Integrated　circuit　systems　are　always　suffering　the　temperature　cycling　environment　caused　by　frequent　power　on　and　off.　Mismatch　in　thermal　expansion　coefficients　among　chip　package　components　will　cause　severe　cyclic　stress　to　the　solder　joints.　During　thermal　fatigue,　β-Sn　grains　tended　to　generate　thermal　fatigue　cracks　at　the　grain　boundaries.　This　work　proposed　a　strategy　of　using　amorphous　Co-15　at.%　P　coating　as　the　interface　layer　to　inhibit　the　initiation　and　propagation　of　thermal　fatigue　cracks　and　improve　the　shear　strength　of　solder　joints　by　numerous　endogenous　CoSn3　laths.　Mechanistically,　based　on　the　amorphous　structure　of　the　Co-P　coating,　Co　atoms　would　massively　and　rapidly　diffuse　into　the　molten　solder　during　the　liquid-solid　diffusion　stage　of　reflow　soldering,　and　the　eutectic　solidification　together　with　β-Sn　spontaneously　precipitated　numerous　laths　of　CoSn3　intermetallic　compounds　(IMC),　which　were　evenly　distributed　and　became　the　skeleton　of　the　solders.　The　skeleton-like　CoSn3　laths　with　high　elastic　modulus　were　the　reinforcing　phase　of　Sn-based　solder,　but　also　refined　the　grains　of　β-Sn.　The　refinement　of　β-Sn　grains　and　the　strengthening　of　CoSn3　laths　played　an　important　role　in　inhibiting　the　initiation　and　propagation　of　thermal　fatigue　cracks.　In　addition,　a　discontinuous　Co-Sn　IMC　layer　was　formed　between　the　amorphous　Co-P　and　the　solder.　The　alternating　appearance　of　CoSn3　and　relatively　soft　β-Sn　at　the　interface　avoided　the　generation　of　cracks　at　the　interface　during　temperature　cycling.　©　2024　Elsevier　Inc.