Effect of massive spalling on mechanical strength of solder joints in Pb-free solder reflowed on Co-based surface finishes

This study investigated the effect of massive spalling of intermetallic compounds (IMCs) on the microstructural evolution and mechanical properties of solder joints after multiple reflows. SAC305 solder was reflowed on substrates with four 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). CoSn3 was the main intermetallic compound (IMC) that formed in the systems, and it massively spalled in the systems with Pd and high P content. Systems with massive spalling have a high shear strength and low shear height owing to solid solution strengthening. Due to strain hardening, high shear velocity caused high strength in all systems. Massive spalling of CoSn3 transformed the failure mode from brittle to ductile and enhanced the shear strength of the joints. [Display omitted] •Massive spalling does not affect or reduce the strength of solder joints.•Adding Pd layer and high P content in electroless Co layer are the two main factors that induce massive spalling.•Discontinuous Co–Sn–P layer resulted in interdiffusion and reaction of Sn and Cu to form Cu6Sn5 intermetallic compounds.•Failure mode transformed form brittle to ductile after the occurrence of massive spalling.