Effect of Ni, Ge and Bi alloying additions to the hot air solder leveling (HASL) coating properties

For several decades, Sn-Pb alloys have been extensively used as a solder coating for surface finish applications in the electronic packaging industry. However, due to toxicity concerns of lead (Pb) in eutectic Sn-Pb solders, researchers focussed on developing Pb-free solder coating for surface finish applications. This solder coating utilises solder material to insulate Printed Circuit Boards (PCB) from oxidation, serving two essential functions; protect the exposed copper and provide a solderable surface when soldering components. There are several surface finishes utilised in PCB manufacturing, with Hot air Solder Leveling (HASL) being a popular surface finish for PCB used for circuitry with significant components in heavy-duty applications and harsh environments. The surface finish applied onto the PCB resulted in improved solderability and corrosion resistance of the copper pads while also connecting the individual components. This study elucidates the relationship between solderability, free solder thickness, and intermetallics compound’s (IMC) thickness of the solder coating on a Cu substrate. An understanding of the interfacial reaction kinetics between the solder and the substrate is essential in order to allow for the maximisation of the service life of the solder joints in electronic assemblies. It is, therefore, important that the effect of long-term storage on manufacturability and reliability of a HASL surface finish be elucidated. In this thesis, a series of Sn-0.7Cu solder alloys, with the addition of Nickel (Ni), Germanium (Ge), and Bismuth (Bi) were fabricated, and the microstructure and soldering behaviours of these alloys were investigated. The results indicated that solderability is related to free solder thickness, where increased ageing time and temperature, the thickness of the free solder decreased due to growth of the interfacial intermetallic compound. The investigation also involved elucidating the performance of the solder alloy in the context of its thermo-mechanical properties and its influence upon the microstructure of the alloy. The samples were characterised using advanced techniques such as synchrotron in-situ imaging and synchrotron micro-XRF, where the former was used to analyse the development of microstructure encompassing the nucleation and growth behaviour of the primary intermetallics occurring in the solders, while the latter technique was used to determine the distribution of the primary intermetallics. The nucleation of primary Cu6Sn5 and β-Sn crystals in solder alloys Sn-0.7Cu-0.05Ni and Sn-0.7Cu-0.05Ni-1.5Bi were then explained. The results of this study demonstrate a detailed understanding of the fabricated of Sn-0.7Cu solder coating and the mechanism of microstructure formation. The results are of industrial significance and have significant implications in controlling the microstructure and improving the performance and reliability of Sn-0.7Cu solder coating for surface finish applications. In conclusion, the addition of Ni, Ge and Bi increases the solderability and improves the thermo-mechanical properties of the solder coating.