Esther Janssen - Philips Research


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Esther Janssen - Philips Research

  1. 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 Esther Janssen, April 24, 2012 2
  2. 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 Esther Janssen, April 24, 2012 4Basics 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. Esther Janssen, April 24, 2012 5
  3. 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• Snells law of refraction: θi • n1 sin (θi) = n2 sin (θr) θr Esther Janssen, April 24, 2012 6Basics 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. • Esther Janssen, April 24, 2012 7
  4. 4. Basics in optics: Total internal reflection• If the incident angle increases past a specific value (Brewsters 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 Esther Janssen, April 24, 2012 8Basics 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 Esther Janssen, April 24, 2012 9
  5. 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 Esther Janssen, April 24, 2012 10Optically 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, … Esther Janssen, April 24, 2012 11
  6. 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, Esther Janssen, April 24, 2012 12Optical 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, Esther Janssen, April 24, 2012 13
  7. 7. Important properties• Absorption spectrum• Refractive index• Adhesion• Elasticity modulus• Resistance to – UV – Temperature (high, low, shock) – Moisture – Vibration – Chemicals• Outgassing Esther Janssen, April 24, 2012 14Optical bonding technologies• Dispensing• Screen printing• Casting• Lamination – Vacuum – Roll• Molding Esther Janssen, April 24, 2012 15
  8. 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 Esther Janssen, April 24, 2012 16Optical bonding applicationsDisplays Photovoltaic energy panels Applications LEDs X-ray detector Esther Janssen, April 24, 2012 17
  9. 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 Esther Janssen, April 24, 2012 18LEDs• 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. Esther Janssen, April 24, 2012 19
  10. 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 Esther Janssen, April 24, 2012 20Photovoltaic 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 Esther Janssen, April 24, 2012 21
  11. 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. Esther Janssen, April 24, 2012 22 Esther Janssen, April 24, 2012 23