<p>The current study investigates the role of In on the compressive yield strength of a Sn-0.7 wt.% Cu (Sn-1.3Cu in at.%) solder alloy and examines the melting temperature–yield strength relation along with the underlying microstructural features. The addition of In to the Sn-1.3Cu alloy up to 5 at.% enhances the compressive yield strength from 20 MPa to 45 MPa, with a concomitant reduction in the melting point by 14°C, resulting in an inverse melting point–strength relationship. The enhancement of the compressive strength of the solder alloy containing an alloying concentration of In up to 1 at.% is attributed primarily to solid solution strengthening (<InlineEquation ID="IEq1"> <EquationSource Format="TEX">\({\sigma }_{ss})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>σ</mi> <mrow> <mi mathvariant="italic">ss</mi> </mrow> </msub> <mrow> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation>. For an intermediate In alloying concentration of 3 at.%, the Orowan precipitate strengthening <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\({(\sigma }_{Or}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mrow> <mo stretchy="false">(</mo> <mi>σ</mi> </mrow> <mrow> <mi mathvariant="italic">Or</mi> </mrow> </msub> </math></EquationSource> </InlineEquation>) mechanism owing to the presence of secondary-phase Cu₆Sn₅ is observed along with solid solution strengthening. For higher In content of 5 at.% in the Sn-1.3Cu solder, solute clustering (Cottrell atmosphere) is determined to be an effective strengthening mechanism. Moreover, the Sn-1.3Cu-5In alloy also exhibits strain softening behaviour which is attributed to dynamic recovery owing to its higher homologous temperature (T/T_m), which promotes dynamic recovery in the alloy.</p> Graphical Abstract <p></p>