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Research Papers

Effect of Intermetallic Compounds on the Thermomechanical Fatigue Life of Three-Dimensional Integrated Circuit Package Microsolder Bumps: Finite Element Analysis and Study

[+] Author and Article Information
Soud Farhan Choudhury

Department of Mechanical Engineering,
University of Connecticut,
Storrs, CT 06269-3139
e-mail: soud.choudhury@uconn.edu

Leila Ladani

Department of Mechanical Engineering,
University of Connecticut,
Storrs, CT 06269-3139
e-mail: lladani@engr.uconn.edu

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received November 3, 2014; final manuscript received August 31, 2015; published online September 25, 2015. Assoc. Editor: Jeffrey C. Suhling.

J. Electron. Packag 137(4), 041003 (Sep 25, 2015) (10 pages) Paper No: EP-14-1099; doi: 10.1115/1.4031523 History: Received November 03, 2014; Revised August 31, 2015

Currently, intermetallics (IMCs) in the solder joint are getting much attention due to their higher volume fraction in the smaller thickness interconnects. They possess different mechanical properties compared to bulk solder. Large volume fraction of IMCs may affect the mechanical behavior, thermomechanical and mechanical fatigue life and reliability of the solder interconnects due to very brittle nature compared to solder material. The question that this study is seeking to answer is how degrading IMCs are to the thermomechanical reliability of the microbumps used in three-dimensional (3D) integrated circuits (ICs) where the microsolder bumps have only a few microns of bond thicknesses. Several factors such as “squeezed out” solder geometry and IMC thickness are studied through a numerical experiment. Fatigue life is calculated using Coffin–Manson model. Results show that, though undesirable because of high likelihood of creating short circuits, squeezed out solder accumulates less inelastic strains under thermomechanical cyclic load and has higher fatigue life. The results show that with the increase of IMCs thickness in each model, the inelastic strains accumulation per cycle increases, thus decreasing the fatigue life. The drop in fatigue life tends to follow an exponential decay path. On the other hand, it was observed that plastic strain range per cycle tends to develop rapidly in Cu region with the increase in IMC thickness which calls for a consideration of Cu fatigue life more closely when the microbump contains a higher volume fraction of the IMCs. Overall, by analyzing the results, it is obvious that the presence of IMCs must be considered for microsolder bump with smaller bond thickness in fatigue life prediction model to generate more reasonable and correct results.

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Figures

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Fig. 1

(a) Microelectronic package, showing the cross section line, (b) SEM image of the cross sectioned area, (c) SEM image of the microsolder bump, revealing the bond structure, and (d) Cu material mapping of the bond area

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Fig. 2

(a) Schematic top view of the package, (b) quarter model, showing the materials used in the model, (c) the global meshed model, and (d) local meshed model with the materials considered in the analysis

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Fig. 3

Temperature cycle used in the FE analysis

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Fig. 4

von Mises Stress contour plot (in MPa) at 125 °C of (a) global slice model and (b) the solder bump area. (c) The equivalent plastic strain contour plots at critical region after three thermal cycles.

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Fig. 5

(a) Surface evolver plot of the predicted bump structure after pressure application and (b) figure showing the cross section of the bump structure at the center

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Fig. 6

(a) Perfect microsolder bump model, (b) squeezed out microsolder bump model, (c) and (d) the von Mises stress contour plot (in MPa), (e) and (f) the equivalent plastic strain contour plots showing the critical regions used to extract results. All results were captured at the end of three thermal cycles.

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Fig. 7

Comparison of (a) equivalent plastic strains and (b) equivalent creep strains accumulation in both perfect and squeezed out microsolder bump model

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Fig. 8

Average shear stress versus Time in the middle of the microsolder bump

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Fig. 9

History of (a) accumulative equivalent plastic strains and (b) accumulative equivalent creep strains at the critical corners of the microbump in solder region for three thermal cycles. (c) and (d) Maximum accumulated equivalent plastic and creep strain, respectively, after each cycle.

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Fig. 10

Total equivalent inelastic strain range per cycle after three thermal cycles for solder and copper regions

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Fig. 11

Contour plot of equivalent plastic strain distribution after three thermal cycles for (a) No IMCs, (b) 20% IMCs, (c) 40% IMCs, (d) 60% IMCs, and (e) 80% IMCs model

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Fig. 12

Predicted mean cycles to failure due to fatigue for Cu and solder with a different volume fraction of IMCs in the model

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