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Five-layer Cu-coated Zn/Al clad solder for die attachment
Keywords: Zn-Al-Cu, Clad material, Pb-free solder
A great deal of effort has been undertaken to develop new bonding technologies that can replace conventional Pb-based solder in the electronics field. One of the motivations for this is to comply with environmental conservation regulations such as the RoHS and ELV directives. Another is to improve the reliability and robustness of products. This is particularly important for future SiC and GaN power devices, which will operate at extreme temperatures above 200C, because even conventional Pb-based solder cannot satisfy the reliability requirements for such devices. Among the many novel bonding approaches that have been suggested are Au-, Bi- and Zn-based solder, transition liquid phase diffusion bonding using Sn-Cu and Ag-In systems, and sintering bonding using Ag nanoparticles. Although these methods have the potential to offer high reliability, they have not come into widespread use due to high materials and process costs, and technical difficulties in achieving an ideal bonding state. On the other hand, Zn-Al solder is relatively inexpensive, exhibits a high thermal conductivity and forms a stable bonding interface with Ni metallization. However, a crucial drawback is its poor wettability due to oxidation of Zn and Al, and suitable countermeasures must be found before Zn-Al solder can be used in practical applications. To overcome this problem, we have developed Zn/Al/Zn and Zn/Al/Cu multilayer clad materials for high-temperature die attachment. The clad structure is used in an attempt to improve the wettability of Zn-Al solder by preventing oxidation of Zn and Al. The materials are produced by clad-rolling of Zn, Al and Cu strips, and they act as a solder following eutectic melting at temperatures above 382C. In the present study, the wettability, bondability and reliability of the clad materials were examined. A three-layer Zn/Al/Zn clad material was first investigated and was found to exhibit superior wettability to conventional Zn-Al solder. Its bondability, however, was insufficient because of oxidation of the Zn outer layers. Five-layer Cu/Zn/Al/Zn/Cu and Cu/Al/Zn/Al/Cu clad materials were next examined from the viewpoint of their oxidation resistance. For the Cu/Zn/Al/Zn/Cu material, Cu from the outer layers was found to diffuse into the Zn layers at high temperature, so that the outer Cu layers completely disappeared. However, for Cu/Al/Zn/Al/Cu, the outer Cu layers were present up to the melting temperature of 382C. This was because the Al layers acted as diffusion barriers between the Cu and Zn. In addition, the outer Cu layers prevented oxidation of the Zn and Al. Thus, Cu/Al/Zn/Al/Cu exhibited superior bondability even under an atmospheric oxygen concentration of 100 ppm. Finally, the reliability of joints formed using Cu/Al/Zn/Al/Cu was compared with that using Pb-Sn-Ag solder in the temperature range of -55 to 150C. The thermal cycle lifetime for Cu/Al/Zn/Al/Cu was found to be longer than that for the Pb-based solder, and no Kirkendall voids were observed near the bonded interface after an aging test at 250C for 1000 h. In conclusion, the Cu/Al/Zn/Al/Cu clad material is a promising candidate for replacing Pb-based solder and for use with high-operation-temperature SiC devices.
Takuto Yamaguchi, Researcher
Hitachi, Ltd.
Yokohama-shi, Kanagawa-ken

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