Due　to　its　high　Young＇s　modulus　and　strong　bond　with　Sn,　Co　has　been　considered　as　one　of　the　most　promising　alloying　elements　to　improve　the　strength　and　reliability　performance　of　Pb-free　solders.　In　this　study,　the　microstructual　characteristics　and　mechanical　properties　of　Sn-3.0Ag-0.5Cu　+　XCo　(X　=　0　wt.%,　0.1　wt.%,　0.2　wt.%,　0.45　wt.%,　and　1.0　wt.%)　solder　joints　on　Cu　substrates　were　investigated.　Moreover,　an　exploration　on　different　joint　fabrication　methods　and　undercooling　of　solder　alloys　with　both　solder　preform　and　paste　was　carried　out.　Experimental　results　indicated　that　the　undercooling　of　the　solder　alloy　can　be　altered　by　Co　additions.　During　the　metallurgical　process,　the　added　Co　can　dissolve　into　the　solder　matrix　before　rolling　the　preform.　In　addition,　Co　can　form　reinforcement　particles　in　the　solder　paste.　The　tendency　for　the　formation　and　growth　of　an　intermetallic　compound　(IMC)　layer　can　be　enhanced　after　incorporation　of　Co　elements　into　the　preform.　However,　a　proper　amount　of　Co　addition　to　the　paste　can　suppress　IMC　growth.　The　shear　strength　of　solder　joints　reinforced　with　0.1　wt.%　or　0.2　wt.%　Co　addition　can　be　increased.　The　solder　joint　made　by　paste　with　0.2　wt.%　Co　addition　exhibited　the　highest　shear　strength　among　all　the　solder　joints.