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Research Papers

Issues on Viscoplastic Characterization of Lead-Free Solder for Drop Test Simulations

[+] Author and Article Information
Etienne L. Bonnaud

Infineon Technologies Sweden AB, Isafjordsgatan 16, SE-164 81 Stockholm, Sweden; Department of Solid Mechanics,  Royal Institute of Technology, SE-100 44 Stockholm, Sweden

J. Electron. Packag 133(4), 041013 (Dec 19, 2011) (9 pages) doi:10.1115/1.4005451 History: Received June 29, 2010; Revised October 19, 2011; Published December 19, 2011; Online December 19, 2011

Reliable drop test simulations of electronic packages require reliable material characterization of solder joints. Mechanical properties of lead-free solder were here experimentally investigated for both monotonic and cyclic loading at different strain rates. With regards to the observed complex material behavior, the nonlinear mixed hardening Armstrong and Frederick model combined with the Perzyna viscoplastic law was chosen to fit the experimental data. This model was subsequently implemented into a commercial finite element code and used to simulate drop tests. Actual drop test experiments were conducted in parallel and experimental results were compared to simulations. Prediction discrepancies were analyzed and explanations suggested.

Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Test specimen dimensions; effective length was shortened to 22 mm and end diameters were increased to 18 mm. Black areas show the shape of the fixture designed to prevent the specimen from sliding out.

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Figure 2

Experimental true stress vs. true strain for monotonous loading up to complete failure at three strain rates (0.2/s, 1/s, 5/s). Necking sets in around ɛ = 0.1.

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Figure 3

Experimental engineering stress vs. engineering strain for cyclic loading (20 cycles) at 0.2/s; cyclic hardening between cycle 1 and 2 followed by cyclic softening between subsequent cycles

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Figure 4

Experimental envelopes (2nd and 20th cycles) of each of the 3 sets of 20 cycles at 0.2/s. A-B: cycles 2–20; C-D: cycles 22–40; E-F: cycles 42–60. Resting time between sets: 1 min.

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Figure 5

True stress vs. true strain for monotonous loading up to necking at three strain rates (0.2/s, 1/s, 5/s): experiment (dotted lines) and model prediction (solid lines)

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Figure 6

Engineering stress vs. engineering strain for cyclic loading (three first cycles) at strain 0.2/s: experiment (dotted lines) and model prediction (solid lines)

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Figure 7

Layout of the test component BGA 345. Solder joint ball diameter is 0.3 mm and pitch is 0.5 mm. Plastic strains are monitored at bottom left pad of each of the six components.

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Figure 8

Jedec drop test card; note that only the four corner holes are used for mounting on the drop test fixture. Due to symmetry, only six of the 15 component locations need to be monitored; numbering is used in Figs.  101112.

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Figure 9

Quarter FEM model for drop test simulations. According to standards, components are placed underneath the PCB. The bottom plate consists of a single rigid element.

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Figure 10

FEM simulations of displacements in solder joints (locations 1–6 are defined in Fig. 8)

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Figure 11

Armstrong and Fredrik model: FEM simulations of effective plastic strains in solder joints (locations 1–6 are defined in Fig. 8)

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Figure 12

Armstrong and Frederick model: comparison of the peeling stress σz at location 3 (dotted line) where failure occurs in practice and location 6 (dashed line) for which the simulated stress is maximum

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Figure 13

Comparison of an extrapolated experimental curve (dotted line) with the model prediction (solid line) for a strain rate of 200/s strain rate. The discrepancy shows the inability of the model to predict results outside the curve fitting range.

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Figure 14

(a) Cross-section of SnAgCu solder on a Cu pad with intermetallics (IMC) in between. (b) Cracks in SnAgCu run between the Cu layer and the intermetallics (IMC).

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Figure 15

(a) Cross-section of SnAgCuNi solder on a Cu pad with intermetallics (IMC) in between. (b) Cracks in SnAgCuNi run between the intermetallics (IMC) and the solder material.

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