Interfacial Reaction Between Cu Substrate And Lead-free Solders Under Miniaturization Trend

Currently, with the rapid development of3D packaging technology, the concerning flip-chip solder bumps decrease remarkably in dimensions. Under this miniaturization trend, the interfacial reaction in micro solder joints would alters during soldering and the afterward serving process, especially the proportion of interfacial IMC in micro bumps would greatly increased. As the assurance of reliable metallurgical bonding, the growth behavior of interfacial IMC is crucial to the reliability of solder joints. Meanwhile, the current and stress which micro bumps bear accordingly arises, which would challenge the solder joint reliability as well. Additionally, with the shrinkage of solder bump size, the effects of UBM grain size and crystalline orientation on the interfacial reaction become more significant, and varied microstructure states of UBM would lead to differing substrate dissolution and interfacial IMC growth behavior. While due to the requirement of lead-free in consumer electronic market, the usage of lead-free solders with higher melting point and Sn content further exacerbates the interfacial reaction in micro solder joints. Therefore, in this dissertation, the interfacial reaction between copper substrate and lead-free solders under miniaturization trend was investigated from two aspects.Firstly,(111) single crystal Cu was chose as the substrate metal to investigate its solid state interfacial reaction with three solder alloys, i.e. pure Sn, Sn-0.7Cu and Sn-0.7Cu-0.1Ni, in order to study the morphological evolution and growth kinetics of interfacial IMC. Results showed that prism Cu6Sn5grains formed at the as-soldered interface between (111) single crystal Cu and pure Sn or Sn-0.7Cu solder, and elongated along three directions with an intersection angle of60°. After aging for a short duration (10h) at150℃. the regular arrangements of Cu6Sn5prisms disappeared and were taken place by the random arranged Cu6Sn5facet scallops. This morphological change could be ascribed to the fast formation of Cu.^Sn at the primitive Cu6Sn5/(111) single crystal Cu interface, which broke the initial interface thermodynamic balance. Meanwhile, the addition of0.7wt.%Cu into the solder slightly stimulated the growth of interfacial IMC (Cu6Sn5and Cu6Sn) during aging. The morphology of interfacial IMC at the as-soldered interface significantly changed with0.1wt.%Ni additions, as well as its growth behavior during aging. On one hand the adding Ni improved the interfacial Cu6Sn5growth during solid/liquid interfacial reaction; on the other hand it retarded the growth of Cu6Sn5and inhibited the formation of Cu3Sn. Due to the higher affinity of Ni to Sn compared with Cu, the adding Ni gave an impetus to the nucleation of interfacial IMC and elevated the chemical stability of Cu6Sn5phase.Then, Cu/Sn/Cu fine pitch interconnect (about40μm) was prepared by the method of diffusion bonding. And the morphological evolution and growth kinetics of its interfacial IMC were studied during soldering at250℃for various durations. Results showed that Cu6Sn5scallops formed at the Cu/Sn interface and ripened with prolonging durations. The dependence of average grain number on duration/followed t-0.70relationship. During solid-liquid interfacial reaction, a different growth behavior of interfacial Cu6Sn5scallops presented along the direction vertical to the interface, compared to the direction parallel to it. And according to the variance of R/H, three growth periods were divided, i.e. the initial period, the ripening period and the thickening period. The growth of Cu6Sn5scallops was controlled by the grain boundary or channel diffusion, and its thickening kinetics was similar to volume diffusion controlled growth. While the growth of Cu3Sn was purely volume diffusion controlled.