Electrodeposition and characterisation of lead-free solder alloys for electronics interconnection

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.