The end of life vehicle (ELV) and the restriction of hazard substances (RoHS) directives have proposed the phase-out of lead in transportation vehicles and electrical devices by the years 2013 and 2014, respectively. These policies have stimulated the development of alternative high-temperature lead-free solders, as necessary replacements for the conventional Sn-Pb solders. For the development of a successful high-temperature lead-free solder, a comprehensive understanding of the formation and deformation behaviour of intermetallic compounds (IMCs) is essential because the IMCs layer continues to grow during service and may initiate cracks. Unfortunately, detailed knowledge on the formation, and thermo-mechanical properties of the IMCs at elevated temperature, especially in high-temperature lead-free solder joints, is lacking.
The continuous demand on the performance of modern electronic devices has increased the current density through solder joints and is accompanied by Joule heating of the joint. Moreover, the recent progress of 3 dimensional (3D) packaging technologies of modern electronic products has led to an increase in the volume fraction of IMCs in solder joints, to such a degree that the solder alloys are completely consumed and a solder joint is composed of only a few grains of IMCs. More commonly, solder balls of 500µm diameter or less are used in arrays for the mechanical and electrical connection between printed circuit boards (PCB) and integrated circuits (ICs). In this case the IMCs are the major feature of the microstructure at the solder/substrate interfaces of a solder joint. In both scenarios, the properties such as the relationship between crystal structure and thermo-mechanical properties of the IMCs must be understood to ensure the in-service reliability of these micro joints.
Cu6Sn5 is the most important and common IMC in electronic packaging and forms during the interface reactions between most Sn-based solders and Cu substrates. The Cu6Sn5 plays a critical role in the crack formation and propagation at the solder/substrate interfaces in the lead-free solder of electrical devices. The Cu6Sn5 also determinately affects the deformation of micro-bumps in recent high-density 3D electrical packages, which have higher operating temperatures and large volume fractions of Cu6Sn5 in the interfacial microstructure. In this research, the thermo-mechanical properties and growth orientation of Cu6Sn5 subject to the effect of trace additions of Ni was studied. The effects of hexagonal to monoclinic polymorphic phase transformation on the thermal expansions of Cu6Sn5 and (Cu,Ni)6Sn5 were characterised using dilatometry in addition to laboratory and synchrotron X-ray diffraction (XRD). It was found that there is a volume shrinkage during the hexagonal to monoclinic polymorphic phase transformation. The Ni addition was found to stabilize the hexagonal Cu6Sn5 in a temperature range of -100 to 250 oC and preferentially reduce the thermal expansion along the a-axis of Cu6Sn5 unit cell.
Using nanoindentation and energy dispersive spectrometer (EDS) techniques, the relationship between temperature, Ni content and the mechanical properties and creep of Cu6Sn5 were elucidated. The elastic modulus and hardness of Cu6Sn5 decreased linearly and exponentially, respectively, as temperature increased. The addition of Ni increased the elastic modulus and hardness, but showed variable effects on the creep of Cu6Sn5, at room and elevated temperatures.
The morphology, crystal orientation and mechanical properties of Cu6Sn5 and (Cu,Ni)6Sn5 intermetallics, formed between Sn-3/4/7Cu-0/0.05/0.1/0.3Ni (wt%) hyper-eutectic high-temperature lead-free solder alloys and poly-crystal Cu substrates, were investigated using scanning electronic microscope (SEM), X-ray diffraction (XRD) pole figures and nanoindentation. The Cu6Sn5 formed in hyper-eutectic Sn-Cu/Cu solder joints was found to have a (001) growth texture and trace addition of Ni was observed to alter the (001) growth texture to a (101) growth texture of Cu6Sn5.
Taking the crystallographic effects on mechanical properties of Cu6Sn5 into account, the mechanical properties, creep and crack patterns on different crystal planes of directionally solidified Cu6Sn5 and (Cu,Ni)6Sn5 were systematically investigated using electron back-scattered diffraction (EBSD) and nanoindentation techniques. Strong anisotropy in elastic modulus, hardness and creep of Cu6Sn5 was observed. The Ni addition was found to have limited effects on the anisotropy in elastic modulus, but make the hardness and creep of Cu6Sn5 more isotropic and increase the crack resistance of Cu6Sn5. Therefore, this research aims to address some critical issues of fundamental importance to understand the deformation of high-temperature Sn-Cu/Cu solder joints, as well as the micro-bumps in 3D integrated circuits.