Kirkendall void formation

Kirkendall void formation can, however, be prevented from occurring by choosing the right metal species. For example, whereas platinum coating on copper is subject to the Kirkendall void creation process, the same coating on electrodeposited nickel is free of it even if heated to as high as 600°C for many hours (more than 10 hours ). 

Kejun Zeng, Roger Stierman,Tz-Cheng Chiu, Darvin Edwards, Kirkendall void formation in eutectic SnPb solder joints on bare Cu and its effect on joint reliability, /ourwa/ of Applied Physics,2005, 97,024508. 

The Kirkendall effect (8) is time and temperature dependent, and with some metal couples, it takes place even at room temperature. For instance, adhesion of solder to gold is damaged by heating to about 150°C for about 5 minutes, due to the formation of Kirkendall voids. Naturally, the formation of Kirkendall voids is accelerated by increased temperature and dwelling time. 

The Kirkendall effect is commonly accompanied by the Frenkel effect, the void formation in the diffusion zone. In foreign literature, the Frenkel effect is often referred to as Kirkendall voiding, which is rather confusing, as Kirkendall and Frenkel effects are competitive vacancies annihilating at the dislocation kinks and causing the Kirkendall shift, cannot be used for Kirkendall voiding, and vice versa. 

An interesting point with the second case (thermomigration-driven Kirkendall shift) is that there is no void formation or hillock extrusion at all. 

Nanoshells, produced in the diffusive reactions of nanoparticles within the ambient phase with a simultaneous formation of Kirkendall voids inside, are unstable in principle, but the shrinkage time can be very large due to their cubic dependence on the radius of the nanoparticle. The mechanism of shrinkage is the out-diffusion of vacancies from the void due to the curvature effect. 

FIGURE 58.18 Solder joint intermetallic formation, (a) The constituent material stack up in a BGA substrate land pad. (b) After the ball is attached, a number of intermetallics are formed between the copper pad and the SnPb solder joint. Kirkendall voids are known to form at the phosphorus-rich Ni layer and Sn-Ni intermetallic interface, (c) After board level reflow, the interface region thickens and several more intermetallics are formed. (Reprinted with permission from Renesas Technology Corp./ [2003] lEEEECTC)... 

Work is also needed to further characterize the creep and fatigue behavior of the lead-free solder alloy. The interfacial interactions (including intermetallics formation and growth, and the formation of Kirkendall voids on various PWB surface finishes) and their impact on reliability also warrant further investigation (Ref 2-3). 

The compound interface is the type of interface found in reactive systems such as oxygen-active metal films on oxide substrates, where a mixed oxide interphase material is formed, or in intermetallic-forming metal-on-metal systems such as Au-Al and Al-U. In the case of Au-Al the interdiffusion and reaction form both Kirkendall voids and a brittle intermetallic phase termed purple plague , which allows easy bond failure. When materials react, the reaction can be exothermic, where energy in the form of heat is released, or endothermic, where energy is taken up. Table 10.2 lists some heats of formation of various materials in forming compounds. An exothermic reaction is indicated by a negative heat of formation and an endothermic reaction is indicated by a positive heat of reaction. 

A binary phase system that forms an intermetallic compound normally provides good adhesion because of the mutual attraction between atoms of different species. Examples of this type of system include Ti-Cu, Ni n, and Sn-Cu (Fig. 29) [105]. Intermetallic compound formation occurs during high-temperature process steps, and the consequences are usually undesirable, resulting in increased stress levels, impurity snowplowing, Kirkendall voids, and an increase in electrical resistance. These reactions must be controlled either by hmiting the amount of reactants available or by controlling the time and temperature of the reaction. 

FIG. 33 Schematic representation depicting the formation of Kirkendall voids during the reaction of copper with tin to form intermetallic compounds. The imbalance in the diffusion fluxes between copper and tin atoms results in the generation of atomic vacancies that condense to form voids. 

