Interfacial voids formation, intermetallics

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-... 

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. 

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). 

It should be remembered that diffusion and reaction may continue during the deposition process, particularly if an elevated deposition temperature and long deposition times are used. For example, with aluminum on platinum, an Al-Pt intermetallic is formed and, as the intermetallic layer thickness increases, it removes the aluminum preferentially from grain boundaries at the Al/Al-Pt interface. This leads to void formation at the aluminum grain boundaries and the formation of capillary voids . As diffusion proceeds, the interfacial boundary becomes rough . 

Pb-Sn alloy (2) the electromigration of minor constituents such as fast diffusers, e.g., Cu or Ni. Both of these can be important. In the first case, electromigration of a major constituent can lead to voids or extrusions that can increase electrical resistance and produce open or short circuits [11-14]. When fast diffusing species are incorporated in the under bump metallurgy (UBM), fast diffusion can lead to the dissolution of the UBM materials, and the formation of interfacial intermetallic compounds layers that can lead to interfacial fracture, and ultimately, circuit failure [15]. Both of these failure modes have been observed. 

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