On the influence of ag content on the creep behaviour of sn-ag-cu solder alloys

This thesis presents an investigation of the influence of silver content on the creep behaviour of ternary tin-silver-copper (SAC) solder alloys. To this end, the creep behaviour of Sn98.5Ag1.0Cu0.5 (SAC105) solder alloy was characterised and compared to that of Sn96.5Ag3.0Cu0.5 (SAC305). Both solder alloys were tested at the joint-scale (~180 μm) using specially designed specimens on a shear testing facility developed at the Stokes Institute. With the use of new lead-free solder alloys in electronic manufacturing processes, there are two critical considerations: reliability and cost. The reliability of electronic assemblies is linked to the mechanical performance of solder joints. SAC105 is more cost effective than SAC305 due to its lower silver content than SAC305, but it is known to exhibit inferior reliability to SAC305 under thermal fatigue. An Anand viscoplastic constitutive model is developed in this thesis for thermally preconditioned SAC105 from a series of creep tests under varying constant shear stresses (5MPa to 15MPa), constant shear strain rates (1E-6 (1/s) to 1E-2 (1/s)) and temperatures (20ºC to 100ºC), in order to increase the understanding of the creep behaviour of the alloy. An extensive comparison is made of the creep characteristics of SAC105 solder alloy to that of SAC305. The microstructure of both alloys is found to be a beta-Sn matrix with precipitates of Ag3Sn and Cu6Sn5; SAC305 is noted to feature more Ag3Sn precipitates and smaller Sn dendrites than SAC105, which are the primary reasons for its superior creep resistance. In terms of practical usage, SAC105 is found to be preferable for applications which may experience high strain rate stimuli (portable electronic devices) as the alloy undergoes plastic flow at lower stress levels than SAC305, minimizing the risk of interfacial failures. SAC305 is superior for applications which feature thermo-mechanical fatigue (desktop computers, for example, or servers) as it accumulates less strain than SAC105. SAC105 is found to be significantly less creep resistant than SAC305. With the Anand model parameters developed here, however, practitioners using finite element modelling can determine the suitability of the lower cost SAC105 alloy for applications which induce thermal fatigue.