Elsevier

Microelectronics Reliability

Volume 54, Issue 11, November 2014, Pages 2440-2447
Microelectronics Reliability

Reliability and optical properties of LED lens plates under high temperature stress

https://doi.org/10.1016/j.microrel.2014.05.003Get rights and content

Highlights

  • Thermal degradation of Bisphenol A Polycarbonate at 100–140 °C is studied.

  • Increasing the exposure time is associated with yellowing.

  • Two stages are observed in the yellowing of polycarbonate plates.

  • Oxidation of plates leads to yellowing.

Abstract

In this investigation the thermal degradation mechanisms of Bisphenol A Polycarbonate (BPA-PC) plates at the temperature range 100–140 °C are studied. The BPA-PC plates are currently used both in light conversion carriers in LED modules and optical lenses in LED-based products. In this study BPA-PC plates are aged at elevated temperature of 100–140 °C for a period up to 3000 h. Optical and chemical properties of the thermally-aged plates were studied using UV–Vis spectrophotometer, FTIR–ATR spectrometer, and integrated sphere. The results show that increasing the thermal ageing time leads to yellowing, loss of optical properties, and decrease of the light transmission and of the relative radiant power value of BPA-PC plates. The results also depict that there is not much discoloration within the first 1500 h of thermal ageing. The rate of yellowing significantly increases at the end of this induction period. Formation of oxidation products is identified as the main mechanism of yellowing. An exponential-based reliability model is also presented to calculate the rate of degradation reaction and to predict the life-time of BPA-PC plates.

Introduction

Solid-state lighting technology is expected to replace conventional incandescent and fluorescent light sources, due to as the high efficiency, long service time, small volume, and low power consumption [1]. Producing white light in LEDs can either be done by discrete colour mixing i.e. with mixing different LEDs of different colours (red. green, and blue LEDs) or by using phosphor to convert light to white colour [2], [3], [4], [5], [6], [7]. Multi-chip white LEDs have a good colour rending index (CRI) and have a higher efficiency in white light generation. However multi-chip LEDs also have some disadvantages. First problem is that the efficiency of red, green and blue LEDs change with time with different rates, so the quality of white light degrades over times. The second problem is the complexity of a different LED package including electrical connections and complicated optics for blending the discrete colours [7].

Currently white LED light is produced by combining colour LEDs and wavelength conversion materials instead of using different LEDs. White LEDs made by combining the blue-emitting diode chips with phosphor (YAG:Ce3+) are the most commercially available LEDs due to their better performance. In this system, the phosphor layer can be either deposited directly on the chip or incorporated into a lens disc [5], [6], [7].

Several elements such as semiconductor chip, bond wires, lead frames, heat slug, solder joints, and optical materials are combined to make high-power white LED products. The life time of white LED may exceed 50 kh but the light intensity could drop significantly in long term operation [2]. The decrease in light intensity and degradation of these LEDs could be attributed to the die-, interconnected-, and/or encapsulants-related failures [2], [3], [4], [5], [6], [7].

Among different degradation mechanisms in LED package, discoloration and yellowing are the most common failure mechanisms, resulting in the reduction in the transparency of encapsulants/lens and decrease in the LED light output [7]. The yellowing of encapsulant/lens could be ascribed to prolonged exposure to short wavelength emission (blue/UV radiation), temperature, and the presence of phosphors, with temperature having a very crucial influence [2].

Thermoplastic Bisphenol A Polycarbonate (BPA-PC) plates are widely used in LED-based products, due to their lightweight, toughness and transparency. Any change in chemical structure of BPA-PC, induced by thermal- and/or photo-degradation, could significantly change these properties. Various studies have been performed to understand the different mechanisms of degradation [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. During last decades the photo-degradation mechanisms of BPA-PC, under UV irradiation, have been extensively studied [8], [9], [10], [11], [12], [13], [14], [15], [16]. It is known [8], [9], [11] that under UV radiation side chain and ring oxidation could occur by the photo-Fries mechanism, resulting in chain scission. The effect of blue light radiation on the optical properties of BPA-PC encapsulants has also been comprehensively addressed in our previous work [17]. BPA-PC is quite stable at air at temperatures below glass transition. However, discoloration of BPA-PC is a major problem during thermal ageing, resulting in a decrease in light transmission of BPA-PC plates in visible and near UV range. Discoloration of BPA-PC plates, measured by changes in yellowness index, is very much dependent on temperature.

