In　advanced　chip　packaging,　the　miniaturization　of　solder　joints　has　increased　the　requirements　for　diffusion　barrier　layers.　Cobalt-phosphorus　(Co–P)　coatings,　with　superior　solderability　and　diffusion　resistance,　emerge　as　an　ideal　alternative　to　under-bump　metallization　(UBM).　However,　the　absence　of　systematic　research　on　the　optimal　thickness　of　Co–P　coatings　hinders　further　solder　joint　miniaturization.　This　study　investigated　the　diffusion　resistance　of　various　UBMs,　focusing　on　how　the　thickness　of　nanocrystalline　Co–4P　coating　affects　their　diffusion　resistance　and　mechanical　properties.　SEM,　EPMA,　and　shear　test　were　used　to　investigate　the　morphology,　composition　of　intermetallic　compounds　(IMCs),　and　mechanical　properties　in　solder　joints.　First-principles　calculations　were　used　to　study　the　interface　bonding　of　Co–P/Sn–Ag　and　the　effect　of　Cu　diffusion　concentration　on　it.　After　aging　at　180　°C　for　15　days,　nanocrystalline　Co–4P　with　a　thickness　of　2.6　μm　exhibited　enough　diffusion　resistance,　which　inhibited　the　excessive　generation　of　brittle　Cu–Sn　IMCs　and　enhanced　the　mechanical　properties　in　joints,　which　is　a　UBM　alternative　with　high-performance,　economy,　and　resource-saving.　The　simulation　results　showed　that　the　bonding　properties　at　the　Co–P/Sn–Ag　interface　were　better　than　that　of　Ni/Sn–Ag　and　Cu/Sn–Ag.　When　Cu　diffusion　was　below　9.375　at.%,　the　limited　Cu　diffusion　due　to　Co–4P　coating　thinning　had　minimal　impact　on　interface　bonding,　which　was　according　to　EPMA　and　shear　test　results.　This　work　provides　a　scientific　basis　for　the　application　of　ultra-thin　nanocrystalline　Co–P　coatings　and　offers　prospective　guidance　for　coating　design　in　package　solder　joints.　©　2025　The　Authors