Chloride ions penetration into passive film

Fig. 7.5 Schematic representation of pit initiation by chloride ion penetration into passive film. Source Ref 6...

Three main mechanisms for passive film breakdown and pit initiation have been suggested in the literature through penetration, adsorption, or film breaking [20—22]. These mechanisms apply to pure metal systems because they do not consider second-phase particles in the passive film matrix, which very often initiates pitting. For example, as already discussed, dissolution of MnS inclusion at the MnS/matrix is the initial pit formation step in steel [15]. In the absence of chloride ions, the protective hydrated iron passive film slowly converts into dissolved ferric ions ... 

According to this theory, aggressive anions such as chloride penetrate into the passive film, under the influence of the electric field, where they exchange positions in the crystal lattice with ions (Figure 6.40(a)). This process is believed to modify the film properties, such as increasing its conductivity and facilitating its dissolution. 

In summary for anodic corrosion, water molecules permeate through the encapsulation material to reach the chip surface. Delamination between the lead frame and encapsulation material must accelerate moisture penetration into devices. Delamination between chips and encapsulation material allows water film formation. If halides, such as chloride ion are present in the water film, the corrosion of aluminum is accelerated. Corrosion takes place mainly on aluminum of the bonding pad, since the chip is usually coated with passivation film. But when passivation defects exist, moisture penetrates through them and aluminum corrosion of the wiring conductor occurs. Then improvements in adhesion, and lowering molding stress and chloride content are important. 

Penetration of chloride ions this mechanism [first discussed by Hoar et al. (1965)] involves, following the adsorption of Cl" on the passive film surface, the entry of Cl" into the film and its transport through the passive film to the metal/oxide interface, where it causes breakdown of the passive film. The accumulation of Cl" at the interface or the formation of metal chloride may cause the film breakdown. Support of this mechanism is provided by the observation of chlorides in the inner oxide part of the passive film on nickel (Marcus and Herbelin, 1993), Fe-Cr (Yang et al., 1994), and aluminum (Natishan etal., 1997). 

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