Grain Boundary Phase Control

It is not the attack of the matrix Si3N4, which controls aqueous corrosion but that of the grain-boundary phase. Often this grain-boundary phase is a oxidic silicate glass with or without small nitrogen contents. To predict the corrosion resistance in water, acids, and bases the reader is thus referred to the literature on glass and glass corrosion [118,119]. 

Nevertheless, some correlation s between the microstructure and the corrosion behaviour are known. The corrosion behaviour of silicon nitride materials in liquids is mainly controlled by the stability of the grain-boundary phase. Therefore the corrosion resistance of the silicon nitride materials in dilferent media can be altered by more than two orders of magnitude by changing the composition of the material. The corrosion behaviour can be organised into a few main classes (see Table 5). 

In general, compared with solid state sintering, liquid phase sintering allows easy control of microstructure and reduction in processing cost, but degrades some important properties, for example, mechanical properties. In contrast, many specific products utilize properties of the grain boundary phase and, hence, need to be sintered in the presence of a liquid phase. Zinc oxide varistors and SrTiOs based boundary layer capacitors are two examples. In these cases, the composition and amount of liquid phase are of prime importance in controUing the sintered microstructure and properties. 

During recent decades, extensive efforts have been made to control the grain boundary phase and to improve the heat resistance of silicon nitride, and this has led to significant improvements in high-temperature mechanical reliability. For example, some grades of commercial silicon nitrides have shown excellent creep resistance, even at temperatures above 1400 °C (45, 46]. Subsequent XRD analyses of these materials have revealed Lu2Si207 and Lu4Si2N207 as secondary phases. 

CO gas has no chance to produce YN Eq. (11) because the formation of YN is hindered when CO gas is introduced to the furnace (49). This grain boundary phase elimination reaction is not a diffusion controlled process because the rate of it is remarkably sensitive to sample atmosphere (e.g. with different using setter or gas purity) and time dependency of grain boundary phase elimination rate (Fig. 27). 

Schaefer, M. Foumelle, R.A. Liang, J.J. Theory for intermetallic phase growth between Cu and liquid Sn-Pb solder based on grain boundary diffusion control. J. Electron. Mater. 1998, 27, 1167. Gur, D. Bamberger, M. Reactive isothermal solidification in the Ni-Sn system. Acta Metall. Mater. 1998, 46 (14), 4917-4923. 

An interface is a surface that forms a common boundary between two bodies or phases. A number of different types of interfaces are discussed in this section. Some interfaces form as the result of a solder reaction with chip or chip carrier terminal metallizations. These interfaces separate layers of intermetallic compounds and form boundaries between solder and the unreacted terminal metal. Other interfaces exist between metal and insulation layers. A number of critical interfaces form between an underfill and chip, substrate and solder. The solder contains a number of interfaces including grain boundaries, phase boundaries, and boundaries between intermetallic particles and the solder. Fig. 25 illustrates interfaces that exist in a terminal made with Pb n solder, Ti-Cu-Au under bump metallurgy (UBM) mounted on a chip carrier with Ni Au metallization and reflowed in a hydrogen environment. These interfaces must perform a number of critical roles in the formation of solder joints. The interface supports large structure and composition differences between phases and also contains and controls electrochemical barriers [100]. 

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