nist 2012 mechanical reliability glaesemann

The Strength of Thin Flexible Glass Sheets

Flexible Printed Electronics Metrology September 12 -13, 2012National Institute of Standards and Technology

G. Scott GlaesemannScience and TechnologySullivan Park

Corning Incorporated 2012

Flexible Electronics Applications Continuing to Emerge

Device applications include: Display (e-paper, color filter, OLED, LCD) Touch sensor Photovoltaic Lighting

Processes include: high-resolution and high-registration patterning and printing

Glass substrate opportunities progressing toward flexible sheets and web

Flexible Glass Sheets Flexible Glass Web

2


Sufficient Strength to Survive ProcessingContinuous R2R ITO patterning has been demonstrated

CoolingDrum

ITO Deposition Slot Die Coating

Supply Roll Take-Up Roll

Exposure

Supply Roll Take-Up Roll

Development & Etch

3

S. Garner, et al., Flexible glass substrates for continuous manufacturing, Flexible Electronics and Displays Conference, February 9, 2011.


The strength of glass is controlled by flaws Strength is the stress on the glass at failure Function of flaw depth and fracture toughness

Glass strength is dependent on flaw depth more than mechanical properties Surface flaws from handling-induced damage are the most common Strength is primarily determined by the glass objects handling history

Strength is more likely to go down than up during the life of an object

Glass Mechanical Strength

0.01

0.1

1

10

100

Stre

ngth

(GPa

) Theoretical TypicalOptical Fiber

TypicalBulk Glass

4

=


MaterialMaterialFracture Toughness

(MPa m)Fracture Toughness

(MPa m)Soda-Lime SilicateSilicaGlass CeramicsAl2O3PSZ AlloysWhisker Reinforced Ceramics4340 Steel

Soda-Lime SilicateSilicaGlass CeramicsAl2O3PSZ AlloysWhisker Reinforced Ceramics4340 Steel

0.5 - 0.70.75 - 0.81 - 32 - 66 - >126 - >12

90

0.5 - 0.70.75 - 0.81 - 32 - 66 - >126 - >12

90

Fracture Toughness the resistance to crack propagation

5


Sharp Impact

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How Glass Responds to Sharp ContactB. Lawn, Fracture of Brittle Solids, Cambridge Univ. Press, 1993.

Median crack Lateral crack

Loading Unloading

Idealized Median cracks

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Chatter MarksChatter Marks

Sliding Contact Produces Frictive Damage

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Scratches are accompanied by lateral cracks and chipping

Scratch with Lateral Cracks

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Predicted Strength vs. Flaw Depth

IC f cK Y aV

0

20

40

60

80

100

120

140

160

180

200

0 10 20 30 40 50 60 70 80 90

Flaw Depth, microns

Pred

icte

d St

reng

th, M

Pa

Y=1.12 sqrt(pi)

Y=sqrt (pi)

Y=0.73 sqrt (pi) Newman Raju

10


Printed Electronics Manufacturing

In-Device

Spool Storage and Shipping

Glass Manufacturing

Localized high stress surface and edge flexure

&Potential global bending

Short duration global bend stress (R2R)

Edge flaws: separation and guiding

Surface flaws: deposition, rollers

Long duration global bend stress

Process Step Stress Events Flaw Introduction

Initial Surface and Edge flaws created

Potential for edge and surface damage

Short duration global bend stress

Contact damage during installation

Failure Scenarios

Mechanical Failure

Initial flaws cannot survive manuf bend

stresses

Fatigue failure from previous flaw or shipping flaws

Global bend stress exceeds strength of prev and process

induced flaw populations

*Localized surface stress exceeds strength of process

induced surface flaws*Over stress edge flaws from

separation process

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Bending: Most Common Mode for Generating Stress

Bend stress can be generated by: Bulk bending Localized contact induced flexure

Maximum bend-induced tensile stress at surface, y=t/2 Dependent on thickness, modulus, and radius

RyE

bt

E2R

Compression

Tension

M Mt/2y

Youngs Modulus (approximate values)Steel 200 GPaCopper 110-128 GPaAluminum ~70 GPaGlass 50-90 GPaPolymer


RyE

Bending induced stress

bt

E2R

Constant Bend Radius

compression

tension

M Mt/2y

Maximum bend-induced tensile stress at surface, y=t/2

0

50

100

150

200

250

300

350

400

450

500

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Bend Radius, mm

Ben

d St

ress

, MPa

1 mm

0.7 mm

0.5 mm

0.3 mm

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Reliability Design Diagram for Glass in Bending

hours

1050

900

750

600

450

300

150

seconds

700

600

500

400

300

200

100

0

50

100

150

200

250

300

350

400

0 5 10 15 20

Bend Radius, cm

Bend

Stre

ss, M

Pa

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Req

uire

d In

ert S

treng

th, M

Pa

10 micron25 micron50 micron75 micron100 micron125 micronhoursseconds125 proof

lifetime

thickness

E = 70 GPa

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Glaesemann and Gulati, Opt Engr (30) 6 1991

What is the strength of flexible glass sheets?


