Effects of humidity and phosphor on silicone/phosphor composite in white light-emitting diode package
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Silicone/phosphor composite, which serve as mechanical protection and light conversion material, is an integral part of white light-emitting diode (LED) package. In this paper, a comprehensive study is conducted to investigate the effect of humidity and phosphor on moisture absorption, hygroscopic swelling, mechanical behavior, as well as thermal properties of silicone/phosphor composite in comparison with the pure silicone. SEM/EDAX and FTIR were performed to identify the phosphor and silicone compositions. Through moisture sorption test, it has been observed that the addition of phosphor significantly lowers the capacity of moisture absorption, but accelerates the diffusivity of moisture absorption. The hygroscopic swelling test showed that the phosphor has little effect on the swelling compared to the pure silicone sample. Both moisture absorption/desorption and hygroscopic swelling/de-swelling are reversible. Strain ramp test revealed that the phosphor enhances the stiffness of the composite. The moisture absorption, however, has negligible impact on mechanical stiffness for both pure and composite samples. Finally, thermal expansion test showed that the coefficient of thermal expansion does not change with the addition of phosphor into pure silicone.
From the recent study [6, 7, 8, 9], it has been observed that most degradation of white LED products is mainly caused by the degradation of encapsulants. Some researchers [10, 11, 12, 13] reported that high temperature and high humidity usually contributed to LED’s failure. Although silicone encapsulant exhibits better thermal stability than epoxy-based resins, the darkening and carbonization phenomenon were observed under high temperature and high humidity condition [14, 15]. Absorption of moisture from environment causes the change in silicone properties like coefficient of thermal expansion and elasticity modulus [16, 17]. The water trapped inside silicone capsule may cause bubble generation , dissolution of the phosphors , and light scattering . Fan et al. [20, 21] studied the interaction of silicone and phosphor, and showed that moisture could be the reason behind degradation of silicone/phosphor composite due to pH change. It further causes the degradation of light-conversion efficiency of phosphors and the transmittance of silicone. Due to this reason it suffers a high temperature treatment during operation [22, 23]. Higher temperature at the phosphor/binder interface can cause discoloration, decohesion, and cracking of the interface layer . Zhang et al.  reported that heat treatment of multicolor phosphor films leads to serious degradation of luminous intensity. According to the study of Wang et al. , the introduction of phosphor powder into the silicone matrix shortens the cure rate, while ultraviolet radiation and temperature accelerate the cure of silicone/phosphor composite by increasing the rate of cross-linking. Khalilullah et al.  tested the silicone/phosphor composite under high moisture condition and found irreversible swelling despite the reversibility of moisture absorption/desorption. Another study  revealed that addition of phosphor with a high temperature aging will stiffen the silicone matrix significantly. Increased Young’s modulus of the silicone was observed with aging time which causes severe stress conditions and cracks in the material. Recently, Fan et al.  investigated the curing mechanism of phosphor/silicone composites and evaluated their mechanical properties after a high moisture ageing test. Tang et al. studied the reliability of the chip-scale packaged LEDs under overdriving condition .
In this paper, a comprehensive study is conducted to investigate the effect of humidity and phosphor on moisture absorption, hygroscopic swelling, mechanical behavior, as well as thermal properties of silicone/phosphor composite in comparison with the pure silicone. Specifically, in situ hygroscopic swelling/de-swelling behavior is restudied for the repeatability and to confirm the non-existence of irreversible swelling observed in the previous study . SEM/EDAX and FTIR were performed to identify the phosphor and silicone compositions. A series of tests, including in situ moisture absorption/desorption, hygroscopic swelling, strain ramp test, as well as thermal expansion measurement, were conducted.
2 Experimental method
Silicone and silicone/phosphor composite materials were used in this study. For both samples, a general manufacturing procedure was provided by a manufacturer company, keeping the chemical composition of the samples unknown. The silicone/phosphor samples were prepared by mixing the phosphor powder with silicone, then a curing process was performed as follows: 1-hour prebake at 100 °C, 2 h curing at 150 °C and then cooling to ambient temperature. The pure silicone sample was prepared similarly but without the addition of phosphor. In this study, the thickness of all test samples is made as ~ 0.5 mm, which is a typical thickness in actual LED package. The concentration of phosphor in composite is ~ 9% in mass fraction.
Identification of the phosphor in the sample was achieved using energy dispersive spectroscopy (EDS). Samples were placed in a Hitachi S-3400 N (SEM) equipped with a back-scatter detector and an AMETEK EDAX EDS system. Samples were prepared by securing small pieces cut from the bulk silicone phosphor composite using conductive carbon tape and copper tape to reduce charging of the electrically insulating silicone. To increase the sensitivity of the EDAX analysis the elemental mapping was used to identify subsurface phosphor particles based on threshold X-ray emission at characteristic wavelengths.
Attenuated total reflectance fourier transform infrared spectroscopy (ATR-FTIR) was used to characterize the silicone composite and pure silicone samples. A perkin elmer spectrum Two FTIR was used. Samples were prepared by cutting small pieces from the bulk. Samples were placed in the ATR accessory and the spectra were captured.
2.4 TGA sorption
Dynamic mechanical analysis (DMA) was performed using at TA DMA Q800 equipped with the humidity control (see Fig. 2). The DMA-RH accessory enables analyzing the hygroscopic swelling behavior by controlling temperature and relative humidity. Dry nitrogen was used to dehumidify the sample chamber. The samples were cut into 10 × 3 mm size. One of the 3 mm wide ends was held by a fixed clamp and other end was held by movable clamp. Stress–strain curves were measured for pure silicone and silicone/phosphor composite samples with and without the effect of humidity.
