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Investigation of new silver alloys and invention of Ag foil bonding and Ag-Ag direct bonding through in-situ reduction of surface oxides for high power and high temperature electronics


Despite the superior physical properties of silver (Ag) and its alloys such as high electrical and thermal conductivities and high ductility, their applications in electronics have been quite limited due to corrosion and tarnishing concerns. To expand their applications in electronic packaging, developing new materials and processes and investigating new materials properties are essential. This PhD research aims to design and grow novel Ag alloys and address the intermetallic compound (IMC) issues associated with Ag wire bonds, followed by producing Ag and alloys in various forms for solid state bonding processes valuable for high temperature electronics.

Firstly, the growth of homogeneous Ag solid solution phase with zinc (Zn) is conducted at two different compositions and their mechanical properties are evaluated by tensile test. According to the experimental results, Ag solid solution phase with Zn at either composition show tempered yield strength, high tensile strength and large uniform strain compared to those of Ag. Moreover, the tarnish resistance is improved with the alloying of zinc. Secondly, Ag3Al has been reported to be the weakest part in the Ag-Al wire bonds due to its low toughness and low corrosion resistance. A method to suppress Ag3Al through alloying indium (In) into Ag is devised. The working mechanism is studied through transmission electron microscopy (TEM) and thermodynamic modeling. Reaction kinetics and evolution of defects are further discussed.

A new approach for die attachment through solid-state bonding using fine-grained Ag foils is developed. The bonding is conducted at 300 ℃, assisted by low pressure. The strength of the joints far exceeds the requirement specified in military standards. No degradation can be observed after aging test. This approach avoids the oxidation issue on bonding interfaces in present sintered-silver technique.

Lastly, an Ag-Ag direct bonding process is invented using in-situ decomposition of silver oxide. The bonding is performed at 210 ℃ under 0.1 torr vacuum and assisted by only 1.4 MPa pressure. Encouragingly, the resulting Ag-Ag joints are strong with only sparse voids in nano-scale. Cross-sectional study shows that the bonding is formed by grain growth across the original interface. No residual oxygen is trapped. This new process is a breakthrough in Ag-Ag bonding at low temperature and low pressure that produce pure Ag joints. Its applications are wide open.

The Ag solid solution phases investigated here show great potential not only in wire bonding and other mechanical component in electronic devices but also photonic applications. The novel bonding processes achieved in this study should be highly valuable for applications in advanced electronic and photonic packages where performance and reliability are highly preferred.

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