Research Papers

Display Light Guide Swelling Due to Moisture Absorption

[+] Author and Article Information
A. K. Cousins

1 Microsoft Way,
Redmond, WA 98052
e-mail: ancousi@microsoft.com

G. H. Thiel

1 Microsoft Way,
Redmond, WA 98052
e-mail: gethiel@microsoft.com

E. Wolak

1 Microsoft Way,
Redmond, WA 98052
e-mail: edwolak@microsoft.com

G. Limaye

1 Microsoft Way,
Redmond, WA 98052
e-mail: gelimaye@microsoft.com

S. Tiku

1 Microsoft Way,
Redmond, WA 98052
e-mail: stiku@microsoft.com

1Corresponding author.

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received July 25, 2017; final manuscript received September 19, 2017; published online October 5, 2017. Assoc. Editor: Yi-Shao Lai.

J. Electron. Packag 139(4), 041006 (Oct 05, 2017) (7 pages) Paper No: EP-17-1070; doi: 10.1115/1.4037971 History: Received July 25, 2017; Revised September 19, 2017

Display light guides, commonly constructed of a polymer such as polymethyl methacrylate (PMMA), are known to be susceptible to moisture absorption, swelling, and warping in the field when exposed to elevated ambient humidity levels. This work presents a quantitative theoretical framework for calculating water absorption over time. In addition, a simple theory is laid out, which connects mass absorption of water to linear expansion of the polymer. Finally, a simple quantitative analysis of out-of-plane warping of the light guide is presented. Experimental data from laboratory PMMA coupons and two different sizes of large displays are used to establish the values of key parameters of the theoretical model. General purpose results for any size of PMMA display are presented. The approach can easily be adapted to light guides fabricated using other polymers.

Copyright © 2017 by ASME
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Fig. 1

Progressive light guide swelling and deformation due to prolonged humidity exposure. Buckling occurs due to the confinement by the display.

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Fig. 2

Permanently warped light guide after moisture absorption

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Fig. 3

Light guide bowing after moisture absorption. The upper curve is for a light guide not constrained by an LCD; the lower curve is for a light guide constrained by an LCD.

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Fig. 4

Light guide broken by moisture-induced swelling

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Fig. 6

PMMA water absorption capacity as a function of humidity and temperature

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Fig. 7

Comparison of water absorption curves from various sources

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Fig. 8

Linear strain versus fractional mass gain due to water absorption

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Fig. 11

Light guide swelling and deformation

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Fig. 10

Transient water absorption—experiment versus theory, based on experimental data from large-display applications. A diffusion coefficient of 1.25 × 10−8 cm2/s was found to best match the data.

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Fig. 9

Universal curve for water absorption over time




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