Massive spalling and morphological change of intermetallic compound affected by adding Pd in Co-based surface finishes

This study investigates the interfacial reactions and microstructures of SAC305 solders on four different Co-based surface finishes, electroless Co (EC), electroless Co/immersion Au (ECIG), electroless Co/electroless Pd (ECEP), and electroless Co/electroless Pd/immersion Au (ECEPIG). The evolution of microstructure at different reflow conditions revealed that Pd not only changed the morphology of the intermetallic compound but also generated higher interfacial energy between solders and Co-based substrates. High interfacial energy and thick reaction phase could induce compressive stress within the (Co,Cu,Pd)Sn₃ layer during the reflow process after exhausting Co(W,P). The reduction of the stress could be the possible driving force of massive spalling of the (Co,Cu,Pd)Sn₃ intermetallic compound.