In　the　advanced　microelectronic　packaging,　Cu/Sn　joints　require　high-quality　diffusion　barriers　to　inhibit　Cu-Sn　intermetallic　compounds　(IMCs)　growth　and　improve　the　interface　brittleness.　Cobalt-Phosphorous　(Co-P)　alloy　coatings　are　ideal　candidates　due　to　their　good　wettability,　promising　diffusion　resistance　and　low　interfacial　brittleness.　In　this　work,　the　microstructural　evolution　of　IMCs　between　crystalline/amorphous　Co-P　coatings　and　lead-free　solders　during　the　solid-state　diffusion　were　systemically　investigated.　The　microstructure,　phase　distribution,　and　grain　characteristics　of　coatings　and　IMCs　were　carefully　characterized　by　SEM,　EPMA,　EBSD,　and　TEM.　It　was　found　that　the　consumption　rate　of　crystalline　Co-P,　21.2　nm/h,　was　significantly　slower　than　that　of　amorphous　Co-P,　96.4　nm/h,　and　exhibited　an　excellent　diffusion　resistance,　which　was　attributed　to　the　nanocrystalline　structure　and　the　P-rich　layer　which　performed　the　diffusion　barriers　for　Sn　and　Cu　atoms.　Compared　with　Ni,　amorphous　Co-P　coatings　could　better　improve　the　interfacial　brittleness　with　soft　and　ductile　Co-Sn　compounds　(hardness　of　3.0　GPa)　growing　with　the　rapid　diffusion　of　Co,　and　the　toughened　Co-Sn-P　attributing　to　the　diffusion　of　Sn.　Kinetic　analysis　showed　that　the　growth　rate-controlling　process　of　Co-Sn　IMCs　was　jointly　controlled　by　interfacial　reaction　and　volume　diffusion,　and　the　effect　of　the　interfacial　reaction　is　more　pronounced　in　the　amorphous　system,　which　leads　to　coarse　Co-Sn　grains　that　are　beneficial　to　the　softness　and　ductility　of　IMCs.　Crystalline　and　amorphous　Co-P　coatings　can　be　applied　to　the　surface　treatment　for　die　pads　and　print-circuit-board　pads　according　to　their　own　characteristics,　respectively.(c)　2022　The　Author(s).　Published　by　Elsevier　B.V.　This　is　an　open　access　article　under　the　CC　BY-NC-ND　license　(http://creativecommons.org/licenses/by-nc-nd/4.0/).