of the former, while big cracks were observed in the latter. Because the intermetallic compound layer with Cu electrodes of Sn-Bi-Sb-Zn solder changed from η’-Cu6Sn5 to γ-Cu5Zn8, and brittle Bi rich layer did not form around the joint interface with copper electrode, dramatic improvement in drop impact resistance of the BGA package was observed even after aging at high temperatures.
Keishiro Okamoto,
Fujitsu Laboratories Ltd.
Atsugi, Kanagawa

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