User:Wuhudasima/Gold–aluminium intermetallic

Lead
A gold–aluminium intermetallic is an intermetallic compound of gold and aluminium that occurs at contacts between the two metals.

Article body
Gold–aluminium intermetallic have different properties from the individual metals depending on their composition, such as low conductivity and high melting point. These properties can cause problems in wire bonding in microelectronics. The main compounds formed are usually Au5Al2 (white plague) and AuAl2 (purple plague), which both form at high temperatures, then Au5Al2 and AuAl2 react with Au to form more stable compound, Au2Al.

Au5Al2, also known as White Plague, has low electrical conductivity and relatively low melting point. 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. Au5Al2's formation at the joint causes increase of electrical resistance, which can lead to electrical failure. AuAl2, which is also known as Purple plague, is a brittle. AuAl2 is a bright-purple compound, its composition is about 78.5% Au and 21.5% Al by mass. AuAl2 is the most thermally stable specie of the Au–Al intermetallic compounds, it has a melting point of 1060 °C (see phase diagram), which is similar to the melting point of pure gold. Due to the difference of density between the metal and intermetallics, the growth of the intermetallic layers causes reduction in volume, and therefore creates gaps in the metal near the interface between gold and aluminium. The production of gaps lowers the strength of the metal compound, which can cause mechanical failure at the joint, fostering the problems that the intermetallics causes in metal compounds. At composition of 93% of Au and 7% of Al by mass, AuAl2 is replaced by Au2Al, a tan-colored substance. It is also a poor conductor and can cause electrical failure of the joint, which further lead to mechanical failure.

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, leads to both increased electrical resistance and mechanical weakening of the wire bond. 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.

Among all the species of gold-aluminum intermetallics, AuAl2 is widely researched and used in many different field, such as decorative coating and resitors. AuAl2 has a beautiful purple color, this color is quite rare in the metal and intermetallic compounds. Therefore, it is often used as a coating material, which can last for a long time on the surface due to its stength. Additionally, being used as a resistor is also a major application of AuAl2. AuAl2 has high resitivity and high melting point, which is a desirable resistor in many field, due to its strength at high temperature.