Purpose　-　The　purpose　of　this　paper　is　to　investigate　the　creep　properties　of　Sn-0.7Cu　composite　solder　joints　reinforced　with　optimal　nano-sized　Ag　particles　in　order　to　improve　the　creep　performance　of　lead-free　solder　joints　by　a　composite　approach.　Design/methodology/approach　-　The　composite　approach　has　been　considered　as　an　effective　method　to　improve　the　creep　performance　of　solder　joints.　Nano-sized　Ag　reinforcing　particles　were　incorporated　into　Sn-0.7Cu　solder　by　mechanically　mixing.　A　systematic　creep　study　was　carried　out　on　nano-composite　solder　joints　reinforced　with　optimal　nano-sized　Ag　particles　and　compared　with　Sn-0.7Cu　solder　joints　at　different　temperatures　and　stress　levels.　A　steady-state　creep　constitutive　equation　for　nano-composite　solder　joints　containing　the　best　volume　reinforcement　was　established　in　this　study.　Microstructural　features　of　solder　joints　were　analyzed　to　help　determine　their　deformation　mechanisms　during　creep.　Findings　-　The　creep　activation　energies　and　stress　exponents　of　Ag　particle-enhanced　Sn-0.7Cu　lead-free　based　composite　solder　joints　were　higher　than　those　of　matrix　solder　joints　under　the　same　stress　and　temperature.　Thus,　the　creep　properties　of　nano-composite　solder　joints　are　better　than　those　of　Sn-0.7Cu　solder　joints.　Originality/value　-　The　findings　indicated　that　nano-sized　Ag　reinforcing　particles　could　effectively　improve　the　creep　properties　of　solder　joints.　A　new　steady-state　creep　constitutive　equation　of　nano-composite　solder　joints　was　established.　Deformation　mechanisms　of　Sn-0.7Cu　solder　and　nano-composite　solder　joints　during　creep　were　determined.