Paper No P41: Predicting Change in Cell Gap in LCD Panels Subjectedto Touch Force

K. Hemanth Vepakomma

Corning Incorporated, Corning, New York 14831

Manoj Pandey

Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai, TamilNadu India

Tomohiro Ishikawa

Corning Incorporated, Corning, New York 14831

Ramji Koona

Department of Mechanical Engineering, Andhra University, Andhra Pradesh, India

AbstractLiquid crystal displays (LCDs) with touch-screen capabilities arebecoming increasingly popular. When a user pushes a screentoo hard, the cell gap changes and causes a blur (known astouch mura) around the push location. This paper uses finite ele-ment analysis to predict the change in cell gap when an LCDpanel is subjected to a touch force.

Author Keywordstouch mura; LCD panel; thin glass; finite element analysis

1. IntroductionLiquid crystal displays (LCDs) with touch-screen capabilities are

becoming increasingly popular. In a rudimentary form, an LCDpanel consists of a color-filter (CF) glass and a thin-film transistor(TFT) glass with liquid crystal and photospacers sandwichedbetween them. The CF and TFT glasses are glued together alongthe perimeter using epoxy. The epoxy also acts as a seal and con-tains the liquid crystal from leaking. For the proper functioning ofan LCD panel, it is necessary to maintain a certain prescribed cellgap between the CF and TFT glasses. This is obtained by uniformlydistributing photospacers inside the liquid crystal. Generally, thisgap varies between 2.5 and 4 mm. In this study, a cell gap of 4 mmwas assumed. A panel contains other layers like alignment film,ITO layers, color filter pixels, thin-film transistors, polarizers, etc.However, their bending stiffness is an order of magnitude lowerthan that of the glass and so they were ignored.

When a user pushes the screen too hard, the cell gap changesand causes a blur (known as touch mura) around the push loca-tion (Figure 1). This paper discusses a finite element approach tomodel this change in cell gap. Aspect ratio is the main challengein this type of problem. The length and width of an LCD panelis in the order of tens to hundreds of microns whereas the cellgap is in the order of a few microns. The glass thickness canrange from 0.2 to 0.7 mm. In this study, only the cover glass,CF glass, TFT glass, liquid crystal, and photospacers were mod-eled. Unless, all the layers and their material properties are mod-eled accurately, the calculated change in cell gap should not beused as an absolute value, but should be used as a relative num-ber for comparing analogous cases to understand the trends.

2. Finite Element ModelFigure 2 shows a schematic of an LCD panel which was used

as a basis to build a finite element model. A cover glass meas-

uring 210 3 165 3 0.7 mm3, CF and TFT glasses measuring205 3 160 3 0.35 mm3, a cell gap of 4 microns, a spacer cross-section of 18 3 8 mm2 and a distance between spacers of0.2 mm were used as inputs to the model. For material proper-ties, the glasses were assigned a Young’s modulus of 73,000MPa and a Poisson’s ratio of 0.23. A Young’s modulus of 5000MPa was used for the spacers. All four sides of the cover glass,CF glass, and TFT glass were fixed. A touch force of 20 N wasapplied on a circular area of radius 1.5 mm at the center of thecover glass.

Figure 3 shows the type of elements used to model differentregions of the panel. The cover glass, CF glass and TFT glasswere modeled using continuum shell (solid shell) elements [1].These are three-dimensional solid elements with eight nodes(three degrees of freedom at each node) and can be directly con-nected to other continuum elements. These elements are free oflocking in bending-dominant situations. For extremely thin appli-cations, they use a suite of special kinematic formulations toavoid locking [1].

The liquid crystal was modeled using three-dimensional hydro-static fluid elements. These elements are used for modelingenclosed fluids and are well-suited to model the pressure–volumerelationship for coupled problems involving fluid–solid interac-tion [1]. These elements do not model the flow of the fluid andso viscosity of the fluid cannot be defined. It is also assumedthat the fluid is incompressible and there is no pressure gradientinside the enclosure. The epoxy that contains the liquid crystalfrom escaping was modeled using rigid shell elements.

The photospacers are grown on the CF side using photolithog-raphy and are not attached to the TFT side. So they were mod-eled using three-dimensional compression-only spar elements.These do not have any bending or tensile stiffness and only pro-vide resistance when the CF glass is pushed towards the TFTglass. Lack of bending stiffness ensures that the CF glass willslide before the spacers get a chance to bend. Lack of tensilestiffness becomes important when some part of the CF glass lifts(away from the touch region) in response to the applied touchforce.

The glasses were divided into three different zones (Figure 4),namely, the “touch region,” the “modeled spacer region” and themain glass. For each glass, the mesh was not continuous acrossthe “touch region” and the rest of the main glass. Bonded contactwas defined between the “touch region” and the rest of the glass.

