| Based on technology of Chip Scale Package?CSP?,LEDs mainly have the advantages of small package size,low cost of materials and processes,good heat dissipation and so on.However,during the preparation of fluorescent gel used for CSP package,it is founded that phosphor inside disperses unevenly and tends to precipitate,which results in affecting the luminous efficiency of LED and the appearance of color spots or color circles.Moreover,the above situation also accounts for the existing phenomena of being easy to aging,shedding,cracking and so on in LED packaging materials,which may lead to LED failure.Therefore,the preparation of fluorescent gel with excellent performance as a packaging material is in great need,occupying great significance in the development of domestic LED industry.In this paper,moderate modification research is executed aiming at the modification of particle deposition,aging and other problems that often occur in fluorescent gel used for CSP package.At the same time,the aging properties of fluorescent gel are characterized and the service life prediction is studied.The experimental samples were mainly subjected to infrared analysis,nuclear magnetic analysis,particle size analysis,SEM scanning,thermomechanical analysis?TMA?,dynamic mechanical analysis?DMA?,thermogravimetric analysis?TGA?,rheological analysis,mechanical properties analysis,and X-ray photo-electron spectroscopy analysis?XPS?.And finally,the dielectric properties of the fluorescent gel under high temperature accelerated aging are tested by high-frequency impedance analyzer.Firstly,the effect of the different content of fumed silica on the phosphor precipitation in fluorescent gel is studied by using fumed silica as filler.The results show that the anti-precipitation effect of fumed silica reaches to the best when the amount of fumed silica is 1.5 wt%2 wt%,and the settling velocity of the fumed silica is 2.11×10-88 m·s-1 with a reduction of about 23.3%.Besides,silane coupling agent KH-570 is used to modify the surface of SiO2,which is able to reduce the average particle size of SiO2 to 6.422?m with a reduction of about 46%.Furthermore,Combined with the analysis of the storage modulus G'at low strain and the scanning pictures of fracture surface of fluorescent gel,it can be concluded that SiO2 particles modified by 5 wt%KH-570 distributes more uniformly inside the silicone rubber with a better compatibility between the colloidal powder particles and silicone rubber,which can significantly ameliorate the situation of the precipitation of phosphor.Next,the base material consisted of synthetic vinyl raw rubber and hydrogen silicone oil is also modified by different content of phenyl vinyl silicone resin.The results of measurement show that the transmittance and refractive index of silicone rubber increases firstly and then decreases with the increase of the amount of phenyl silicone resin.The highest transmittance of the silicone rubber reaches 98.9%and the refractive index reaches 1.534.And its coefficient of thermal expansion can be reduced to 2.69×10-44 K-1 corresponding to a decrease of about 30%.Besides,the G'of the modified silicone rubber can reach to its maximum of 20.837 MPa,which is about 176%higher than the unmodified one.In addition,based on the heat accelerated aging test at 200?for 200h,the results show that the silicone rubber modified by 5 wt%7 wt%phenyl vinyl silicone resin still possesses stable optical properties and thermomechanical properties.And then,the sol-gel method and the hydrolysis condensation method are applied to the modification of embedding Ti element into the small molecule silicone respectively.Modified by sol-gel method,the transmittance of silicone rubber is up to99.232%,of which the highest refractive index reaches 1.56,the lowest coefficient of thermal expansion is 2.41×10-44 K-1 and the highest thermal decomposition temperature is 304.8?.The orthogonal design is used to determine the optimal modification scheme of the dehydration-condensation method,and the best scheme is that phenyl vinyl siloxane modified by 7 wt%TBT reacts at 90?for 24 h with a catalyst loading of 0.9 wt%,which can acquire silicone rubber of better optical properties with a light transmittance of 99.402%,a refractive index of 1.6,a thermal expansion coefficient of 2.4×10-44 K-11 and an initial thermal decomposition temperature of 314.7?.In the comparison of the effects of two Ti-based methods on the optical properties and the thermomechanical properties of silicone rubber treating at 120?for 200h,the results show that both of them can keep the transmittance of silicone rubber above 99%after heat aging at high temperature.The refractive index of the silicone rubber modified by the sol-gel method reduces from 1.56 to 1.55 and the thermal expansion coefficient decreases to 2.35×10-44 K-1.While the refractive index and the thermal expansion coefficient of the silicone rubber modified by the hydrolysis condensation method remain invariant before and after aging,which indicates more stable optical properties.Furthermore,the G'of the silicone rubber modified by the hydrolysis condensation method obviously increases at room temperature.The G'of the silicone rubber before modification is 7.55 MPa,while the G'of the silicone rubber modified by the Ti element is 33.30 MPa,which increases by about 341%.Finally,FT-IR and XPS analysis are used to characterize the changes of the functional groups and the changes of the elements in the fluorescent paste during aging.And based on the results it can be inferred that the aging reaction of the fluorescent gel is mainly the oxidation of the side groups.Besides,under the conditions of oxygen and high temperature,the methyl and vinyl groups on the side groups of the silicone rubber in the fluorescent gel may undergo an oxidation reaction and further cause cross-linking,leading to cross-linking hardening and cracking of the material.Furthermore,the characteristic parameters of the aging properties of the silicone rubber before and after the modification were determined by research.The variation of the dielectric constant of the two silicone rubbers with the aging time can be fitted by the Arrhenius equation ???.With a 20%drop in light transmittance,the use of CSP packaging material is critical,and the dielectric constant at this time is also a critical value.Calculate the aging time corresponding to the critical value of the dielectric constant at different temperatures before and after modification.It can be found that tc and 1/T can be fitted by the first-order kinetic equation ???In the unmodified silicone rubber,tc was fitted to 1/T.The a value was-0.49355,and the b value was 1363.19 K.The corresponding aging activation energy E was26.10 KJ/mol.The service life of unmodified silicone rubber at room temperature?25??was calculated as 12589 h,524 d.After Ti-modified silicone rubber tc and 1/T fitting,a value of 1.90758,b value is 828.97 K,aging activation energy E is 15.87KJ/mol,and the service life at room temperature?25??is 63095 h,2628 d. |
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