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. The effect can be observed for example by placing insoluble markers at the interface between a pure metal and an alloy containing that metal, and heating to a temperature where diffusion is possible; the boundary will move relative to the markers. For example, using molybdenum as a marker between copper and brass (a copper-zinc alloy), the region occupied by the brass will expand until it includes the molybdenum. This occurs because zinc diffuses more rapidly than the copper, and thus diffuses out of the alloy down its concentration gradient, thus expanding the area occupied by the brass. Such a process is impossible if diffusion is by the direct exchange of atoms.

The Kirkendall effect was named after Ernest Kirkendall (1914–2005) assistant professor of chemical engineering at Wayne State University from 1941 to 1946. He discovered the effect in 1947.

The Kirkendall effect has important practical consequences. One of these is the prevention or suppression of voids formed at the boundary interface in various kinds of alloy to metal bonding. These are referred to as Kirkendall voids.

In 1972, C.W. Horsting of the RCA Corporation published a paper which reported test results on the reliability of semiconductor devices in which the connections were made using aluminium wires bonded ultrasonically to gold plated posts. His paper demonstrated the importance of the Kirkendall effect in wire bonding technology, but also showed the significant contribution of any impurities present to the rate at which precipitation occurred at the wire bonds. Two of the important contaminants that have this effect, known as Horsting effect (Horsting voids) are bromine and chlorine.

Both Kirkendall voids and Horsting voids are known causes of wire bond fractures, though historically this cause is often confused with the purple colored appearance of one of the five different gold-aluminium intermetallics, commonly referred to as "purple plague" and less often "white plague".