Critical Concerns in Soldering Reactions Arising from Space Confinement in 3-D IC Packages

Six critical issues relating to interfacial reactions arising from space confinement in 3-D integrated-circuit (3-D IC) packaging are presented in this paper. The first issue arises from the concern that intermetallics (IMCs) may occupy a large portion of the solder joint volume. It will be demonstrated that this concern is real even for Ni under bump metallurgy (UBM) or surface finish, which reacts very slowly with solders. The second issue relates to impingement and ripening of IMC grains. When IMCs occupy a large portion of a joint, the IMC grains growing from the opposite sides of a joint will eventually touch each other. The morphology evolution from this point on determines the final microstructure of a joint. Structural defects might form as a result of the impingement and ripening of IMC grains. The third issue is the rise of impurity concentration due to solder consumption. Many of the impurities in solders, such as those from electroplating, are not soluble in IMCs. As Sn reacts to form IMCs, these impurities are rejected from IMCs into the remaining solder. Consequently, the concentrations of these impurities increase with the progress of reaction. Eventually, these impurities are trapped between IMCs growing from the opposite directions. The impact of these trapped impurities on the properties of solder joints is an important issue. The fourth issue is similar to the third except that the role of impurities is replaced by inert alloy elements of solders. The most obvious element is Ag. The fifth issue arises from the fact that, as the size of a joint becomes smaller, the surface-area-to-volume ratio increases. This makes the impact of thin-film layers on UBM and surface finish become ever higher. One well-known element is Au. The so-called Au embrittlement issue may become relevant again. The last issue is the volume shrinkage from chemical reactions. Internal stress or structure defects may form because of this shrinkage. Experimental evidence will be used in this paper to illustrate these six issues, and the implications of these issues will be discussed.