Measurement of Glass strength Ring-on-Ring

Ring-on-Ring test (ASTM C1499) Maximum stress in region below inner ring on other side

Limitations: Thin glass (with deflection greater than half thickness at failure) shows significant non-linear effects

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Ring-on-Ring Testing and Large Deflections

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Gulati et al., Overview of Strength Tests for LCD Substrates and Panels


Large Deflections during Ring-on-Ring Strength Test

Stress(MPa)

Load (N)

17


Ball on Clamped Ring

0 200 400 600

0

500

1000

Stre

ss (M

Pa)

Load (N)

Stress under ball

Membrane Stress

18


Ball on Clamped Ring Contact Area

4 mm

19


0 100 200 300 400 500 600-200-100

0100200300400500600700800900

1000110012001300140015001600

Stre

ss (M

Pa)

Load (N)

Measured Stress Under Ball

0.3mm

MaximumBend-InducedStress

20

Wrapping around Ball

Membrane


Glass Surface Strength is Independent of ThicknessNew test methods developed for flexible substrates

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G.S. Glaesemann, et al., The Strength of Thin Fusion Drawn Glass Sheets, 11th ESG Conference 2012, Maastricht, The NetherlandsStrength, MPa

Failu

re P

roba

bilit

y, %

100 1000010001.E-1

5.E-1

1

5

10

50

90

99

1.E-1

100 Pm

200 Pm

300 Pm

Strength (MPa)

Failu

re P

roba

bilit

y (%

)

21


Measurement of Glass strength 4 point bend

Four point bend strength (ASTM C158) Maximum stress in region between loading points (opposite

surface) Edges are often weaker than surface Effectively tests only two bottom edges

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Two Point Bend Concept

Upper Platen

Lower Platen

Glass Specimen

Upper Platen

Lower Platen

Glass Specimen

Bend a piece of glass between two platens

Elastic Beam Theory describes basic behavior

Concept is not new (its been around for decades)

Used extensively in the fiber industry

New concept: Use high speed video to capture failure location

Upper Platen

Lower Platen

Glass Specimen Failure Location

23


Two Point Bend Test Setup

Specimen goes here

CameraLight Source

Mirror

Load Frame

Upper Platen

Lower Platen

Camera Rail

Screen

24


Strength, MPa

Failu

re P

roba

bilit

y, %

10 10000100 10001.E-1

5.E-1

1

5

10

50

90

100

1.E-1

Edge Strength after Mechanical Score and Break of 100 m thick Glass

Scoreside

Breakside

25

Surface failures as suspensions


The Fracture Surface

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Branching

Mirror Mist Hackle BranchingMirror Mist Hackle Branching

Mirror

Mist

Hackle(points to origin)

Flaw Origin

The Fracture Surface

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Velocity Mist / Mist Hackle

Mist hackles occur beyond the mirror region and appear as a gray-matte surface. These markings occur when the fracture approaches terminal velocity. This feature is not present on low stressed breaks.

The shape of the hackle is indicative of the stress that was associated with the break. The above sketches are the extremes - the shape of the mist region can vary.

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Origin

Bending Mist / Velocity Hackle

Bending Mist Hackle

Secondary Wallner Lines

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540 MPa 320 MPa

Strength Determined from Mirror Radius

30


250 MPa 210 MPa

Return to Mirror

Strength Determined from Mirror Radius

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Glass Mechanical Reliability Key Principles

Every glass object has strength-limiting flaws Machining, polishing Glass chips scratching surface Flaws potentially introduced at every handling step

Strength is not a pure, material property Controlled by flaws Determined by manufacturing and handling history Statistical in nature

Strength is always a characterization of a flaw population We never measure flawless glass Strength is always measured after some damage has been done

Surface flaw due to contact

Edge flaw due to cutting

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Strength Testing Flexible Glass

Match strength testing to the failure mode of interest Failure mode 1st

Strength testing 2nd

Novel strength testing methods Surface Strength Ball on Clamped Ring Edge Strength Modified 2 point Bend

Fractography Cause of failure Foundational method for improving strength

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nist 2012 mechanical reliability glaesemann

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