Effects of Cooling Rate on Creep Behavior of a Sn-3.5Ag Alloy F. OCHOA 1 , X. DENG 1 , and N. CHAWLA 1,2 1.—Department of Chemical and Materials Engineering, Ira A. Fulton School of Engineering, Arizona State University, Tempe, AZ 85287-6006. 2.—E-mail: nchawla@asu.edu The effect of cooling rate on microstructure and creep behavior of bulk, eutectic Sn-3.5Ag solders was studied. The cooling rate is an important processing vari- able that significantly affects the microstructure of the solder and therefore determines its mechanical behavior. Controlled cooling rates were obtained by cooling specimens in different media: water, air, and furnace, which resulted in cooling rates of 24°C/s, 0.5°C/s, and 0.08°C/s, respectively. The cooling rate de- creased the secondary dendrite arm size and the spacing of the Sn-rich phase, as well as the morphology of Ag 3 Sn. The Sn-dendrite arm size and spacing were smaller at fast cooling rates, while slower cooling rates yielded a nearly eutec- tic microstructure. The morphology of Ag 3 Sn also changed from relatively spherical, at faster cooling rates, to needlelike for slower cooling. The effect of cooling rate on creep behavior was studied at 25°C, 60°C, 95°C, and 120°C. Faster cooling rates were found to increase the creep strength of the solder due to the refinement of the solder microstructure. Stress exponents, n, indicated that dislocation climb was the controlling mechanism. Activation energies, for all cooling rates, indicated that the dominant diffusional mechanism corre- sponded to dislocation pipe diffusion of Sn. Grain boundary sliding (GBS) mea- surements were conducted, using both scanning electron microscopy (SEM) and atomic force microscopy (AFM). It was observed that GBS had a very small contribution to the total creep strain. Key words: Sn-3.5Ag alloy, creep behavior, cooling rate Journal of ELECTRONIC MATERIALS, Vol. 33, No. 12, 2004 Special Issue Paper 1 (Received March 9, 2004; accepted June 25, 2004) INTRODUCTION The microstructure and mechanical properties of solders are very sensitive to processing and geomet- rical parameters that affect solidification conditions. Among these processing parameters, the cooling rate has a significant effect on the microstructure and mechanical behavior of the solder. 1–5 For exam- ple, the size, geometry of the solder, and location in the electronic package will have an effect on cooling rate. 1,2 Thus, understanding the effects of cooling rate on microstructure and mechanical behavior of solders is extremely important. Furthermore, such an understanding may be used to tailor solder microstructure and optimize the mechanical prop- erties. Several studies have examined the effects of cooling rate on microstructure and mechanical prop- erties of Pb-Sn solders. 6–8 Not as much is known about the effects of cooling rate on microstructure, tensile, and creep behavior of Sn-rich solders. 9–11 Additionally, there appears to be a lack of consensus on the fundamental deformation mechanisms and effects of cooling rate on the tensile and creep behavior of the Sn-rich solders. 1,2,9–16 This study presents a systematic evaluation of the effects of controlled cooling rates on creep behavior of a Sn-3.5Ag solder. A large portion of this study was directed to the understanding of the effects of cooling rate on the creep behavior of the solder, as well as to explaining the fundamental deformation mechanisms taking place during creep. Creep stress exponents, activation energy for creep, and micro- structural observations were used to elucidate the deformation processes taking place during creep of these materials. In addition, the role of phase/ GBS during creep deformation was studied by scan- ning electron microscopy (SEM) and atomic force microscopy (AFM). The contribution of grain bound- ary sliding (GBS) to the total creep strain was also evaluated.