Electroplating　crystalline　cobalt-phosphorus　(Co-P,　with　1　similar　to　8　at.%　P)　is　a　potential　interfacial　layer　for　electronic　packaging　solder　joints,　however,　the　optimal　content　of　P　has　been　lack　of　systematic　research.　In　this　work,　crystalline　Co-P　coatings　with　P　contents　of　2,　4　and　6　at.%　(Co-2P,　Co-4P,　Co-6P)　were　prepared　and　then　bonded　with　Sn-Ag　solder　to　form　Cu/Co-xP/Sn-Ag　solder　joints.　Within　aging　at　180　degrees　C　for　120　h,　the　diffusion　resistance　and　intermetallic　ductility　of　crystalline　Co-P　coatings　with　different　P　contents　were　systematically　studied.　The　microstructure,　phase　composition　and　mechanical　properties　of　Co-Sn　intermetallic　compounds　(IMCs)　were　comprehensively　characterized　using　XRD,　SEM,　EBSD　and　nano-indentation　techniques.　The　XRD　results　showed　that　the　Co-P　coatings　were　crystalline　structure,　with　obvious　sharp　diffraction　peaks　of　beta-Co.　The　Co-4P　system　exhibited　the　slowest　IMCs　growth　rate　and　the　best　diffusion　resistance　was　obtained　in　this　system,　and　the　kinetic　analysis　showed　that　the　Co-Sn　IMCs　growth　at　the　interface　was　controlled　by　interface　reaction　and　volume　diffusion.　It　was　found　that　CoSn3　was　generated　in　the　three　systems　during　the　aging　process,　while　CoSn4　and　Co-Sn-P　were　generated　only　in　Co-6P　system.　Compared　with　Ni-Sn,　Co-Sn　exhibited　significantly　lower　IMCs　hardness,　so　the　brittle　fracture　tendency　of　Co-P　solder　joints　was　lower.　With　the　increase　of　P　content,　the　hardness　of　Co-Sn　IMCs　increased　first　and　then　decreased,　ranging　from　3.20　to　4.94　GPa,　and　the　minimum　value　was　noticed　in　the　Co-6P　system.　Co-4P　coating　would　be　a　promising　barrier　layer　owing　to　its　good　diffusion　resistance,　Co-6P　coating　may　be　used　in　solder　joints　with　high　mechanical　load　owing　to　its　lowest　IMCs　hardness,　and　the　Co-2P　coating　can　maintain　a　balance　in　anti-diffusion　and　mechanical　properties.