Brittle　Cu-Sn　intermetallic　compounds　(I1VIC5)　are　detrimental　to　the　serviceability　of　solder　joints.　Cobalt-phosphorus　(Co　-P)　is　a　coating　choice　with　good　diffusion　barrier　properties.　In　this　work,　Co　-P　coatings　with　4　at.%　and　15　at.%　P　content　representing　crystalline　and　amorphous　were　electroplated　onto　the　Cu　pad　surface,　and　formed　Cu/Co-P/Sn-Ag/Co-P/Cu　joints　by　soldering.　Thermal　shock　test　was　carried　out　for　250　cycles　in　the　range　of　-50　degrees　C　to　150　C,　each　cycle　was　30　minutes.　Under　mechanical　and　thermal　fatigue　loading,　fatigue　cracks　appeared　at　the　interface　of　Cu/Co-4　at.　/0　P/Sn-Ag/Co-4　at.　/0　P/Cu,　while　the　interface　of　Cu/Co-15　at.　/0　P/Sn-Ag/Co-15　at.%　P/Cu　joint　can　keep　basically　stable.　This　work　reveals　the　IMCs　evolution　of　solder　joints　containing　crystalline/amorphous　Co　-P　coatings　during　mechanical-thermal　fatigue.　Combining　the　analysis　of　nanoindentation　test　results,　it　is　found　that　the　amorphous　structure　of　the　Co　-15　at.%　P　coating　leads　to　a　low-hardness　Co-Sn　IMCs,　which　is　the　key　to　maintaining　the　stability　of　the　joint　interface.　The　soft　Co-Sn　IMCs　bear　a　large　amount　of　plastic　deformation　in　the　fatigue　load,　and　improve　the　mechanical　properties　of　the　solder　joint　interface.