High Temperature Creep Response of Lead Free Solders Abdullah Fahim, Sudan Ahmed, Md Mahmudur R. Chowdhury, Jeffrey C. Suhling, Pradeep Lall Department of Mechanical Engineering, and Center for Advanced Vehicle and Extreme Environment Electronics (CAVE3) Auburn University Auburn, AL 36849 Phone: +1-334-844-3332 FAX: +1-334-844-3124 E-Mail: jsuhling@auburn.edu ABSTRACT Lead free solder materials are susceptible to significant creep deformations in harsh high temperature environments including automotive, avionics, military, and oil exploration applications. In addition, dramatic degradations will occur in the creep responses of lead free solder alloys when they are exposed to long term isothermal aging during product applications at high temperatures. Such degradations in the creep compliance of the solder material are universally detrimental to reliability of solder joints in electronic assemblies. In this work, we have characterized the high temperature creep behavior of SAC405 (95.5Sn4.0Ag0.5Cu) lead free solder, which is the most creep resistant of the standard SACN05 alloys. In addition, we have studied the creep behaviors of two doped SAC solders, SAC_Q and Innolot, which have been previously shown to out-perform SAC405 in simple mechanical stress-strain tests at room temperature. Tensile specimens were formed in rectangular cross-section glass tubes using a vacuum suction process, and a water quenched (WQ) solidification profile was utilized to yield fine microstructures and the upper limits of the mechanical properties for each alloy. The samples were then aged for 10 days at room temperature to stabilize their microstructures. After aging, creep testing was performed at two different stress levels (10, 15 MPa) and several different extreme/high testing temperatures (T = 100, 125, 150, 175, and 200 o C). For each set of conditions, the creep performances of the three alloys were compared. The results showed that the doped SAC alloys were more resistant to creep at high temperatures. The creep rates of SAC_Q are roughly 50% of those for SAC405, while the creep rates of Innolot are roughly 33% of those for SAC405. It is likely that the dopants can significantly block the movement of dislocations and thus increase the creep resistance of these solders. KEY WORDS: Lead Free Solder, Creep, Aging, Strain Rate INTRODUCTION Creep becomes a dominant deformation mode in a solder material when it’s homologous temperature, T H = T/T Melt > 0.5. For Sn-Ag-Cu (SAC) lead free solders, this condition occurs at relatively low temperatures (e.g. room temperature, T = 25 o C, T H ≈ 0.6). Lead free electronics are often exposed to more severe high temperature environments including automotive, avionics, military, and oil exploration applications, where service temperatures can approach T = 150-200 o C (T H ≈ 0.85-0.95). Thus, significant creep will easily occur in such harsh environments. In addition, prolonged exposures at such high temperatures can lead to excessive aging induced changes to their microstructure and reductions in their mechanical properties and creep resistance. The literature on lead free solder materials has shown that aging is universally detrimental to their constitutive and failure behaviors [1]. In particular, large degradations have been observed in ball shear strength [2], elastic modulus [3], drop reliability [4], fracture behavior [5], microstructure [6], creep behavior [7-11], thermal cycling reliability [12-16], Anand model parameters [15-16], nanoindentation joint modulus and hardness [17-19], high strain rate mechanical properties [20], uniaxial cyclic stress-strain curves and fatigue life [21-22], and shear cyclic stress-strain curves and fatigue life [23-24]. Adding dopants such as Bi, Ni, In, Mg, Mn, Zn, La, Ce, Co, and Ti to lead free SAC alloys can improve the wettability, melting temperature, shock/drop reliability, creep properties, and microstructure [11]. For example, adding Bismuth (Bi) helps to reduce solidification temperature, increases strength by means of precipitation hardening, and also helps to reduce IMC (Intermetallic Compound) layer thicknesses in lead free solder materials [25]. The Effect of Bi on the mechanical properties of a SAC (Sn3.5Ag0.9Cu) alloy was investigated by Matahir and coworkers [26]. They reported that the shear strength increased with increasing Bi addition up to 2% (wt). Beyond that point, the shear strength decreased with increasing Bi%. The improved shear strength was attributed to the role of Bi on the morphology of the microstructure and distribution of dominant IMC (Ag 3 Sn). Reduction of strength at higher Bi content was due to the evolution of Bi rich phase and fragmentation of the IMC. Pandher, et al. [27] also reported that addition of up to 2% Bi in SAC alloys improved wetting and alloy spreading. Cai, et al. [11, 28] demonstrated that addition of 0.1% Bi in SAC0307 solder dramatically reduces aging effects such as material property degradations by a solid solution strengthening mechanism. Witkin [29] compared mechanical properties of SAC305 with two different Bi-doped alloys, and also concluded that the addition Bi can significantly reduce aging effects. He also showed that Bi is present in the microstructure as a separate phase or goes into solid solution with Sn, and does not form any intermetallic compounds with Sn, Cu or Ag. Reduction of IMC layer thickness after Bi addition has also been reported [30-31] 978-1-4673-8121-5/$31.00 ©2016 IEEE 1218 15th IEEE ITHERM Conference