Thermomechanical analysis (TMA) measures sample displacement as a function of temperature, time and applied force. TMA was used to determine the linear coefficient of thermal expansion (CTE) for pure silicone and silicone/phosphor composite samples were measured from 25 °C to 300 °C.
3 Mathematical model for moisture diffusion
Here, Co is initial concentration in the material and Cs constant moisture concentration at the surface, and h is thickness of the plate.
4 Data regression
Here, SSTotal is the sum of squared differences between average value and the data points. R2 is the measure of how much experimental data fits the theoretical model. The values of R2 ranges between 0 to 1. Value 1 means a perfect association of theoretical model and experimental data, whereas 0 means no association.
5 Results and discussions
Extracted diffusivity and saturated moisture concentration from absorption
COD (coefficient of determination)
Extracted diffusivity and saturated moisture concentration from desorption
COD (coefficient of determination)
In this paper, the effect of humidity and phosphor addition on silicone/phosphor composite was investigated. We performed an in situ TGA moisture absorption/desorption test for both silicone and silicone/phosphor samples. The results showed both samples follow the Fickian diffusion behavior well. Moisture absorption and desorption are also reversible with the addition of phosphor into silicone. However, the composite reduces the significant amount of moisture absorption compared to the pure silicone, and the diffusion rate increases. In-situ hygroscopic swelling test showed that with and without phosphor, the silicone and composite have almost the same amount of swelling upon moisture absorption. Through the careful experimental analysis, it has confirmed that the previous observation on the permanent swelling for silicone/phosphor composite was false information due to instrument setup issue. The hygroscopic swelling is reversible in absorption/desorption cycle. In strain ramp test, it showed that the moisture has no effect on stress–strain behavior for both silicone and silicone/phosphor samples. But the addition of phosphor enhances the stiffness of material. TMA analysis indicates that the phosphor into silicone does not change the coefficient of thermal expansion in a wide temperature range. In summary, the comprehensive experimental characterization provides a full picture of the effect of humidity and phosphor on the mechanical behavior of silicone/phosphor composite.
Jiajie Fan would like to acknowledge the support from the National Natural Science Foundation of China (Grant No. 51805147, 61673037).
- 1.W.D. van Driel, X. Fan (eds.), Solid state lighting reliability: components to systems (Springer, Berlin, 2012)Google Scholar
- 2.W.D. van Driel, X. Fan, G.Q. Zhang (eds.), Solid state lighting reliability part 2: components to systems (Springer, Berlin, 2018)Google Scholar
- 6.C. Jiang, J.J. Fan, C. Qian, H. Zhang, X.J. Fan, W.L. Guo, G.Q. Zhang, Effects of voids on mechanical and thermal properties of the die attach solder layer used in high-power LED chip-scale packages. IEEE Trans. Compon. Packag Manuf. Technol. 99, 1–9 (2018)Google Scholar
- 9.Trevisanello L, De Zuani F, Meneghini M, Trivellin N, Zanoni E, Meneghesso G (2009) Thermally activated degradation and package instabilities of low flux LEDs. In Reliability Physics Symposium, 2009 IEEE International (pp. 98–103). IEEEGoogle Scholar
- 14.Velderrain M (2012) Designing low permeability, optical-grade silicone systems: guidelines for choosing a silicone based on transmission rates for barrier applications. In Advances in display technologies II. International Society for Optics and Photonics, Bellingham, p 82800OGoogle Scholar
- 20.J. Fan, M. Zhang, X. Luo, C. Qian, X. Fan, A. Ji, G. Zhang, Phosphor–silicone interaction effects in high power white light emitting diode packages. J. Mater. Sci. 28(23), 17557–17569 (2017)Google Scholar
- 21.Luo X, Fan J, Zhang M, Qian C, Fan X, Zhang G (2017) Degradation mechanism analysis for phosphor/silicone composites aged under high temperature and high humidity condition. In Electronic Packaging Technology (ICEPT), 2017 18th International Conference on pp. 1331–1336. IEEEGoogle Scholar
- 25.Zhang M, Fan J, Qian C, Fan X, Ji A, Zhang G (2016) Analysis of photoluminescence mechanisms and thermal quenching effects for multicolor phosphor films used in high color rendering white LEDs. In Electronic Packaging Technology (ICEPT), 2016 17th International Conference on p 334–340. IEEEGoogle Scholar
- 26.Wang Z, Fan J, Liu J, Hu A, Qian C, Fan X, Zhang G (2018) Study of ultraviolet assisted cure mechanism of the phosphor/silicone composites used in White LEDs. In 2018 19th International Conference on Electronic Packaging Technology (ICEPT) p 525–530. IEEEGoogle Scholar
- 27.I. Khalilullah, T. Reza, L.B. Chen, M. Placette, M.H.A.K.M. Mazumder, J. Zhou, J.J. Fan, C. Qian, G.Q. Zhang, X.J. Fan, In-situ characterization of moisture absorption and hygroscopic swelling of silicone/phosphor composite film and epoxy mold compound in LED packaging. Microelectron. Reliab. 84, 208–214 (2018)CrossRefGoogle Scholar
- 32.X.J. Fan, E. Suhir (eds.), Moisture sensitivity of plastic packages of IC devices (Springer, New York, 2010)Google Scholar
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