1. Optical adhesive bonding
Ir. Esther Janssen
Philips Research
24 april 2012
Themadag Mikrocentrum: “Verbinden van Kunststoffen”
Contents
• Function of optical bonding
• Basics in optics
• Optically transparent adhesives
• Important properties
• Optical bonding technologies
• Challenges
• Optical bonding applications
• Summary
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2. 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
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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.
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3. 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: θi
• n1 sin (θi) = n2 sin (θr)
θr
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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.
•
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4. 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
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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.
K
Air 1.00
Water 1.33
Silicone 1.4 - 1.6
Glass (typ.) 1.52
PMMA 1.49
PC 1.61
Diamond 2.42
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5. 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
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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, …
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6. 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,
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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,
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7. Important properties
• Absorption spectrum
• Refractive index
• Adhesion
• Elasticity modulus
• Resistance to
– UV
– Temperature (high, low, shock)
– Moisture
– Vibration
– Chemicals
• Outgassing
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Optical bonding technologies
• Dispensing
• Screen printing
• Casting
• Lamination
– Vacuum
– Roll
• Molding
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8. 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
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Optical bonding applications
Displays Photovoltaic
energy panels
Applications
LEDs
X-ray detector
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9. 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
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LEDs
• 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.
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10. 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.
X-ray
Scintillator
Optical adhesive
Photo detector
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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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11. 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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