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<p> Low-temperature, lead-free solders mitigate heating-induced warpage caused by the  differences in coefficient of thermal expansion between printed circuit boards (PCBs), substrates,  and dies during package assembly. Eutectic and near-eutectic Sn-Bi solders are promising low  temperature candidates because they show high reliability at low strain rates during thermal  cycling. However, Sn-Bi low temperature solder (LTS) has poor performance at high strain rates  during drop-shock testing. Alloying additions such as Ag, Cu, and Sb have been shown to increase  the ductility and strength of eutectic Sn-Bi and therefore improve the overall reliability during both  thermal cycling and drop-shock. Small Sb additions to Sn-Bi LTS are of particular interest because  these additions significantly increase ductility while maintaining the tensile strength. This increase  in ductility was previously attributed to small SnSb intermetallic particles that form within the Sn  phase on the interface of Sn and Bi in 1.0wt% Sb containing samples. Despite the fact the no SnSb  intermetallic compound (IMC) particles have been found in 0.5Sb-42Sn-Bi samples in any  previous studies or in our own studies, it was thought that the SnSb IMC particles were responsible  for the improved reliability and ductility of Sn-Bi.  This work encloses our efforts to understand how small Sb additions to eutectic Sn-Bi  impact the solder microstructure and the resulting mechanical properties of the solder alloy. We  began by studying possible solidification pathways through phase diagram analysis in Thermo?Calc to understand how the microstructure is predicted to develop and compared these models to  the literature data. Next, we analyzed the microstructures of our custom Sb-containing alloys  through a combination of scanning electron microscopy (SEM), energy dispersive spectroscopy  (EDS), and electron probe microanalyzer-wavelength dispersive spectroscopy (EPMA-WDS) and  determined that no SnSb IMC particles were found in the 0.5Sb-42Sn-Bi alloy and at 0.5 wt% the  Sb remained in solid solution with Sn. Nanoindentation was then used to evaluate the strain rate  sensitivity of Sn-Bi LTS with Sb additions and we found that, while the alloy hardness remains  sensitive to different strain rates, the Sb in solid solution with Sn altered the deformation behavior  of the alloy and decreased the amount of planar slip during indentation. To study the stability of  the microstructure and the alloy behavior in use, shear testing was performed before and after  isothermal aging. Our results suggest that Sb in solid solution with the Sn-rich phase contributes  significantly to the changes in the eutectic microstructure and the mechanical properties. </p>

Conventional tin-lead solder alloys have been widely used in electronics interconnection owing to their properties such as low melting temperature, good ductility and excellent wettability on copper and other substrates. However, due to the worldwide legislation addressing the concern over the toxicity of lead, the usage of lead-containing solders has been phased out, thus stimulating substantial efforts on lead-free alternatives, amongst which eutectic Sn-Ag and Sn-Cu, and particularly Sn-Ag-Cu alloys, are promising candidates as recommended by international parties. To meet the increasing demands of advanced electronic products, high levels of integration of electronic devices are being developed and employed, which is leading to a reduction in package size, but with more and more input/output connections. Flip chip technology is therefore seen as a promising technique for chip interconnection compared with wire bonding, enabling higher density, better heat dissipation and a smaller footprint. This thesis is intended to investigate lead-free (eutectic Sn-Ag, Sn-Cu and Sn-Ag-Cu) wafer level solder bumping through electrodeposition for flip chip interconnection, as well as electroplating lead-free solderable finishes on electronic components. The existing knowledge gap in the electrochemical processes as well as the fundamental understanding of the resultant tin-based lead-free alloys electrodeposits are also addressed. For the electrodeposition of the Sn-Cu solder alloys, a methanesulphonate based electrolyte was established, from which near-eutectic Sn-Cu alloys were achieved over a relatively wide process window of current density. The effects of methanesulphonic acid, thiourea and OPPE (iso-octyl phenoxy polyethoxy ethanol) as additives were investigated respectively by cathodic potentiodynamic polarisation curves, which illustrated the resultant electrochemical changes to the electrolyte. Phase identification by X-ray diffraction showed the electrodeposits had a biphasic structure (β-Sn and Cu6Sn5). Microstructures of the Sn-Cu electrodeposits were comprehensively characterised, which revealed a compact and crystalline surface morphology under the effects of additives, with cross-sectional observations showing a uniform distribution of Cu6Sn5 particles predominantly along β-Sn grain boundaries. The electrodeposition of Sn-Ag solder alloys was explored in another pyrophosphate based system, which was further extended to the application for Sn-Ag-Cu solder alloys. Cathodic potentiodynamic polarisation demonstrated the deposition of noble metals, Ag or Ag-Cu, commenced before the deposition potential of tin was reached. The co-deposition of Sn-Ag or Sn-Ag-Cu alloy was achieved with the noble metals electrodepositing at their limiting current densities. The synergetic effects of polyethylene glycol (PEG) 600 and formaldehyde, dependent on reaching the cathodic potential required, helped to achieve a bright surface, which consisted of fine tin grains (~200 nm) and uniformly distributed Ag3Sn particles for Sn-Ag alloys and Ag3Sn and Cu6Sn5 for Sn-Ag-Cu alloys, as characterised by microstructural observations. Near-eutectic Sn-Ag and Sn-Ag-Cu alloys were realised as confirmed by compositional analysis and thermal measurements. Near-eutectic lead-free solder bumps of 25 μm in diameter and 50 μm in pitch, consisting of Sn-Ag, Sn-Cu or Sn-Ag-Cu solder alloys depending on the process and electrolyte employed, were demonstrated on wafers through the electrolytic systems developed. Lead-free solder bumps were further characterised by material analytical techniques to justify the feasibility of the processes developed for lead-free wafer level solder bumping.

<div> <div> <div> <p>Transient liquid phase sintering (TLPS) technologies are potential low-temperature solders for sustainable replacements of lead-based solders and high-temperature lead-free solders. Compared to solid-state sintering and lead-free solders, TLPS uses lower temperatures and is, thus, suitable for assembling temperature-sensitive components. TLPS is a non- equilibrium process and determining the kinetics is critical to the estimation of processing times needed for good joining. The tin-indium (Sn-In) system with a eutectic temperature of 119°C is being considered as the basis for a TLPS system when combined with tin. Most models of TLPS include interdiffusion, dissolution, isothermal solidification, and homogenization and are based on simple binary alloys without intermediate phases. The Sn-In system has two intermediate phases and thus the reaction kinetics require additional terms in the modeling. Differential Scanning Calorimetry (DSC) has been used to measure the response of Sn-In alloys during the transient liquid phase reaction. Preparation of tin indium alloys for microstructural analysis is challenging due to their very low hardness. This study uses freeze-fracturing of the tin indium alloys to obtain sections for microstructural analysis. The combination of DSC and microstructure analysis provides information on the reaction kinetics. It was observed that the solid/liquid reaction does not proceed as quickly as desired, that is, substantial liquid remains after annealing even though the overall composition is in the single-phase region in the phase diagram. </p> </div> </div> </div>