The Effect of Bismuth Content on Mechanical Properties of SAC+Bi Lead Free Solder Materials and Determination of Anand Parameters

The reliability of the electronic packages depend largely on the selection of suitable interconnect materials based on their mechanical properties. In selecting suitable alternative of Sn-Pb soldering materials, a number of different alloys were proposed as an alternative to the Sn-Pb solder. Most of these are Sn based solder where Sn is the main constituent along with one, two or even three other minor elements. If added in small amount, Bi can improve the wetting ability and reduce melting temperature of lead free solder alloys. It also increases strength of the bulk solder and inhibit the large Ag
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Sn formation in the bulk solder. The Anand viscoplastic constitutive model is a popular commercial finite element program built in to many viable FEA packages like ANSYS and ABAQUS. This model uses nine material parameters, and the reliability prediction results are often highly sensitive to the Anand parameters. In this investigation, we have studied the mechanical behavior, and reliability of several different SAC+Bi alloys with various levels of Bismuth. Our goal has been to identify optimal SAC+Bi alloy compositions for various applications and usage environments. The alloys considered were based on SAC305 and SAC405, with various levels of Sn replaced by Bi. The percentages of Bi considered include 1.0%, 2.0%, and 3.0%.In this work, the EDS method was used to determine the approximate chemical composition of the materials. Then uniaxial tensile stress-strain tests were carried out on this SAC+Bi specimens using a micro tension/torsion testing machine with three strain rates (0.001, 0.0001 and 0.00001 (1/sec)), and five different testing temperatures (T = 25, 50, 75, 100 and 125 °C). The temperature dependent mechanical properties of the various new SAC+Bi solders were measured including effective modulus, yield stress, and ultimate tensile strength, and then were compared with those for standard SACN05 (N = 1, 2, 3, 4) lead free alloys. Our findings show that solder alloys with Bi content showed better mechanical properties compared to SACN05, and as the Bi content decreased their strength also decreased correspondingly. The Anand parameters were calculated from each set of stress-strain data, and we used the constitutive model to predict the stress-strain curve at each particular temperature and strain rate used in the experimental testing to evaluate the goodness of fit of the constitutive model to the test data. Good correlation was observed between the experimental curves and the model predictions.