Abstract
In this work we present a novel optical polymer system based on polyurethane elastomer components, which combines excellent UV transparency with high thermal stability, good hardness, high surface tension and long pot life. The material looks very promising for encapsulation and microlensing applications for chip-on-board (CoB) light-emitting diodes (LED). The extinction coefficient k, refractive index n, and bandgap parameters were derived from transmission and reflection measurements in a wavelength range of 200-890 nm. Thermogravimetry and differential scanning calorimetry were used to provide glass transition and degradation temperatures. The surface tension was determined by means of contact angle measurements. As proof of concept, a commercial InGaN-CoB-LED is used to demonstrate the suitability of the new material for the production of microlenses.
Highlights
Current requirements and the market volume of light emitting diodes (LEDs) and miniaturized CMOS image sensors have led to an increasing demand for low-cost optics and a notable growth of production capabilities [1,2,3,4]
In this work we present a novel optical polymer system based on polyurethane elastomer components, which combines excellent UV transparency with high thermal stability, good hardness, high surface tension and long pot life
The material looks very promising for encapsulation and microlensing applications for chip-on-board (CoB) light-emitting diodes (LED)
Summary
Current requirements and the market volume of light emitting diodes (LEDs) and miniaturized CMOS image sensors have led to an increasing demand for low-cost optics and a notable growth of production capabilities [1,2,3,4]. The combination of materials combines the advantages of conventional polymers such as low cost, easy processing, excellent mechanical properties and good adhesion with the advantages of silicone to improve toughness, internal stresses and thermal stability of encapsulations [17,25]. Aliphatic isocyanates result in polyurethanes that do not discolor under the influence of light or heat and are highly transparent and thermally stable [29,30] These media have long pot life, that simplifies the manufacturing process. The targeted application of the novel PU is Chip-on-Board LED micro lens packaging In this scenario, the refractive index n is essential for (i) refractive index matching with the LED material, (ii) addressing the color dispersion and (iii) optimizing the focal length and the directional characteristics of the polymer micro lenses. It was derived from contact angle measurements on various substrates based on the harmonic mean approach of Wu [37,38,39]
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