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
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

 and an alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...

 containing that metal, and heating to a temperature where diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...

 is possible; the boundary will move relative to the markers. For example, using molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...

 as a marker between copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 and brass
Brass is an alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties.In comparison, bronze is principally an alloy of copper and tin...

 (a copper-zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...

 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
Ernest Kirkendall
Ernest Oliver Kirkendall was an American chemist and metallurgist. He is known for his 1947 discovery of the Kirkendall effect.-Life and works:...

 (1914–2005) assistant professor of chemical engineering
Chemical engineering
Chemical engineering is the branch of engineering that deals with physical science , and life sciences with mathematics and economics, to the process of converting raw materials or chemicals into more useful or valuable forms...

 at Wayne State University
Wayne State University
Wayne State University is a public research university located in Detroit, Michigan, United States, in the city's Midtown Cultural Center Historic District. Founded in 1868, WSU consists of 13 schools and colleges offering more than 400 major subject areas to over 32,000 graduate and...

 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
Reliability (statistics)
In statistics, reliability is the consistency of a set of measurements or of a measuring instrument, often used to describe a test. Reliability is inversely related to random error.-Types:There are several general classes of reliability estimates:...

 of semiconductor devices in which the connections were made using 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....

 wires bonded ultrasonically
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...

 to 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...

 plated posts. His paper demonstrated the importance of the Kirkendall effect 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...

 technology, but also showed the significant contribution of any impurities present to the rate at which precipitation
Precipitation (chemistry)
Precipitation is the formation of a solid in a solution or inside anothersolid during a chemical reaction or by diffusion in a solid. When the reaction occurs in a liquid, the solid formed is called the precipitate, or when compacted by a centrifuge, a pellet. The liquid remaining above the solid...

 occurred at the wire bonds. Two of the important contaminants that have this effect, known as Horsting effect (Horsting voids) are bromine
Bromine ") is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists, Carl Jacob Löwig and Antoine Jerome Balard, in 1825–1826...

 and chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...


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
Gold-aluminium intermetallics
A gold-aluminium intermetallic is an intermetallic compound of gold and aluminium that occurs at contacts between the two metals. These intermetallics have different properties than the individual metals which can cause problems in wire bonding in microelectronics...

, commonly referred to as "purple plague" and less often "white plague".

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