Intermetallic Creation and Growth
Intermetallic compounds form when two unlike metals diffuse into one another creating species materials which are combinations of the two materials. For the Au/Al bonding case, there are five different intermetallic compounds that can form. AuAl2 is often referred to as "purple plague" due to its purple color. The other four compounds are white or tan. Consequently, these intermetallics are observed as gray, brown or black when they grow, mixed together through the several phases. These intermetallic compounds are always present in Au/Al bonds and should not be associated with the cause of weak bonds. Their appearance and location can often be indicators of the conditions and results of the bonding event (incomplete bonds and contaminated bond materials for example).
Intermetallic growth is the result of the diffusion of one material into another via crystal vacancies made available by defects, contamination, impurities, grain boundaries and mechanical stress. The diffusion rate for Au in Al is different than that for Al into Au. These rates are a function of temperature. If one material overwhelms the other in volume, and diffusion occurs rapidly enough, the minority material can appear to have been completely "consumed" by the majority material. Diffusion is enabled by the movement of atoms of one material into the crystal vacancies of the other material. The vacancies will appear to be the moving feature and they can tend to coalesce and become visible in the form of voids or pores. Diffusion and void creation during the formation of intermetallic materials was studied by Kirkendall and so these voids have often been called Kirkendall voids. Excessive Kirkendall voiding can result in out-of-tolerance wire bond resistance and weakened wire bonds. A clarifying explanation of the Kirkendall effect can also be found here: http://www.msm.cam.ac.uk/phase-trans/kirkendall.html
The intermetallic compound growth and the ratio of Au-Al in each phase will vary depending on the diffusion rate and amount of material available. Intermetallic layer thickness can be estimated by following equation:
Where X is the intermetallic layer thickness, t is the time and K is the rate constant which is calculated by following:
Where C is the rate constant (There are nine different ones listed by Philofsky/3), e is the activation energy, K is the Boltzmann constant, and T is the temperature in absolute scale
Intermetallic growth is accelerated with temperature and contamination. Horsting /2 studied excessive voiding in Au-Al wire bonds and found that the contamination plays a major role in the collection of voids along the diffusion front. These voids are often confused with Kirkendall voids. Here we call them Horsting voids. Kirkendall voiding and Horsting voiding are the most common causes of bond fracture though historically these failures have been attributed to "purple plague" misleading the public into understanding that the problem was rooted in the presence of the purple colored intermetallic material
Paper of interest: Effect of Vacuum on High-Temperature Degradation of Au-Al Wire Bonds in PEMS (Ref. 142) and High-Temperature Degradation of Wire Bonds in Plastic Encapsulated Microcircuits (Ref. 143)