Mechanical Behavior of Eutectic Sn-Ag and Sn-Zn Solders
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HAREESH MAVOORI*, SEMYON VAYNMAN*, JASON CHIN**, BRIAN MORAN**, LEON M. KEER**, MORRIS E. FINE* * Dept. of Materials Science & Engineering ** Dept. of Civil Engineering Northwestern University, Evanston, IL 60208
ABSTRACT Eutectic Sn-Ag and Sn-Zn solders are currently being investigated as the basis for replacement of Sn-Pb solders. Some mechanical properties of these solder systems - fatigue, tensile, stress relaxation and creep are presented here along with some damage feature observations for eutectic Sn-Ag solder. Semi-empirical modeling, constitutive modeling and numerical simulation based on damage models have been carried out for lifetime prediction. Preliminary agreement of this numerical simulation with experimental results appears encouraging, but the models need further refinement. 1. INTRODUCTION Eutectic Sn-Ag and Sn-Zn solders are promising lead-free candidates for replacing eutectic SnPb solder in electronics. This work aims to establish a mechanical property database for these two lead-free solders along with methodologies for lifetime and reliability prediction. Since straincontrolled fatigue has been reported to be an important factor leading to interconnect failure in SMT [1], the fatigue tests presented here were run under total strain control. Bulk specimens have been tested under uniaxial loading conditions. It has been shown in the past that mixed mode failure is predictable in solders from pure mode loading by using the Von Mises strain criterion [2] or the energy formulation [3]. Physically, in uniaxial tensile and fatigue tests of these solders, most of the damage is seen to occur in localized shear bands oriented at approximately 450 to the tensile loading direction. Also, bulk specimen isothermal fatigue data has been found to correlate well with data from tests on simulated solder joints [3,4]. So, the results of this study are thought to be applicable to cases where loading is mixed, as in real electronic devices. 2. EXPERIMENTAL PROCEDURE Bulk, chill cast specimens were subjected to a standard heat treatment (150°C for 24 hrs followed by room temperature aging for 6-10 days) and tested on a servo-hydraulic machine under total strain control. Triangular strain waveforms were used for the fatigue tests without hold and trapezoidal waveforms for those with hold at maximum strain. A 50% drop in stress was used as the failure criterion as it correlates well with other more fundamental criteria for eutectic Sn-Ag and Sn-Zn [5]. Creep tests were done under constant load conditions. The thermomechanical fatigue (TMF) equipment developed at Northwestern University employs temperature control through the grips under the control of a programmable power controller. It is equipped with a heating capacity of 2100 Btu per hour and a cooling capacity of 6000 Btu per hour. The schematic is shown in Figure 1. This equipment was used for the elevated temperature tensile tests also.
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Mat. Res. Soc. Symp. Proc. Vol. 390 ©1995 Materials Research Society
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