In　advanced　microelectronic　packaging　technology,　the　interfacial　brittleness　of　Cu/Sn　solder　joints　puts　forward　higher　requirements　for　high-performance　diffusion　barriers.　Cobalt–Phosphorus　(Co-P)　alloy　coating　becomes　ideal　candidates　for　its　good　wettability　and　diffusion　resistance.　In　this　work,　for　Co-P　with　different　crystallinity,　the　barrier　properties　of　electrodeposited　Co-P　coatings　as　well　as　the　mechanical　properties　of　Co-Sn　intermetallic　compounds　(IMCs)　at　the　Co-P/Sn-Ag　interface　were　investigated.　According　to　the　XRD　results,　the　crystallinity　of　the　Co-P　coating　was　directly　related　to　the　phosphorus　(P)　content,　and　Co-P　were　classified　into　three　types:　crystalline　state,　mixed　state　and　amorphous　state.　The　morphology,　composition,　phase　characterization,　and　mechanical　properties　of　the　Co-Sn　IMCs　at　the　interface　were　investigated　by　SEM,　EPMA,　TEM,　and　nanoindentation　methods,　respectively.　After　180　°C　aging　test　for　120　h,　the　thickness　of　CoSn3　generated　at　the　interface　of　crystalline　Co-4.2　at.%　P/Sn-Ag　was　the　smallest　among　a　total　of　18　Co-P　coatings,　where　P　content　ranged　from　0.9　at.%　to　20.8　at.%,　showing　the　best　barrier　properties.　The　hardness　of　(Co,Cu)Sn3　at　the　amorphous　Co-P/Sn-Ag　interface　was　the　lowest　(3.2　GPa),　which　was　only　47%　of　that　of　Ni3Sn4,　exhibiting　the　lowest　brittleness　tendency.　The　mixed　crystalline-amorphous　Co-P　could　greatly　improve　the　diffusion　barrier　properties　while　sacrificing　a　small　degree　of　brittleness　comparing　to　the　amorphous　coatings,　and　had　the　potential　to　become　a　general-purpose　alloy　coating.　This　work　could　provide　a　scientific　basis　for　the　selection　of　Co-P　barrier　layers　with　excellent　barrier　properties　and　mechanical　properties　leading　to　different　application　scenario　in　electronic　packaging.　©　2022　Elsevier　Inc.