Impression creep testing and microstructurally adaptive creep modeling of lead free solder interconnects

Creep plays an important role in the reliability of solder joints under thermo-mechanical fatigue conditions encountered by a microelectronic package during service. In addition, the fine intermetallic precipitates (Ag₃Sn and/or Cu₆Sn₅) in the microstructures of the new lead-free solders (Sn-Ag and Sn-Ag-Cu) can undergo significant in situ strain-enhanced coarsening during TMC, resulting in in-service evolution of the creep behavior of the joints. Since there are significant microstructural/compositional differences between bulk solder samples and tiny microelectronic solder joints, it is critical to develop accurate creep testing methodologies on tiny life-sized solder joints and microstructurally adaptive constitutive creep models for the emerging Pb-free solder alloys. In this paper, we present creep data obtained from tests conducted on individual Sn₄Ag_0.5Cu ball grid array (BGA) solder balls attached to a packaging substrate, using a newly developed miniaturized impression creep apparatus, which affords high test throughput with minimal sample preparation. Coarsening of intermetallic particles is demonstrated to influence creep behavior in two ways. At low stresses, the creep rate increases proportionately with precipitate size. At high stresses, precipitate coarsening influences creep response by altering the threshold stress for particle-limited creep. Based on the experimental observations, a microstructurally adaptive creep model, which accounts for the effects of coarsening on the creep response of solder joints, and is capable of adjusting itself as solder joint microstructures evolve during service, is presented, along with experimental determination of the relevant coarsening kinetics parameters.