Interfacial Reactions Between Single Crystal Cu (Ni) Substrates And Lead-free Solders

The reliability of solder joints is an important issue in chip package technology. With the world wide trends of miniaturization of consumer electronics and lead-free solders, the solder joint will experience a harsher service environment. The thicknesses of the intermetallic compounds (IMCs) formed in solder joints have great effects on their reliability. The morphology of IMCs affects its growth kinetics dramatically. Thus, it is important to research the morphological evolution of IMCs and their growth kinetics to effectively control their thicknesses.The pad size will shrink with the decreasing of the solder joint size. Thus, in this study, the single crystal metal substrates were used to investigate the interfacial reactions in ultra fine solder joint condition (only one grain in the pad). Furthermore, it is essential to study the interfacial reactions between single crystal substrates and lead-free solders to explore the nature of the interfacial reactions.In this dissertation, (001) single crystal Cu was used to investigate its solid/solid interfacial reactions with Sn-xCu (x=0,0.7) solders at both high (150℃) and low (20℃and 40℃) temperatures. The effects of Cu concentration and aging temperature on the morphological evolution of Cu6Sn5 and growth kinetics of IMCs were studied. The results showed that the as-soldered Cu6Sn5 grains exhibited the prism-type morphology. A Cu3Sn layer appeared in short time aging (less than 10 h) at 150℃. During 150℃aging, the edges of Cu6Sn5 grains turned flat, then to a scallop-type with increasing aging time, and its thickness decreased first and then increased. The thickness of Cu3Sn and total IMC (Cu6Sn5+Cu3Sn) both increased with increasing aging time. The growth kinetics of total IMCs (Cu6Sn5+Cu3Sn), Cu3Sn and stable period of Cu6Sn5 all followed parabolic rules. In a short aging time (less than 10 h), the thickness ratio of Cu3Sn/total IMC reached a stable value, which was 0.68-0.74 for pure Sn solder and 0.58-0.68 for Sn-0.7Cu solder. The addition of 0.7 wt.%Cu dramatically deceased the growth kinetic of Cu3Sn, but increased the growth kinetic of Cu6Sn5; furthermore, the addition of 0.7 wt.%Cu had little influence on the growth kinetics of total IMCs. When being aged at low temperatures the Cu6Sn5 grains still remained the prism-type morphology after long time aging (3000 h for 20℃and 40℃for 500 h), The formation of Cu3Sn layer led to the morphological transition of interfacial Cu6Sn5 grain (from prism-type to scallop-type). (001) Ni single crystal was used to investigate its solid/liquid interfacial reaction with pure Sn, Sn-0.7Cu, Sn-0.7Cu-0.1Ni, Sn-1.5Cu and Sn-1.5Cu-0.1Ni solders at both 250℃and 300℃. The effects of solder content, reaction temperature and reaction time on the regular arrangement of interfacial (Cu,Ni)6Sn5 grains were studied. Results showed that the regular arrangement of (Cu,Ni)6Sn5 grains occurred in Sn-0.7Cu-0.1Ni, Sn-1.5Cu and Sn-1.5Cu-0.1Ni solders, while IMC grains appeared isotropic irregular arrangement. The rationality of the regular arrangement formed on (001) Ni single crystal was explained in terms of (1) the crystallographic orientation relationship between the interfacial (Cu,Ni)6Sn5 and (001) Ni and (2) the solubility limit of Cu in molten solders based on the Sn-Cu-Ni phase diagram. The regular arrangement of (Cu,Ni)6Sn5 grains faded with increasing dipping time.Preferred orientation with lowest misfit existed between single crystal substrate and interfacial IMC formed on it. IMC preferred to form aligning these directions because of the smallest nucleation driving force. The formation of a new IMC between the single crystal substrate and the initial IMC was the reason for the disappearance of regular arrangement.