Gold-aluminium intermetallics
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Gold-aluminium intermetallics are intermetallic compounds of gold and aluminium that occur at contacts between the two metals. These intermetallics have different properties than the individual metals which can cause problems in wire bonding in microelectronics. 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 is a brittle, 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. As the denser faster-growing layers consume the slower-growing ones, cavities form. This process, known as Kirkendall voiding, leads to increase of electrical resistance of the wire bond and its mechanical weakening. 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), or by designing circuitry in such a way as to avoid aluminium-to-gold contact using aluminium-to-aluminium or gold-to-gold junctions.
