Interfacial Properties of Phosphor Filled in Silicone

Phosphor Concentration in Silicone and Its Effect on the Mechanical and Interfacial Properties of Phosphor-Filled Silicone

Xing Chen1, 2, Simin Wang1, 2, Xiaogang Liu1, 2, Sheng Liu1, 2, *

1 Institute of Microsystems, State Key Laboratory for Digital Manufacturing Equipment & Technology, School of Mechanical & Engineering, Huazhong University of Science & Technology, Wuhan, China, 430074

2 Division of MOEMS, Wuhan National Laboratory for Optoelectronics, Wuhan, China, 430074 * [email protected]

Abstract

The phenomenon that phosphor particles tend to settle in silicone is widely known. Recent researchers have discussed the effects of phosphor concentration on luminous efficacy of LED packaging. But to produce reliable products, mechanical and interfacial considerations are also essential. In this paper, the mechanical behaviors and interfacial strength of silicone with different levels of phosphor concentration are studied. In our experiments, four groups of silicone samples with different levels of phosphor concentration are prepared and are subjected to the uniaxial tensile loads. The results of tensile tests indicate that, for the same level of phosphor addition, higher phosphor concentration will result in lower tensile strength of the phosphor-filled silicone. SEM cross-section images show that the phosphor particles concentrate in the bottom area of the silicone layer. The fractograph of the material indicates that the crack initiates among the bottom area where phosphor particles settle. Interfacial tests of phosphor-filled silicone and GaN substrate are also conducted. The results demonstrate that phosphor concentration has negative effect on interfacial strength of materials. Therefore, for LED package, uniformly distributed phosphor in silicone is extremely demanded for mechanical and interfacial considerations.

Introduction Light emitting diodes (LEDs) continues to be of interest

for their intrinsic high energy efficiency, lack of pollution, high quality, as well as compact size [1, 2]. Nevertheless, further improvements in the lifetime, stability, and light conversion efficiency are urgently demanded for LED devices being adopted as next-generation lighting sources. To improve the reliability of LED module, package materials should be considered. Among various materials used in the LED packaging, phosphor-filled silicone layer plays a crucial role on optical performance, as well as long-time reliability, of the LED module. Though the optical properties of phosphor coating layer are frequently discussed by researchers [3-6], the mechanical properties, which are closely connected to the reliability of packaging, are rarely concerned. Lack of mechanical behavior and interfacial property of phosphor-filled silicone material highly limits the quantitative assessment of the reliability of LED package.

It is known that the phosphor particles tend to settle inside the phosphor encapsulant mixture during the curing process. The non-uniform distribution of phosphor throughout the cured encapsulant will significantly affect the consistency of correlated color temperature (CCT) of LEDs

[7-9]. And we suppose such this phenomenon as phosphor concentration could also affect the mechanical properties of phosphor coating material. For these considerations, we are motivated to investigate phosphor concentration in phosphor-filled silicone from mechanical and interfacial perspectives. In our experiments, the well-distributed phosphor-filled silicone is left to stand for various lengths of time (0h, 1h, 3h and 10h) in order to prepare the silicone with different levels of phosphor concentration. Tensile tests are adopted to obtain mechanical properties of test samples, and engineering stress-stain curves of samples are automatically recorded during the tests. Fractograph of samples are investigated after tensile tests by SEM method. Furthermore, interfacial tests of phosphor-filled silicone on GaN substrates are conducted to study the interfacial strength of material. Finally, the conclusions are summarized and suggestions are proposed.

Experimental Tensile tests are conducted to obtain mechanical

properties at first. SEM image of fracture surface of test samples are investigated afterwards. Interfacial shearing tests are carried out finally.

Phosphor particles are mixed into silicone with mass ratio of phosphor-silicone of 0.3:1. After stirring and vacuum-pumping, the phosphor-filled silicone is divided into four groups. One group of silicone is cured immediately and the other three groups are left to stand for 1 hour, 3 hours and 10 hours, respectively. After curing, the samples are mechanical cut and smoothed according to ASTM 1708 standard (as shown in Fig. 1 and Fig. 2).

Fig. 1 Dimensions of Test Samples (Unit: mm)

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Fig. 2 Photograph of Test Samples

Tensile tests are conducted by a mechanical testing machine (Fig. 3). This machine, which has a portal frame structure, provides a mechanical testing platform with load range of 500N and displacement resolution of 0.08μm. The testing machine is calibrated according to ASTM standards to guarantee accuracy and consistency.

Fig. 3 Photograph of Mechanical Testing Machine

Fracture surfaces of samples are observed by SEM after

tensile tests. Frctographs of phosphor-filled silicone are investigated.

In interfacial test part, samples are cured in square frames on GaN substrates. The dimensions of the frame are shown in Fig. 4. The phosphor-filled silicone is restricted to the inner region of the frame so that the contact areas of silicones and GaN substrates are the same for all the samples (as shown in Fig. 5).

Fig. 4 Dimensions of the Frame (Unit: mm)

Fig. 5 Samples for Shearing Tests

The force-displacement curves of test vehicles are recorded by computer during shearing tests (Fig. 6).

Fig. 6 Photographs of Interfacial Shearing Tests

Experimental Results Results of tensile tests will be presented firstly.

Fig. 7 Stress-strain Curves of Samples

The tensile curves obtained by mechanical testing

machine indicate that longer standing time of the sample results in lower tensile strength of the material (as shown in Fig. 7 and Fig. 8). Moreover, the non-linear behaviors of phosphor-filled silicones become significant when the standing time of the uncured samples reaches 10 hours.

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Fig. 8 Tensile Strengths of Test Samples

Studies of fractographs of tested samples demonstrate that

the crack initiates around region where phosphor particles concentrate, and the crack propagates with a radial pattern through the material till final collapse (as shown in Fig. 9).

Fig. 9 Fractograph of the Test Sample

It can be seen from Fig. 10 (a)-(d) that more phosphor

particles concentrate around bottom area of silicone as the standing time of silicone sample increases. This phenomenon inevitably results in the reduction of tensile strength of phosphor-filled silicone since such material is weakened by the non-uniform distribution of phosphor particles. Moreover, the highly non-uniform distribution is the reason for the non-linear behaviors of the test samples.

Fig. 10 Fractographs of Four Groups of Samples (Standing Time: (a) 0h, (b) 1h, (c) 3h, (d) 10h)

Results of interfacial shearing tests of phosphor-filled silicones on GaN substrates also indicate that phosphor concentration has negative effect on the interfacial strengths of test samples. As shown in Fig. 11 and Fig. 12, the critical shearing forces decrease along with the increase in standing time of uncured samples.

Fig. 11 Shearing Curves of Samples

Fig. 12 Critical Shearing Forces of Samples

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Since phosphor particle has poorer adhesion than pure silicone, when phosphor particles are gathered in interfacial region where the silicone and GaN substrate contact directly, the adhesion property of the test sample will degrade [10, 11].

Conclusions Mechanical and interfacial properties of phosphor-filled

silicone are studied in this paper. SEM method is adopted to investigate the microstructure of test samples.

The results of experiments demonstrate that phosphor concentration has negative effects on both the mechanical property and the interfacial strength of phosphor-filled silicone.

Therefore, the uniform distribution of phosphor particles in silicone is highly demanded for mechanical and interfacial considerations in LED packaging.

Acknowledgments This work is supported by Nature Science Foundation of

China (NSFC) Key Project under grant number 50835005, Guangdong Real Faith Optoelectronics Inc. and Shanghai Fine MEMS Inc.

The authors gratefully acknowledge the assistance from Lei Chen and give special thanks to Dr. Han Yan.

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