During last few years a number of research groups have investigated the thermal degradation of polycarbonate plates [14], [18], [19], [20], [21], [22], [23], [24], [25] and reported that the thermal degradation of BPA-PC leads to the loss of mechanical and optical properties. Montaudo et al. [23] carried out direct pyrolysis of BPA-PC samples under high vacuum conditions and used different sophisticated analytical techniques, including direct pyrolysis-MS (DPMS), thermogravimetry, inherent viscosity, and aminolysis of the pyrolysis residue, to analyze the volatile and non-volatile degradation products and to study the degradation mechanisms at temperatures higher than 300 °C. Lee [26] also studied the thermal oxidation of BPA-PC above 300 °C and showed that there are 3 steps in thermal degradation of BPA-PC at high temperatures. First step is oxidation which takes place at 300–320 °C, followed by the depolymerisation reaction in the range 340–380 °C. Finally, there is a complex random chain scission at the temperature range 480–600 °C, Davis and Golden [24] observed that PC have high degree of thermal stability and only processing at relatively high temperatures could lead to thermo-oxidation degradation. Montaudo and Puglisi [23] reported that molecular rearrangement could also occur at the higher temperature range 500–700 °C. The degradation mechanisms of BPA-PC under hydrolysis reaction condition has also been studied extensively [24], [25].

In most previous studies, the thermal ageing behaviour of BPA-PC is studied at relatively high temperatures. Besides, most studies aim at understanding the effects of temperature on the mechanical properties and structure of BPA-PC. Another important issue is that a lot of studies are either deep in the chemistry of the thermal degradation or have a complete optical approach. Works done by Rivaton [8], Clark and Munro [22], Lemaire et al. [10], Factor and Chu [12], Gorellove and Miller [27], and Davis and Golden [24] are a few examples of those papers with emphasis on the chemistry, whereas papers, published by Trevisanello et al. [4], and Mueller-Mach and Mueller [7] are clearly more into the optical properties of plates with very little information about the chemical background of the problem. Besides, there is not much information about the correlation between thermal degradation at relatively lower temperature, i.e. 100–150 °C, (a temperature lower than its transition temperature) and the chemical and optical properties of BPA-PC on the other side. Understanding the evolution of the optical and chemical properties of BPA-PC during thermal ageing at low temperature is obviously of crucial importance, since it is closer to the real operational conditions. In addition to that, when it comes to the interpretation of the results of accelerated photo-degradation experiments at high temperatures, understanding and consequently ruling out the effect, temperature becomes very much important. In this paper thermal-degradation mechanisms of almost pure BPA-PC, thermally aged at 100, 120, and 140 °C, and their correlation with optical properties (discolouration, light transmission and relative radiant power value, and yellowing index) of BPA-PC plates are studied and discussed. This work is possibly one of a few (if not the only one) in which the chemistry of degradation and optical properties of degraded plates are correlated. Any improvement in the quality and the lifetime of LED lens plates necessitates a deep understanding of the correlation between the chemistry of degradation and the optical characteristics of plates. Understanding the mechanisms and the products of degradation is crucial to modify the types/amounts of additives (including anti-oxidation and heat stabilizer agents) to the base polymer. The presented reliability model could also be very useful for the producers of BPA-PC LED encapsulant plates, when it comes to the prediction of the life-time of their products. Besides, this will give them the opportunity to do accelerated ageing tests at much shorter times and extrapolate the results to more realistic temperature range. This obviously saves lots of money and time in this industry.

Section snippets

Materials and methods

Pure 3 mm thick BPA-PC plates with industrial purity, manufactured by injection moulding, were heated in a furnace at 100, 120, and 140 °C up to 3000 h. All the optical and chemical tests on degraded specimens were performed at room temperature. The kinetics of degradation reaction at practical operational temperatures is very slow, meaning that any ageing experiment would take a very long time and therefore it will not be feasible. In order to perform experiments at a more reasonable time scale,

Reliability model

Reliability model for the lumen depreciation is defined with exponential luminous decay [30]. The exponential luminous decay is a common model for calculating the lumen depreciation. This model is also applicable for the chemical reaction which leads to the discoloration and yellowing. Reaction rate or lumen depreciation is calculated based on both experimental data’s and exponential model. The exponential luminous decay equation to calculate time-to-failure as given in by [30]Φ(t)=βexp(-at),

Chemical analyses

UV–Vis spectrophotometric scans of thermally-aged BPA-PC plates, heated at 100, 120, and 140 °C up to 3000 h, are shown in Fig. 1. As is seen, the absorbance below 400 nm overall increases significantly with increasing thermal ageing time for samples aged at 120 and 140 °C. Clearly, this increase for the sample, aged at 140 °C, is much more pronounced. The sample, heated at 100 °C, however, did not show any significant increase of the absorbance within the selected period of thermal exposure (3000 h).

Discussion

Thermal degradation mechanisms and its effects on the optical and chemical properties of pure BPA-PC plates at 100, 120, and 140 °C are studied. Thermal ageing of BPA-PC lens could significantly deteriorate the optical properties of LEDs. Rearrangement and oxidation in polycarbonate could result in discolouration and yellowing of BPA-PC encapsulant materials [20], [21], [22]. Discoloration due to the formation of oxidation products and rearrangement (Fries) products or a combination of them

Conclusions

Different experimental methods are used to study the effects of thermal exposure on the degradation of Bisphenol A Polycarbonate (BPA-PC) and its optical properties. The aim was to investigate the relationship between the evolutions of optical and chemical properties of BPA-PC plates after thermal ageing, in order to identify the predominant yellowing mechanism. The results show that increasing the ageing time is associated with discolouration and consequently with the degradation of optical

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