A gold–aluminium intermetallic is an

intermetallic An intermetallic (also called an intermetallic compound, intermetallic alloy, ordered intermetallic alloy, and a long-range-ordered alloy) is a type of metallic alloy that forms an ordered solid-state compound between two or more metallic elemen ...

compound of

gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile met ...

and

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...

that occurs at contacts between the two metals. These intermetallics have different properties from the individual metals, which can cause problems in wire bonding in

microelectronics Microelectronics is a subfield of electronics. As the name suggests, microelectronics relates to the study and manufacture (or microfabrication) of very small electronic designs and components. Usually, but not always, this means micrometre-sc ...

. The main compounds formed are Au₅Al₂ (white plague) and AuAl₂ (purple plague), which both form at high temperatures. White plague is the name of the compound Au₅Al₂ as well as the problem it causes. It has low electrical conductivity, so its formation at the joint leads to an increase of electrical resistance which can lead to total failure. Purple plague (sometimes known as ''purple death'' or Roberts-Austen's '' purple gold'') is a brittle, bright-purple compound, AuAl₂, or about 78.5% Au and 21.5% Al by mass. AuAl₂ is the most stable thermally of the Au–Al intermetallic compounds, with a melting point of 1060°C (see phase diagram), similar to that of pure gold. The process of the growth of the intermetallic layers causes reduction in volume, and hence creates cavities in the metal near the interface between gold and aluminium. Other gold–aluminium intermetallics can cause problems as well. Below 624°C, purple plague is replaced by Au₂Al, a tan-colored substance. It is a poor conductor and can cause electrical failure of the joint that can lead to mechanical failure. At lower temperatures, about 400–450°C, an interdiffusion process takes place at the junction. This leads to formation of layers of several intermetallic compounds with different compositions, from gold-rich to aluminium-rich, with different growth rates. Cavities form as the denser, faster-growing layers consume the slower-growing ones. This process, known as Kirkendall voiding, leads to both increased electrical resistance and mechanical weakening of the wire bond. When the voids are collected along the diffusion front, a process aided by contaminants present in the lattice, it is known as

Horsting void The Kirkendall effect is the motion of the interface between two metals that occurs as a consequence of the difference in diffusion rates of the metal atoms. The effect can be observed for example by placing insoluble markers at the interface betwee ...

ing, a process similar to and often confused with Kirkendall voiding. All problems caused by gold–aluminium intermetallics can be prevented either by using bonding processes that avoid high temperatures (e.g. ultrasonic welding), or by designing circuitry in such a way as to avoid aluminium-to-gold contact using aluminium-to-aluminium or gold-to-gold junctions.

External links

Harvard: Gold Aluminium Intermetallics

Aluminium aurate – purple gold
Corrosion Gold Aluminides Integrated circuits Intermetallics {{corrosion-stub

See also

External links