Browsing by Author "Olofinjana, A"

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    The effects of Bi addition on the mechanical properties of eutectic Sn-Ag-Cu lead-free solder alloy
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Olofinjana, A; Shewchenko, L; Mata, JP; Haque, R
    Sn-Ag-Cu (SAC) solders are gaining momentum as the choice of Pb free electrical interconnect materials because of the legislative restrictions on the use of hazardous materials in electronic devices. However, SAC has high melting temperatures around 217oC. The role of Bi substitution to lowering solder joint processing temperatures are widely considered because of the low-temperature Sn-Bi eutectic. However, the embrittling effects of Bi addition require clarification on the maximum limits of substitution. In this work, the mechanical properties of eutectic SAC with gradual Bi substitution up to 10wt.% Bi was followed. It is shown that fracture strength (σ_f) increases from 50MPa plateauing at 60MPa between 1.4 and 1.8%Bi representing the limits of solid solution strengthening. Over this substitution range, strain at fracture (ε_f) dropped from 30% to 10% and reduction in area (RA) dropped from 80% to less than 5%. The σ_f was nearly 80MPa for 2%Bi and this increases gradually with %Bi concentration peaking at 93MPa for 7%Bi. Results of thermal analysis suggested that solidification went off eutectic after Bi concentration exceeded 2%. Using neutron diffraction techniques, the lattice parameter measurements suggest that the solubility limit of Bi in β Sn (in the multicomponent Sn-Ag-Cu) is about 2wt. %. The existence of Bi rich clusters was responsible for observed brittleness after 2wt.% Bi substitution. With the aid small angle neutron Scattering (SANS) and ultra-small angle neutron scattering (USANS), it was found that the intensity changes with respect to scattering factor (Q) especially for high Q region when the precipitations sizes are below 12.5 nm after concentration exceeded 2%Bi that confirms the existence of primary nanosized Bi precipitates that starts to grow into well-defined Bi phases for higher Bi content.
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    The effects of Bi substitution for Sn on mechanical properties of Sn-based lead-free solders
    (Springer Nature, 2021-08-02) Raza, M; Shewchenko, L; Olofinjana, A; Kent, D; Mata, JP; Haque, R
    Pb-free solders are gaining ground as the optimum choice for electrical interconnect materials, however, their higher melting temperature around 217 °C is still an issue that restricts wider adoption. The potential to employ Bi substitution for Sn to lower solder joint processing temperatures has been widely considered. In this work, the mechanical properties of eutectic SAC with gradually increasing Bi substitution up to 10 wt% Bi was studied. It is shown that fracture strength (σf) increases with Bi additions from 50 MPa plateauing at 60 MPa between 1.4 and 1.8% Bi which represents the limits of solid solution strengthening. Over this substitutional range, strain at fracture (εf) dropped from 30 to 10% which was also evidenced by smaller percentage reduction in area (%RA). The σf was nearly 80 MPa for 2% Bi increasing gradually with increasing Bi concentrations and peaking at 93 MPa for 7% Bi whilst maintaining 10% elongation at fracture. X-ray diffraction and DSC thermal studies suggests that the solubility limit of Bi in β-Sn (in the multicomponent SAC) is less than 2 wt% Bi. With the aid of small-angle neutron Scattering (SANS) and ultra-small-angle neutron scattering (USANS), it was found that the scattering intensity changes for alloys with Bi content in the range 0.8 – 1.5wt% compared to ternary SAC with less than 0.8% Bi at low scattering factors (Q > 10 - 2Å - 1) signifying microstructural differences at length scales of the order of 10–100 nm. There were no differences observed in scattering for alloy samples with more than 2 wt% Bi. © 2021, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.