The Interfacial Reaction And Dissolution Behavior Of Single Crystal Cu In Lead-free Solders

With the development of electronic package technology, the demand for finer pitch interconnections with higher current desities as well as increasing tension which lead to stringent requirement for reliability issues, on the other hand, due to the environmental concern of the traditional Pb-bearing and mandatory requirement by RoHS regulations, soldering technology encounter enormous challenge. All the lead-free solders are high tin-containing and high melt-point alloys comparing with the traditional Pb-bearing which lead to more substrate metal dissolution and thicker IMC layer formation than those of Sn-Pb alloys. So the formation of the intermetallic compound (IMC) and the dissolution of substrate which are integrated processes of the same overall mechanism of soldering reaction attract a lot of attention around the world. The formation of the IMC is desirable for interconnects but excessive metal dissolution and uncontrolled growth of the IMC is detrimental to the reliability of the joint. Because single crystal has no grain boundary, all the atoms grow along one direction, single crystal Cu was chosen as substrate in this study, which will provide insight into understanding the growth mechanism of IMC and the dissolution of substrate.In this dissertation, single crystal (001) Cu was used as substrate and pure Sn, Sn-0.7Cu, Sn-1.5Cu and Sn-3Cu were used as high-Sn solder. The morphology aspects of Cu6Sn5 intermetallic compound (IMC) grains formed between (001) single crystal Cu and as-mentioned high-Sn solder bath at 250℃and 300℃for different durations ranging from 10 s to 10 min were investigated. The results shown the prism-type interfacialη-Cu6Sn5 grains formed in Sn, Sn-0.7Cu and Sn-1.5Cu baths at 300℃in the early stage of reaction. Then the initial prism-typeη-Cu6Sn5 grains transformed into scallop-type morphology with the increasing reaction time at 300℃, but The higher the Cu content in the solder was, the earlier the transition occurred. Sn-3Cu is supersaturated with Cu at both 250 and 300℃. The Cu6Sn5 grains formed on Sn-3Cu/(001) Cu at 300℃and those in all the solders at 250℃displayed the common scallop-type. In a word, the interfacialη-Cu6Sn5 grains tended to be the prism-type with the increase of reaction temperature and the decrease of the Cu content in the solder, otherwise tended to be the scallop-type.Then, the morphological transition between prism-type and scallop-type was studied. The results shown the morphological transition between prism-type and scallop-type grains was reversible. The essence of nucleation and growth for prism-type is as same as the scallop-type grains.At last but not the least, the dissolution of single crystal (001) and (111) Cu in the pure Sn and Sn-0.7Cu baths were researched, and then the results were compared with those of polycrystalline Cu substrate. The studies shown that the dissolution of single crystal (001) and (111) Cu in the pure Sn and Sn-0.7Cu baths were faster than that of polycrystalline. The phenomenon was related with a step and kink dissolution model. In addition, the dissolution of single crystal (001) was faster than that of (111) Cu which was as a result of different surface erengy for single crystal (001) and (111) Cu. The growth kinetics of prism-type interfacialη-Cu6Sn5 grains which formed in pure Sn and Sn-0.7Cu was studied. The growth of prism-type interfacialη-Cu6Sn5 grains which formed in pure Sn and Sn-0.7Cu were fitted for t0.088 and t0.089, respectively.