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Research Papers

Regional Stiffness Reduction Using Lamina Emergent Torsional Joints for Flexible Printed Circuit Board Design

[+] Author and Article Information
Bryce P. DeFigueiredo

Department of Mechanical Engineering,
Brigham Young University,
Provo, UT 84602
e-mail: bdefig@gmail.com

Trent K. Zimmerman

Department of Mechanical Engineering,
Brigham Young University,
Provo, UT 84602
e-mail: trentzim@gmail.com

Brian D. Russell

Department of Mechanical Engineering,
Brigham Young University,
Provo, UT 84602
e-mail: brian.russ247@gmail.com

Larry L. Howell

Professor
Department of Mechanical Engineering,
Brigham Young University,
Provo, UT 84602
e-mail: lhowell@byu.edu

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received November 15, 2017; final manuscript received May 25, 2018; published online July 3, 2018. Assoc. Editor: Satish Chaparala.

J. Electron. Packag 140(4), 041001 (Jul 03, 2018) (9 pages) Paper No: EP-17-1120; doi: 10.1115/1.4040552 History: Received November 15, 2017; Revised May 25, 2018

Flexible printed circuit boards (PCBs) make it possible for engineers to design devices that use space efficiently and can undergo changes in shape and configuration. However, they also suffer from tradeoffs due to nonideal material properties. Here, a method is presented that allows engineers to introduce regions of flexibility in otherwise rigid PCB substrates. This method employs geometric features to reduce local stiffness in the PCB, rather than reducing the global stiffness by material selection. Analytical and finite element models are presented to calculate the maximum stresses caused by deflection. An example device is produced and tested to verify the models.

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References

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Figures

Grahic Jump Location
Fig. 1

Current techniques for producing flexible electronics allow for uncontrolled deflection of the entire substrate. When the circuit is (a) undeflected, components and solder joints experience low stresses. When the circuit is (b) deflected, components and solder joints experience high stresses.

Grahic Jump Location
Fig. 2

(a) A single LET joint and (b) an array of LET joints. LET joint geometry lowers stiffness by transferring an applied bending load over the joint to torsional loads in the legs.

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Fig. 3

(a) A single LET joint with its corresponding spring system and (b) geometric parameters for Eqs. (1)(14)

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Fig. 4

Deflected LET joint with parameters from Eq. (1) labeled

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Fig. 5

Detail view of LET joint array

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Fig. 6

The stress distribution for a 1 × 1 joint using the dimensions and material properties listed in Table 1. The shear stress resulting from a 30 deg angular displacement load is shown.

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Fig. 7

Comparison of analytical and FEA model results for various 1 × n joints. Shear stress at the center of the torsion members is plotted versus the number of joints in series (n), due to a 30 deg angular displacement load.

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Fig. 8

(a) Prototype solar array with LET joint hinge in folded and (b) unfolded configurations, along with (c) a detail view of the LET joint array surrogate hinge

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Fig. 9

(a) Diagram and (b) photograph of fatigue testing setup

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Fig. 10

Map-fold origami pattern: (a) Origami fold pattern. Solid lines represent mountain folds, and dashed lines represent valley folds. Labels A, B, and C designate the three folding steps in order of folding sequence. (b) The fold area to be made flexible with surrogate hinges is shown shaded. (c) Surrogate hinge areas are shaded. Arrows show the direction of the fold axis. Labels a, b, and c designate regions of optimized surrogate fold geometry. Shaded regions without arrows show areas where material is removed.

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Fig. 11

Origami-like folding structure designed with optimized surrogate hinge LET joint arrays and fabricated from a single sheet of copper-clad FR-4

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Fig. 12

Possible form factors for various applications of this regional stiffness reduction technique. Circuit boards could be folded to stow in small spaces or to conform to an arbitrary shape.

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