Purple plague (intermetallic)
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- For other meanings, see Purple plague (disambiguation).
Purple plague is a brittle, bright-purple intermetallic compound of aluminium and gold, AuAl2, formed on the contact of these two metals. The process of the growth of the intermetallic layers leads to creation of voids in the metal lattice, which leads to failures of wire bonding in microelectronics.
The other principal aluminum-gold compound is Au5Al2, known as white plague.
The compound, the Roberts-Austen's purple gold, is composed of 79%Au-21%Al, with melting point of about 750 °C. It forms at high temperatures, and can be prevented by bonding processes that avoid high temperatures (for instance, ultrasonic welding), or by using aluminium-to-aluminium or gold-to-gold junctions instead of the aluminium-gold ones.
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 precedes the mechanical failure.
At lower temperatures, about 400–450 °C, an interdiffusion process takes place on the junction of aluminium and gold. 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.
See also white plague (intermetallic).
