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Research Papers

Investigations on Flexural Fatigue Strength of Conductor Paths Fabricated by LPKF-LDS® Technology

[+] Author and Article Information
Hagen Mueller

Hahn-Schickard,
Allmandring 9 b,
Stuttgart 70569, Germany
e-mail: Hagen.Mueller@Hahn-Schickard.de

Tobias Groezinger

Hahn-Schickard,
Allmandring 9 b,
Stuttgart 70569, Germany
e-mail: Tobias.Groezinger@Hahn-Schickard.de

Sascha Weser

Hahn-Schickard,
Allmandring 9 b,
Stuttgart 70569, Germany
e-mail: Sascha.Weser@Hahn-Schickard.de

Wolfgang Eberhardt

Hahn-Schickard,
Allmandring 9 b,
Stuttgart 70569, Germany
e-mail: Wolfgang.Eberhardt@Hahn-Schickard.de

André Zimmermann

Institute for Micro Integration IFM,
University of Stuttgart,
Hahn-Schickard,
Allmandring 9 b,
Stuttgart 70569, Germany
e-mail: Andre.Zimmermann@Hahn-Schickard.de

1Corresponding author.

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MICRO- AND NANO-MANUFACTURING. Manuscript received June 22, 2017; final manuscript received October 27, 2017; published online December 14, 2017. Assoc. Editor: Ulf Engel.

J. Micro Nano-Manuf 6(1), 011004 (Dec 14, 2017) (9 pages) Paper No: JMNM-17-1041; doi: 10.1115/1.4038320 History: Received June 22, 2017; Revised October 27, 2017

Reliability aspects are crucial for the success of every technology in industrial application. Regarding interconnect devices, several methods are applied to evaluate reliability of conductor paths like accelerated environmental tests. Especially, molded interconnect devices (MID), which enable numerous applications with three-dimensional (3D) circuitry on 3D shaped injection-molded thermoplastic parts are often under particular stress, e.g., as component of a housing. In this study, a new test method for evaluating the flexural fatigue strength of conductor paths produced by the laser-based LPKF-LDS® technology is presented. For characterization of test samples, a test bench for flexural fatigue test was built up. A result of the flexural fatigue test is a characteristic Woehler curve of the metal layer system. Applying this new test method, essential influencing parameters on the reliability of MID under mechanical load can be identified. So, the metal layer system as well as the geometric parameters of the metal layer is crucial for the performance. Furthermore, test specimens are tested under different types of mechanical load, i.e., tensile stress and compressive stress. For a holistic view on reliability of MID, experimental results are discussed and supported by simulations. An important finding of the study is the advantage of nickel-free layer systems in contrast to the Cu/Ni/Au layer system, which is often used in MID technology.

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References

Franke, J. , 2014, Three-Dimensional Molded Interconnect Devices (3D-MID)—Materials, Manufacturing, Assembly and Applications for Injection Molded Circuit Carriers, Hanser Publishers, Munich, Germany, pp. 192–195. [CrossRef]
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Figures

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Fig. 4

Test specimen for experiments on flexural fatigue strength

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Fig. 3

Test bench for experiments on flexural fatigue strength

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Fig. 2

Flexural fatigue strength: schematic elongation Woehler curve of metals (left) and schematic stress Woehler curve (right)

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Fig. 5

Validation of test method by testing various elongations of the layer system Cu/Ni/Au on LCP substrate resulting in a Woehler curve [7]. The horizontal line marks the fatigue strength of the layer system.

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Fig. 6

Effect of metal line roughness on flexural fatigue strength

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Fig. 7

Effect of metal layer thickness on flexural fatigue strength

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Fig. 9

SEM micrographs of Cu/Ni layer on test specimen: MidPhos nickel (left), HighPhos nickel (right)

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Fig. 15

Cross section polish of Cu/Ni/Au layer on LCP substrate after experiment on flexural fatigue strength with crack in copper layer initiated by crack in nickel layer

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Fig. 10

Effect of phosphorous content of nickel layer on flexural fatigue strength

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Fig. 8

Effect of metal adhesion on flexural fatigue strength

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Fig. 14

Cross section polish of Cu/Ni/Au layer on LCP substrate after experiment on flexural fatigue strength showing cracks in nickel layer

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Fig. 11

Number of stress cycles to failure of copper conductor paths (V06–V09) and Cu/Ni/Au conductor paths (V01–V05, V10–V19)

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Fig. 12

Effect of nickel-free layer systems Cu/Ag and Cu/Pd/Au on flexural fatigue strength compared to Cu/Ni/Au

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Fig. 13

Modeling of the effect of roughness on stress (equivalent von Mises stress in MPa) in metal layer under deflection of 2 mm: (a) smooth surface and (b) roughness Rz = 40 μm

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Fig. 16

Result of stress simulation (equivalent von Mises stress in MPa) in copper and nickel layer of conductor path (deflection 1 mm)

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Fig. 17

Correlation of flexural fatigue strength and ratio roughness/copper layer thickness (Cu/Ni/Au layer system)

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