In　recent　years,　environmental　concerns　have　attracted　more　and　more　attention　in　many　countries　due　to　poisonous　elemental　of　Pb　in　the　traditional　lead-based　solders.　Many　countries　are　about　to　prohibit　the　use　of　lead-bearing　solders　in　microelectronic　packaging.　Replacement　of　tin-lead　eutectic　solders　has　become　a　worldwide　trend.　Among　the　candidate　lead-free　solders,　SnAg　and　SnAgCu　are　the　foremost　candidates　because　of　their　non-toxic　nature　and　better　processing　performance.　Eutectic　Sn-3.5Ag　solder　shows　excellent　mechanical　properties　with　comparable　wetting　characteristics　with　SnPb　solders.　Its　higher　melting　point　makes　it　more　suitable　for　higher　temperature　applications.　Failure　of　microelectronic　packaging　most　likely　happens　due　to　the　poor　creep　and　thermomechanical　fatigue　properties　of　solder　alloy.　Published　papers　show　that　composite　solders　offer　improved　properties　as　compared　to　non-composite　solders.　Plating　Ni　on　substrate　is　commonly　used　for　preventing　the　interfacial　IMC　growth　in　solder　joints　serving.　Ni　reinforcement　particles　were　incorporated　to　solder　alloys　in　current　research　in　order　to　stabilize　the　microstructure.　In　this　study,　composite　solders　were　prepared　by　mechanically　dispersing　approximately　4um　size　Ni　reinforcement　particles　into　the　Sn-3.5Ag　solder　paste.　In　order　to　study　the　effects　of　volume　fraction　of　the　reinforcing　phases,　composite　solders　with　1,　3,　5,　8　and　10　volume　percent　Ni　particles　were　studied.　The　single　shear　lap　joints　were　fabricated　using　the　composite　solders.　Influence　of　Ni　volume　fraction　on　mechanical　properties,　wettability　and　microstructure　of　solder　joints　were　investigated.　The　morphology,　size　and　distribution　of　the　reinforcing　phases　were　characterized　metallographically.　The　experimental　results　show　that　shear　strength　of　solder　joints　were　increased　with　the　proportion　of　Ni　particles.　However,　beyond　5%Ni,　the　increasing　rate　slows　down　with　the　Ni　volume　fraction.　Calculation　shows　that　the　shear　strengths　of　composite　solder　joints　with　5　vol%　Ni　particles　could　increase　about　30%　as　compared　to　the　Sn-3.5Ag　solder　joint.　The　morphology　of　the　intermetallic　compounds　(IMC)　both　at　the　Cu-solder　interfaces　and　around　Ni　reinforcement　particles　were　analyzed.　The　spreading　areas　of　composite　solder　with　different　Ni　particles　additions　on　Cu　substrate　were　calculated.　Sn3.5Ag　composite　solder　reinforced　with　1%　Ni　particles　spread　into　the　most　circular　shape　and　its　visual　surface　qualities　were　better　as　compared　to　other　composite　solders　on　Cu　substrate.　The　spreading　areas　increased　more　than　15%　under　this　condition.　The　spreading　area　decreased　with　increasing　Ni　volume　fraction　beyond　l%Ni.　When　the　proportion　increased　to　5%,　the　spreading　area　of　composite　solders　dropped　back　to　that　of　Sn-3.5Ag　solder.　©　2006　IEEE.