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Research Papers

On the Strain Rate- and Temperature-Dependent Tensile Behavior of Eutectic Sn–Pb Solder

[+] Author and Article Information
B. L. Boyce1

L. N. Brewer, M. K. Neilsen

 Sandia National Laboratories, P.O. Box 5800, Albuquerque, NM 87185-0889

M. J. Perricone

 Sandia National Laboratories, P.O. Box 5800, Albuquerque, NM 87185-0889; RJ Lee Group, Inc. 350 Hochberg Road, Monroeville, PA 15146

1

Corresponding author.

J. Electron. Packag 133(3), 031009 (Sep 23, 2011) (14 pages) doi:10.1115/1.4004846 History: Received September 14, 2010; Revised May 13, 2011; Published September 23, 2011

The present study examines the thermomechanical strain-rate sensitivity of eutectic 63Sn–37Pb solder over a broad range of strain-rates from 0.0002 s–1 to 200 s–1 , thus encompassing failure events between 1 h and 1 ms, at temperatures ranging from −60 °C to + 100 °C. A newly developed servohydraulic tensile method enabled this broad range of strain-rates to be evaluated by a single technique, eliminating ambiguity caused by evaluation across multiple experimental methods. Two solder conditions were compared: a normalized condition representing a solder joint that has largely stabilized ∼30 days after solidification and an aged condition representing ∼30 years at near-ambient temperatures. The tensile behavior of both conditions exhibited dramatic temperature and strain-rate sensitivity. At 100 °C, the yield strength increased from 5 MPa at 0.0002 s–1 to 42 MPa at 200 s–1 , while at −60 °C, the yield strength increased from 57 MPa at 0.0002 s–1 to 71 MPa at 200 s–1 . The room temperature strain rate-dependent behavior was also measured for the lead free SAC396 alloy. The SAC alloy exhibited thermal strain-rate sensitivity similar to Sn–Pb over this temperature and strain-rate regime. Microstructural characterization using backscatter electron imaging and electron backscatter diffraction showed distinct, morphological changes of the microstructure for different thermomechanical conditions as well as some systematic changes in the crystallographic texture. However, very little intergranular rotation was observed over the range of thermomechanical conditions, suggesting the dominance of a grain boundary sliding (GBS) deformation mechanism. Finally, a recently developed unified-creep-plasticity constitutive model for solder deformation was found to describe the observed behavior with much higher fidelity than the common Johnson–Cook model.

Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Experimental load train configuration to minimize inertial ringing effects

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Figure 2

Cylindrical threaded tensile bar design. Dimensions are in inches.

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Figure 3

Tensile stress-strain curves for (a) normalized and (b) normalized + aged conditions of eutectic Sn–Pb solder

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Figure 4

Room temperature strain rate-dependent property trends comparing the effect of aging condition. For comparison, a strain-rate sensitivity dataset from a companion Sn–Ag–Cu study is included.

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Figure 5

Tensile stress-strain curves for normalized + aged eutectic Sn–Pb solder at three different test temperatures

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Figure 6

Strain rate-dependent property trends comparing the effects of test temperature for the normalized + aged condition

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Figure 7

Comparison of the fracture surfaces for two extreme cases: (a, b) fast-cold: 200 s–1 at -60 °C and (c, d, e) slow-hot: 0.0002 s–1 at 100 °C

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Figure 8

EBSD crystallographic orientation maps representing the microstructure of eutectic Sn–Pb solder in two conditions: (a) normalized at 100 °C for 24 h and (b) normalized at 100 °C for 24 h then aged at 150 °C for 72 h.

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Figure 9

Left column: Orientation maps (inverse pole figure with respect to the tensile axis) for the Sn phase at three strain-rates; right column: Low angle (2 deg < θ < 10 deg) grain boundaries in white and high angle grain boundaries (θ ≥ 10 deg) for the Sn phase. Black regions indicate lead phase or unindexed pixels.

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Figure 10

Texture evolution (normalized samples) for the Sn phase as a function of strain-rate. M.R.D signifies the multiples of a random orientation distribution.

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Figure 11

Texture evolution (normalized samples) for the Pb phase as a function of strain-rate

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Figure 12

Comparison of microstructures for samples deformed at low temperatures and high strain-rates (top row) versus high temperatures and low strain-rates (bottom row). Left column: Orientation maps (inverse pole figure with respect to the tensile axis) for the Sn phase. Middle column: Low angle (2 deg < θ < 10 deg) grain boundaries in white and high angle grain boundaries (θ ≥ 10 deg) in black for the Sn phase. Right column: kernel average misorientation maps with grain boundaries superimposed. Color scale on KAM map is blue = 0 deg and red = 3.5 deg. Note that magnification of the top and bottom rows is not the same due to differences in phase size.

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Figure 13

Fits with Johnson–Cook plasticity model (Eqs. 1,2, solid lines) to the experimental stress–strain data (symbols). Numbers in figure represent nominal strain-rates.

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Figure 14

Fits with modified Johnson–Cook plasticity model (Eq. 4, solid lines) to the experimental stress–strain data (symbols). Numbers in figure represent nominal strain-rates.

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Figure 15

Zener–Holloman plot of the temperature compensated minimum or steady state strain-rate as a function of stress for Sn–Pb solder. Solid symbols are Stephens and Frear [31] data for 60Sn–40Pb and hollow symbols are our data for 63Sn–37Pb; solid line is Eq. 3.

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Figure 16

Comparison of unified creep plasticity model predictions (solid lines) with experimental data (symbols). Numbers in figure represent nominal strain-rates.

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