Structural grain boundary phase control

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]. 

SCC in alloys is controlled by the alloy sohd solution composition, grain boundary segregation, alloy phase transformation, duplex structure, and cold work. SCC metallurgy is discussed in detail in Chapter 9. 

Another possibility is that the embrittlement of Fe-12Mn was a result of the martensite transformation itself, by mechanisms analogous to those responsible for quench cracking in structural steels, but complicated by the intrusion of the martensite phase. If this hypothesis is correct, it should be possible to suppress the intergranular embrittlement with controlled cooling, to prevent the buildup of internal stresses along grain boundaries. 

The MCS reaction is a solid/gas reaction, so that it is natural that morphological effects of the solid components of the reaction should be important. Workers at Wacker defined a surface area parameter, QF, which relates the ratio of elongated to round structural forms. MCS promoters controlled by employing Si particles for a given surface area defined the ratio QF as the ratio of the area portion of intermetallic phases at grain boundaries to the area portion of intermetallic phases in primary silicon. These workers found that maximum Di was obtained for silicon particles having QF = 18-60. Furthermore, the rate was effected by QF, so that for CuO, ZnO, and Sn and Si with QF = 29.55, the... 

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