The effects of Bi substitution for Sn on mechanical properties of Sn-based lead-free solders

Abstract

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 (${{\varvec{\sigma}}}_{{\varvec{f}}})$ 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 (${{\varvec{\varepsilon}}}_{{\varvec{f}}})$ dropped from 30 to 10% which was also evidenced by smaller percentage reduction in area (%RA). The ${{\varvec{\sigma}}}_{{\varvec{f}}}$ 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 ${\varvec{\beta}}$-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 > ${10}^{-2}{\boldsymbol{\AA }}^{-1}$) 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.