Abstract
The properties of interfacial intermetallic compound (IMC), including morphology, phase stability, mechanical properties and grain structure, significantly affect the reliability of solder joint in flip-chip technology (FCT) and micro-bump in 3D packages. The material selection for solder alloys and under bump metallurgies (UBMs) material is a critical issue to affect the microstructure, phase formation, and the mechanical strength of intermetallic compound. In this study, Ni-doped solder and Cu-Zn UBM were developed and applied on solder joints, micro-bump and transient liquid-phase (TLP) bonding. The effects of Ni and Zn on the IMCs and reliability are investigated and discussed. The combined effects of Ni and Zn on interfacial IMC in solder joints were first investigated. Doping Ni in the solder joint suppressed the growth of Cu3Sn and altered the morphology of the interfacial intermetallic compounds (IMCs) from scallop type to layer type. In comparison with the Cu substrates, the Cu-Zn substrates effectively suppressed the formation of Cu-Sn IMCs. It was revealed that the presence of Ni acted to enhance the effect of Zn on the suppression of Cu-Sn IMCs in SAC305-0.1Ni/Cu-15Zn solder joint. The efficient limitation of IMCs lead to that only layer-type (Cu,Ni)6(Sn,Zn)5 formed at joint interface before and after aging. Furthermore, it was notable that the SAC305-0.1Ni/Cu-15Zn solder joint exhibited good impact reliability after reflow and maintained the bonding strength during thermal aging for a long time. The formation of layer-type IMCs caused by Ni prevents the stress accumulation in IMCs under high speed impact test. Moreover, doping small amount of Zn into (Cu,Ni)6(Sn,Zn)5 inhibited the phase transformation (η-Cu6Sn5 to η’-Cu6Sn5) during aging and thus avoided the occurrence of stress in IMC caused by volume expansion. In SAC305-0.1Ni/Cu-15Zn solder joint, the properties of interfacial IMCs were simultaneously modified by Ni and Zn, resulting in the high bonding strength before and after aging. Thereupon, the advantages of Zn on IMCs suppression was extended to be used in micro-bumps of 3D packages. Due to the small size of micro-bumps, Cu-Sn intermetallic compounds (IMCs) rapidly form from the Cu pads. These brittle IMCs occupy the entire joint and weaken joint reliability. Moreover, Ag3Sn tends to precipitate inside the solder in large plates, which results in the accumulation of thermal stress due to a CTE mismatch between Ag3Sn and β-Sn. It wass demonstrated that doping Zn into one of the Cu substrates effectively suppressesed the growth of Cu-Sn and Ag-Sn IMCs. The mechanisms involved were explained in detail using thermodynamic theories and undercooling effects. To further study the Zn effects on IMC, the evolution of grain structures in IMCs in Zn-doped solder joints, micro-bumps, and TLP bondings were investigated. Due to the small size of micro-bumps and TLP bonding, the grain orientation of impinged intermetallic compounds (IMCs) tends to be homogenous and closely-grouped, allowing cracks to more easily propagate under impact experiences. Replacing the Cu substrate with Cu-Zn causes the grain orientation of Cu-Sn IMCs in both solder joints and micro-bumps to become more random and dispersed, which may hinder crack propagation and enhance the bonding strength of joint interface. Fancifully, in the Zn-doped TLP bonding composed of IMC, the randomly-orientated IMCs formed as interfolding pattern after reflow. According to the Hall-Petch theory, this interfolded structure may inhibit the move of dislocations and propagation of cracks. Besides, Zn in the TLP bonding efficiently retained the multi-orientation structure of Cu-Sn IMCs and suppressed the formation of Cu3Sn during lone-time aging. The formation mechanisms of randomly-orientated IMCs were explained in details using thermodynamic related theories, First-principle simulation, FE-SEM observation and EBSD analyses. In summary, Ni and Zn exhibits good ability on suppression, phase stabilization, refinement, toughness enhancement and grain structure modification of IMCs. All the results in this study provide the solutions for critical issues of reliability in solder joints, micro-bumps and TLP bondings. Therefore, Cu-Zn UBM and Ni-doped solder are expected to be two of the idea materials for stable interconnections in electronic packages.