Gold–aluminium intermetallic

Au5Al2 has low electrical conductivity and relatively low melting point. Au5Al2's formation at the joint causes increase of electrical resistance, which can lead to electrical failure. Au5Al2 typically forms at 95% of Au and 5% of Al by mass, its melting point is about 575 °C, which is the lowest among the major gold-aluminum intermetallic compounds. AuAl2 is a brittle bright-purple compound, with a composition of about 78.5% Au and 21.5% Al by mass. AuAl2 is the most thermally stable species of the Au–Al intermetallic compounds, with a melting point of 1060 °C (see phase diagram), which is similar to the melting point of pure gold. AuAl2 can react with Au, therefore is often replaced by Au2Al, a tan-colored substance, which forms at composition of 93% of Au and 7% of Al by mass. It is also a poor conductor and can cause electrical failure of the joint, which can further lead to mechanical failure. == Voiding ==

At lower temperatures, about 400–450 °C, an interdiffusion process takes place at the junction, leading to formation of layers of different gold-aluminum intermetallic compounds with different growth rates. Gaps are formed as the denser and faster-growing layers consume the slower-growing layers. This process is known as the Kirkendall voiding, which leads to both increased electrical resistance and mechanical weakening of the wire bond. When the voids forms along the diffusion front, this process is aided by contaminants present in the lattice, and is known as the Horsting voiding, which is a similar process to the Kirkendall voiding. == See also==