There were several types of genuine porosities observed by various researchers. The first type was a void observed in the magnetite layers formed on iron and steel at temperatures below 570°C [91,93], where the gas used could be moist air, dry air or CO2. The voids were very fine in size and were observed at grain boundaries as well as inside magnetite grains. This type of porosity is also known as Kirkendall voids [100]. The formation of voids appeared to be associated with the formation of a duplex scale structure [101]. Recently, some theoretical treatments using conventional diffusion theories were made by Maruyama etal. [102] and Ueda etal. [92] to provide a semi-quantitative and quantitative explanation of their formation mechanism and their location in the scale. 

An example where one metal melts before the densihcation process, is the formation of bronze from a 90 10 weight percentage mixture of copper and tin. The tin melts at a temperature of 505 K, and the liquid immediately wets the copper particles, leaving voids in the compact. The tin then diffuses into the copper particles, leaving further voids due to dre Kirkendall effect. The compact is therefore seen to swell before the hnal sintering temperature of 1080 K is reached. After a period of homogenization dictated by tire criterion above, the alloy shrinks on cooling to leave a net dilatation on alloy formation of about 1%. 

The Kirkendall effect alters the structure of the diffusion zone in crystalline materials. In many cases, the small supersaturation of vacancies on the side losing mass by fast diffusion causes the excess vacancies to precipitate out in the form of small voids, and the region becomes porous [11], Also, the plastic flow maintains a constant cross section in the diffusion zone because of compatibility stresses. These stresses induce dislocation multiplication and the formation of cellular dislocation structures in the diffusion zone. Similar dislocation structures are associated with high-temperature plastic deformation in the absence of diffusion [12-14]. 

Consider now the consequences of the pressure difference. If the membrane became free to move, it would move to the left, compressing the left chamber and expanding the right to equilibrate the pressure difference (Fig. 3.6a). However, if the membrane is constrained, the fluid may cavitate in the left chamber to relieve the low pressure, as in Fig. 3.66. This is analogous to the formation of voids in the Kirkendall effect. 

It can be concluded that the formation of the voids in the center of SnOi particles is mainly a result of the Kirkendall effect [8] associated with a faster outward diffusion of Sn atoms as compared to the inward diffiision of oxygen atoms in the process of the surface oxide layer formation. This produces high density of vacancies at the metal side of the metal/oxide interface. Vacancies transform to vacancy clusters which then aggregate into holes. It might be expected from this model that the increase of oxygen content in the ambiance will result in the promotion of an inward diffusion of oxygen atoms into the Sn particles, and therefore, suppress the formation of holes. 

Kirkendall Effect The Kirkendall effect is a phenomenon observed frequently in solid materials [38]. It refers to a vacancy counter diffusion process through an interface of two solid materials, metals in particular, to compensate the unequal material flow formation at the interface [38a]. In metals and metallic alloys, the vacancy is atomic defect, that is, empty lattice site. Combination of excess vacancies can lead to the formation of void within the fast-diffusion side of the interface [39]. While this phenomenon has been known for a very long time, synthesis of hollow nanostructures based on Kirkendall effect was realized fairly recently [40]. Ym studied the time evolution in the formation of hollow nanospheres and found that Kirkendall diffusion followed the Tick s law [41]. This means that the diffusion of atoms and vacancies is driven by the difference in atom concentration. Wu et al. synthesized hollow nanostructures of CoCuPt alloy catalyst by using Co nanoparticles as the sacrificial templates. For this trimetallic system, Co atoms diffused faster than those of Pt or Cu to form core-shell like Co CuPt hollow nanoparticles and then the CoCuPt hollow spheres (Fig. 2.10) [42]. 

There is no Kirkendall shift during phase formation, i.e. all vacancy fluxes go to the formation of Kirkendall (or Frenkel) voids instead of being annihilated by internal sinks hence, they do not causing the lattice shift. 

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