Voids intermetallics

The stabilizing influence of small amounts of B (M/B > 0.25) in the voids of the metal host lattice varies with the mode of filling (partial or complete) of the interstitial, mostly O, sites and whether the compounds develop from the binary-intermetallic host lattice. The structures of B-rich compounds (M/B < 4) are mainly determined by the formation of regular, covalent B polyhedra (O, icosahedron) and the connections between them (B frame structures). Typical metal (M) borides therefore are found within a characteristic ratio of metal to boron 0.125 < M/B < 4. 

In case of electronic applications, where it is common to deposit copper and/or copper alloy and tin in sequence, with a nickel diffusion barrier layer, 0.5 /rm thick, between the layers present, no failure occurs. Without the nickel layers between bronze/copper/tin layers themselves, for instance, intermetallic brittle layer(s) and Kirkendall voids are formed, leading eventually to separation of the coated system and substrate. 

Direct space modeling may be an especially powerful tool in the case of intermetallic and related structures, many of which are derived from close packing of incompressible spheres. Thus, when positions of large atoms in the unit cell are known, the smaller atoms will likely occupy voids of sufficient size. 

In order to provide the required flame retardancy to the molding compound, an encapsulated formulation usually contains brominated resins and antimony oxide. The brominated resins used in the encapsulated formulation are mainly tetrabromobisphenol A (TBBA) based epoxy resin or brominated epoxy novolac. These bromine-containing additives were reported to cause bond degradation at high temperature through accelerated void formation in the gold-aluminum intermetallic phases (1-4). 

The areas concerning monolithic intermetallics which have been studied in recent years are (i) the formation of mctastable aluminas, and their transformation to stable a-alumina, (ii) the formation of interfacial voids and scale adherence and how these are influenced by reactive elements and sulfur, and (iii) accelerated oxidation at intermediate temperatures. Additionally the applications oriented areas of (iv) coatings, (v) oxidation of composites, and (vi) life predictions have received attention. 

Five different main classes are listed in Table 7a. Their characteristic splitting parts are Y, YlSibc, D, F and 1(41), respectively. For all these main classes of the 14-types no representative could be found. Several representatives of subclasses, however, exist and demonstrate the importance of these combinations of complexes and their voids. In Table 7b for the representative stmctures the values of the free coordinates are listed, to allow a survey about the deviations from the ideal structure. The structure types are listed here under the assumption that all atoms or atomic groups (polyhedra) like SO4, CO3, Hg2, H2O of a structure form the Bauverband this is probably adequate for intermetallic compounds like ThgMn23, for nitrides or sulftdes. On the other side one may consider also the framework of the 0 or S or N for itself as Bauverband and the whole structure as an addition compound. In this case these stmctures belong to other families than the 1-family. 

When the melt area solidified, voids were sometimes formed in the eutectic clusters (Fig. 3e). Voids also formed in the body of the intermetallic inclusion that was not converted into a eutectic (Fig. 3f). 

A new failure mechanism associated with this surface finish has been reported. - Referred to as planar microvoids or champagne voids, they are small voids in alignment atop the intermetallic layer of a solder-joint bond line (see Fig. 45.4). 

FIGURE 45.4 Cross-sectional micrograph of SAC EGA solder ball bonded to a silver surface finished PWB bond pad. Planar voids are clearly evident in the section and in the magnified inset. Voids in close adjacency above the intermetallic layer and the BGA ball can significantly detract from the solder-joint strength. (Courtesy of Hewlett-Packard). 

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)... 

Gale, R. J., Epoxy degradation induced Au-Al intermetallic void formation in plastic encapsulated MOS memories. Reliability Physics Symposium Proceedings, 22 (1984) 37. 

The main effect of addition of RE is to improve AI2O3 (or Cr203) scale adhesion. The most widely observed RE effect in intermetallics is suppression of interfacial void formation which otherwise limits contact between substrate and scale (e.g. Gig-gins et al., 1974 Smialek, 1978 Hindam and Smeltzer, 1980). The literature on void formation has been reviewed recently (Pint, 1997b), and it was concluded that the mech-... 

Failure mechanisms include phosphorus segregation, corrosion in immersion gold bath, brittle Ni-Sn intermetallics, and Kirkendall voids. 

Kirkendall Voids, Micro-voids at the interface between Ni(P)" and the Ni-Sn intermetallic were observed (Fig. 13) (Ref 93). These voids were thought to have been introduced by Kirkendall effects. It was proposed that the presence of high tensile stresses in the Ni-P" layer due to the volume reduction as a result of the phase change from the amorphous Ni-P to crystalline M3P in Ni-P, creates mud cracks in the Ni-P" layer. Propagation of these mud cracks along the Kirkendall voids results in the brittle fracture... 

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). 

Fig. 3.20 Micro crack in slip band emanating from the intermetallic phase apex on the surface of 2024-T3 aluminum alloy [15, 16]. The crack started from an elongated inclusion and reached a void left by a globular inclusion (reproduced with permission of ASM). All right reserved...

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