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For the sake of computational efficiency, the spacers were onlymodeled around the “touch region.” To validate this assumption,three different sizes (20.25 mm2, 36 mm2, and 81 mm2) of the“modeled spacer region” were modeled and all of them gave thesame change in cell gap which is shown in Figure 5. In theregion where the spacers were modeled, the CF and the TFTglasses were meshed in a regular grid pattern (Figure 6). The ele-ment lengths and widths were made equal to the distancebetween the spacers. Because the cover glass and the CF glassare glued together using optically clear adhesive, a bonded con-tact was defined between them.

3. Results and DiscussionsA panel undergoes two types of deformation mechanisms (Fig-

ure 7) when a touch force is applied. The first is global bending,where the entire panel bends. The second is a local deformationthat happens right underneath the touch force. The local

Figure 2. Schematic of an LCD panel (not to scale).

Figure 3. Different regions of the panel were assigneddifferent element types.

Figure 1. Change in cell gap causes a blur around thepush location.

Figure 4. All the glasses were divided into three differ-ent zones: (i) touch region, (ii) modeled spacerregion, and (iii) main glass region.

Figure 5. Modeling spacers only around the touchregion is sufficient.

Figure 6. (a) Finite element mesh and (b) zoomed viewof the “modeled spacer region” (cell gap was exag-gerated out of scale to show spar elements).

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deformation is caused because of compression of spacers in thetouch region. This local deformation is equivalent to the changein cell gap.

Figure 8 compares global deformations of three modeled cases:(i) spacers and liquid crystal were considered in the model, (ii)spacers and liquid crystal were ignored and instead frictionlesscontact was defined between the CF and the TFT glasses, and(iii) spacers were considered and liquid crystal was ignored. Bycomparing these three cases, it can be concluded that spacers andliquid crystal do not influence the global deformation of thepanel. This conclusion is in line with references [2–7], which

validate finite element models where spacers and liquid crystalwere replaced with frictionless contact with experimental stress.Figure 9 compares change in cell gap for the case where bothspacers and liquid crystal were considered in the model with acase where only spacers were considered in the model. Both thecases report same change in cell gap proving that liquid crystaldoes not influence the change in cell gap. One additional casewas studied where the back of the TFT glass was fixed. Figure10 compares a case where the back of the TFT is fixed with acase where the back of the TFT is free to bend. In both thecases, spacers and liquid crystal were modeled. Having a clear-ance behind the panel so that it can slightly bend as supposed toconstraining the back of the TFT glass will help in reducing vari-ation in cell gap.

4. ConclusionsThe authors have established a method using finite element

analysis to predict variation in cell gap when an LCD panel issubjected to touch force. This approach takes liquid crystal andspacers into consideration. It is not required to model spacerseverywhere in the panel, modeling them around the touch regionis sufficient. The size of the “modeled spacer region” has to bedetermined by systematically increasing the size of the regionuntil no difference is seen in the results. Spacers and liquid crys-tal do not impact global deformation. Liquid crystal does notimpact cell gap variation. Allowing the panel to bend (globally)slightly may reduce cell gap variation. In other words, having asmall clearance behind the panel may be helpful.

5. AcknowledgementThe authors appreciate the support of this work, as well as per-

mission to present it, by Corning Incorporated and Corning Dis-play Technologies.

6. References

[1] ANSYS User’s Manual, ANSYS, Incorporated.[2] K. H. Vepakomma, J. Westbrook, S. Carley, J. Kim, “Finite

element analysis of four point bending of LCD panels”,J. Disp. Technol., Vol. 9, pp. 82–86, 2013.

[3] K. H. Vepakomma, J. Westbrook, S. Carley, J. Kim, “Finiteelement analysis of ring-on-ring test on LCD panels”,J. Disp. Technol., Vol. PP, pp. 1–5, 2013.

Figure 7. Deformation mechanisms: global bendingand local deformation (change in cell gap).

Figure 8. Spacers and liquid crystal do not influenceglobal deformation.

Figure 9. Liquid crystal does not influence the changein cell gap. Spacers play a dominant role in the varia-tion of cell gap.

Figure 10. Having a clearance behind the panel willhelp in reducing change in cell gap.

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[4] K. H. Vepakomma, S. Carley, J. Kim, “Finite element anal-ysis of ball drop on LCD panels”, SID Symposium Digestof Technical Papers, Vol. 43, pp. 675–678, 2012.

[5] J. T. Westbrook, J. F. Bayne, S. H. Carley, K. H.Vepakomma, J. S. Kim, S. T. Gulati, “Four point bendingof AMLCD panel”, SID Symposium Digest of TechnicalPapers, Vol. 43, pp. 996–997, 2012.

[6] S. T. Gulati, J. T. Westbrook, K. H. Vepakomma, T. Ono, J.S. Kim, “Overview of strength tests for LCD substrates andpanels”, International Display Workshops, Vol. 18, 2011.

[7] S. T. Gulati, J. Westbrook, S. Carley, H. Vepakomma, T.Ono, “Two point bending of thin glass substrate”, SID Sym-posium Digest of Technical Papers, Vol. 42, pp. 652–654,2011.

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