Gold-aluminium intermetallic
A gold-aluminium intermetallic is an intermetallic compound of gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...

 and aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

 that occurs at contacts between the two metals. These intermetallics have different properties than the individual metals which can cause problems in wire bonding
Wire bonding
Wire bonding is the primary method of making interconnections between an integrated circuit and a printed circuit board during semiconductor device fabrication. Although less common, wire bonding can be used to connect an IC to other electronics or to connect from one PCB to another...

 in microelectronics
Microelectronics is a subfield of electronics. As the name suggests, microelectronics relates to the study and manufacture of very small electronic components. Usually, but not always, this means micrometre-scale or smaller,. These devices are made from semiconductors...

. 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 electric 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) is a brittle
A material is brittle if, when subjected to stress, it breaks without significant deformation . Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a snapping sound. Brittle materials include most ceramics and glasses ...

, bright-purple compound of AuAl2. The process of the growth of the intermetallic layers leads to creation of voids in the metal lattice.

Other gold-aluminium intermetallics can cause problems as well. The compound Roberts-Austen's purple gold, is composed of 79%Au-21%Al, with a melting point of about 750°C. Above 624°C, purple plague is replaced by Au2Al, a tan-colored substance. It is a poor conductor and can cause electric 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 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
Kirkendall effect
The Kirkendall effect is the motion of the boundary layer between two metals that occurs as a consequence of the difference in diffusion rates of the metal atoms...

, 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 voiding, 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
Ultrasonic welding
Ultrasonic welding is an industrial technique whereby high-frequency ultrasonic acoustic vibrations are locally applied to workpieces being held together under pressure to create a solid-state weld. It is commonly used for plastics, and especially for joining dissimilar materials...

), or by designing circuitry in such a way as to avoid aluminium-to-gold contact using aluminium-to-aluminium or gold-to-gold junctions.

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