Esther Janssen - Philips Research

Philips Research

Themadag Mikrocentrum: “Verbinden van Kunststoffen”

Optical adhesive bonding

Ir. Esther Janssen

24 april 2012

2 Esther Janssen, April 24, 2012

Contents

• Function of optical bonding • Basics in optics • Optically transparent adhesives • Important properties • Optical bonding technologies • Challenges • Optical bonding applications • Summary


Function of optical bonding

• Optically transparent adhesives are used for the following reasons:

– Extract, transmit or redirect light – Enhance brightness – Reduce glare effects and mirror images – Mechanically join optical components – Bond dissimilar materials, e.g. plastic to glass – Minimize thermal stress by matching the difference in expansion of

2 bonded materials


Basics in optics: Light passing an interface • When light is travelling through an

interface between 2 optical materials, part of the light gets reflected and part of the light changes its direction as it enters the second medium: it is refracted

• This is caused by the difference in refractive index of the 2 materials. The larger the difference, the more reflection.


Basics in optics: Refraction of light

• The speed at which light travels is dependent upon the density of the materials it is traveling through.

• Refractive index is defined as the relative speed (v) at which light moves through a material with respect to its speed in vacuum (c)

– n = c/v

• Snell's law of refraction: • n1 sin (θi) = n2 sin (θr)

θi

θr


Basics in optics: Reflection of light

• Reflectivity of a surface also depends on the refractive index, as described by the Fresnel equations.

• The incident angle of light is the same as the angle of reflected light: – θincidence = θreflection

• The fraction of the intensity of perpendicular incident light that is reflected from the interface is given by reflection coefficient R.

•


Basics in optics: Total internal reflection

• If the incident angle increases past a specific value (Brewster's angle), light is no more refracted into the medium of lower refractive index. The light is captured, which is called total internal reflection.

– sin(θc) = n1/n2

θc


Basics in optics: Reflection loss

• So light is lost due to reflection of light at interfaces between materials with different refractive indices.

• The reflection coefficient for glass in air is for example about 4.1% • Optically transparent adhesives are used to match the refractive indices

of the bonded materials and thereby increasing brightness and contrast. • Refractive index of these adhesives range between 1.3 and 1.6.

� Air 1.00 Water 1.33 Silicone 1.4 - 1.6 Glass (typ.) 1.52 PMMA 1.49 PC 1.61 Diamond 2.42


Optically transparent adhesives

• The following types of materials are available as optically transparent adhesive:

– Silicone – Epoxy – Acrylic – Polyurethane – Cyanoacrylate

• Liquid or tapes • Thermal, moisture or UV-curing


Optically Clear Adhesive tapes (OCA)

• For bonding touch panels or ITO film to displays • Pressure-sensitive, acrylic adhesive • Typically 25-125 micron of adhesive thickness • Roll-lamination • 3M, Nitto Denko, …


UV curable optical adhesives

• Fast bonding or encapsulation of optical components, like fiber optics, lenses, mirrors, prisms

• Can be cured in seconds • Acrylics, epoxies, thiolenes, • Epoxy Technology, Dymax, Delo, Henkel,


Optical grade silicones

• Silicones are very attractive optical grade materials – High and stable optical transparency – Resistant to very high and low temperatures – Resistant to high flux (UV-)light – Properties can be engineered

• Transparent to opaque • High and low viscosity • Refractive index • Soft to hard

• Lenses and encapsulants for LED devices • Dow Corning, Wacker, Momentive,


Important properties

• Absorption spectrum • Refractive index • Adhesion • Elasticity modulus • Resistance to

– UV – Temperature (high, low, shock) – Moisture – Vibration – Chemicals

• Outgassing


Optical bonding technologies

• Dispensing • Screen printing • Casting • Lamination

– Vacuum – Roll

• Molding


Challenges

• Find adhesive with optimum properties for performance, processing and cost

• Realize and maintain the optical performance • No air bubbles • Lifetime due to degradation of the adhesive joint

– Due to UV, X-ray, temperature, moisture,.. • Controlling bond line thickness • Adhesion to low energy surfaces • Dimensional stability due to curing shrinkage


Optical bonding applications

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LEDs

Displays Photovoltaic energy panels

X-ray detector

Applications


Displays

• Bonding a cover plate to an LCD panel improves optical performance and ruggedness:

– Improved view ability in bright ambient conditions – Decreased temperature of display – More resistant to Shock and vibration – Prevents condensation


• Silicones are used as optical interface materials in LED-packages, because of their very high light transmission and resistance to high temperatures and UV-light.

• A phosphor plate is bonded to the LED-chip with a silicone adhesive • A silicone lens is molded on top of this LED-chip plus phosphor plate.

LEDs


X-ray detector

• Computed Tomography (CT) is a medical imaging technology which converts X-rays into visible light and thereby provides a cross sectional image of a patient’s body.

• Optical adhesives are commonly used in the X-ray detectors to optically bond the scintillator to the detector.

Scintillator Optical adhesive Photo detector

X-ray


Photovoltaic energy panels

• Optical bonding in solar panels is used to bond the solar cells to the front glass sheet.

• Ethyl Vinyl Acetate is often used as optical adhesive. It is applied as film and laminated and thermally cured in vacuum.

• A solar panel consists of: – Tempered glass – Encapsulant (EVA) – Interconnected solar cells – Encapsulant (EVA) – Rear-side foil

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Summary

• Optical transparent adhesives are being used to make an optical joint between two or more optical components.

• It prevents loss of light due to reflections. • Optical transparent adhesives are available in different chemistries and

forms. • Several processes can be used to create an optical bond • Transmission and refractive index are important adhesive properties • Optical bonding is used in displays, LEDs, solar panels and X-ray

detectors.


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