Research Papers

Study of Electromigration-Induced Stress of Solder

[+] Author and Article Information
Fei Su

Institute of Solid Mechanics,
Beijing University of Aeronautics and Astronautics,
Beijing 100191, China
e-mail: sufei@buaa.edu.cn

Zheng Zhang, Yuan Wang, Weijia Li

Institute of Solid Mechanics,
Beijing University of Aeronautics and Astronautics,
Beijing 100191, China

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received March 1, 2014; final manuscript received December 22, 2014; published online January 19, 2015. Assoc. Editor: Kaustubh Nagarkar.

J. Electron. Packag 137(2), 021006 (Jun 01, 2015) (6 pages) Paper No: EP-14-1025; doi: 10.1115/1.4029463 History: Received March 01, 2014; Revised December 22, 2014; Online January 19, 2015

This study designed and produced a special microsolder specimen (Sn3.8Ag0.7Cu) to equalize current density under stressing. The specimen was generated to avoid temperature gradient and thermal migration. The inelastic deformation of the solder with electromigration (EM) alone was then measured with moiré interferometry. In addition, the EM-induced solder stress was evaluated using a finite element method (FEM). The precision of the FEM model was verified by comparing the simulated results with the experimental results with respect to EM-induced deformation. Findings indicated that the maximum spherical stress in the solder can reach 50 MPa. Moreover, the vacancy concentration is much higher on the cathode end than on the anode end. The simulation results can illustrate the failure mode of a solder and can therefore provide a basis for the comprehensive evaluation of solder reliability under EM.

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Fig. 1

Specimen for EM test: (a) sketch of the specimen, (b) solder microstructure after reflow, and (c) intermetallic compounds in the solder

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Fig. 2

Uniformity of the electrical current and temperature field of the solder. (a) FEM simulation result of the current density field and (b) temperature field by infrared camera.

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Fig. 3

Morphology of a solder after 240 hr of current stressing: (a) hillocks (Sn whiskers) of the solder, (b) details on the anode side, (c) details at middle of the solder, and (d) development process of a hillock

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Fig. 4

EM-induced inelastic solder deformation, the rectangle indicates the position and contour of the solder

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Fig. 5

Evolutions of normalized vacancy concentrations on the cathode and on the anode under current stressing at 300 A/mm2

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Fig. 6

Simulation of the EM-induced deformation and strain field of the solder after 1000 hr of current stressing at 300 A/mm2: (a) V field of the EM-induced inelastic deformation and (b) U field of the EM-induced inelastic deformation

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Fig. 7

Comparison of simulated and tested stress due to EM: (a) FEM simulation of EM-induced solder stress and (b) experimental findings of EM-induced stress through synchrotron X-ray microdiffraction [12]

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Fig. 8

Simulated compressive spherical stress caused by reduced diffusion mass flux in the middle of the solder



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