Fabrication Process and Performance Analysis of CSP LED Filaments with a Stacked Package Design

applied sciences Article Fabrication Process and Performance Analysis of CSP LED Filaments with a Stacked Package Design Nan Jiang 1, Jun Zou 1,2,*, Changran Zheng 3, Mingming Shi 2, Wenbo Li 4, Yiming Liu 1, Bin Guo 3, Jerry Liu 5, Herry Liu 5 and Xavier Yin 5 1 School of Meterial Scince and Engineering, Shanghai Institute of Technoligy, Shanghai 201418, China; [email protected] (N.J.); [email protected] (Y.L.) 2 School of Science, Shanghai Institute of Technology, Shanghai 201418, China; [email protected] 3 School of Material Science and Engineering, Changchun University of Science and Technology, Changchun 130022, China; [email protected] (C.Z.); [email protected] (B.G.) 4 Zhejiang Emitting Optoelectronic Technology Company, Zhejiang 310000, China; [email protected] 5 Tecore Synchem, INC., Tianjin 300451, China; [email protected] (J.L.); [email protected] (H.L.); [email protected] (X.Y.) * Correspondence: [email protected] Received: 25 September 2018; Accepted: 8 October 2018; Published: 16 October 2018 Abstract: This article presents a chip-scale package (CSP) based filament light-emitting diodes (LEDs) with a stacked-type structure and a segmented-type luminescence design for higher luminous efficiency and better electrical stability. The innovation strategy improves the luminous efficiency by 7.69% and 18.97% than the traditional dispensing packaged LED filament when the current is at 30 mA and 210 mA, respectively, and reduces the use of red phosphors and green phosphors by 66.7% and 26.2%, respectively. With the increase of the current, the luminous efficiency variations of the stacked-packaged filament are lower than that of the traditional dispensing packaged filament. Moreover, the color temperature of the filament is no longer determined by the calculation of the ratio of various phosphors. The stacked structure and the segmented design can help us to adjust the specified color temperature quickly by changing the proportion and position of the blue chips and the red CSP LEDs. Keywords: optical design and fabrication; chip-scale package; light-emitting diode; optical devices 1. Introdution Attributed to their high efficiency, long life, low power consumption, environmental friendliness, tiny size, and directionality, the recently-developed phosphor-converted white light-emitting diode (WLED) have become the most promising substitute to conventional light sources [1–4]. However, there are still many problems, including the improvement of phosphor conversion and luminous efficiency [5], and improvements in color purity for their substitution of cheaper incandescent and fluorescent lamps that currently occupy the global lighting market [6]. Presently, light-emitting diode (LED) filaments are a fervent LED product. They have three-dimensional luminescence and high plasticity, and can control light distribution according to the application environment. In contrast, the shortcomings are also very prominent, such as low color purity, low light efficiency, and low heat dissipation, so it is difficult to meet the standards of indoor lighting. The dispensing method of traditional LED filament packaging technology has a large variation in optical performance and the drift of color temperature under different currents [7,8]. Color purity is the key to improving luminous efficiency and light output. The blue InGaN LED chip pumped by blue/green phosphor is a mature technology; however, these lack red components [9]. Appl. Sci. 2018, 8, 1940; doi:10.3390/app8101940 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 1940 2 of 10 2+ Therefore, the mixing of a red phosphor viz. Sr2Si5N8: Eu and green/yellow phosphor, namely, 3+ 2+ yttrium aluminum garnet (YAG): Ce or SrSi2O2N2: Eu , is one method to overcome the problem of impure color. Nevertheless, low color purity can be caused by spectral overlapping, which happens when the partial reabsorption of emitted light by another phosphor, mixed with the design of the two phosphors, occurs. [10–15]. To tackle this problem, stacked phosphor layers were developed by Appl. Sci. 2018, 8, x FOR PEER REVIEW 2 of 10 stacking green and red phosphor layers, which have been found to have higher efficiencies. This design showedovercome that the the reabsorption problem of ofimpure green color. emissions Neverthele by redss, low components color purity is can reduced, be caused when by aspectral red phosphor is usedoverlapping, under the greenwhich phosphorhappens when layer the [ 16partial]. reabsorption of emitted light by another phosphor, However,mixed with thus the far, design most of of the the two research phosphors, on stacked occurs. package[10–15]. To structures tackle this is focusedproblem, onstacked remote type phosphor layers were developed by stacking green and red phosphor layers, which have been found packages,to have such higher as phosphor efficiencies. in This glass design (PiG), showed fluorescent that thefilm, reabsorption and other of green forms emissions [17,18]. by For red example, 3+ Xiang etcomponents al. [19] foundis reduced, that when screen-printing a red phosphor ais redused phosphor under the gree layern phosphor on LuAG:Ce layer [16]. PiG obtained a thermally-stableHowever, warm thus WLED. far, most Lee of et the al. research [20] designed on stacked a new package phosphor structures design is focused for reducing on remote the spectral 3+ 3+ overlappingtype packages, of PiGs, such by cuttingas phosphor and reassemblingin glass (PiG), commercialfluorescent film, green and Luother3Al 5formsO12: Ce[17,18].(LuAG:Ce For ) example, Xiang et2+ al. [19] found +that screen-printing a red phosphor layer on LuAG:Ce3+ PiG and red CaAlSiN3: Eu (CASN:Eu ) PiGs. Inspired by what has been explained above, our team obtained a thermally-stable warm WLED. Lee et al. [20] designed a new phosphor design for planned to solve the performance problem of LED filaments by combining chip scale package (CSP) reducing the spectral overlapping of PiGs, by cutting and reassembling commercial green Lu3Al5O12: with a traditional dispensing method. In recent years, CSP has become a very popular research Ce3+ (LuAG:Ce3+) and red CaAlSiN3: Eu2+ (CASN:Eu+) PiGs. Inspired by what has been explained topic inabove, the LED our team packaging planned field to solve [21 ,the22]. performance The package problem size isof onlyLED filaments slightly largerby combining than an chip LED chip, with anscale area package ratio of(CSP) 1.5:1 with [23 a ].traditional CSP has dispensing the advantages method. In of recent having years, a small CSP has package become sizea very and high assemblypopular density research [21]. topic Thus, in inthe this LED work, packaging we report field [21,22]. a new The phosphor package designsize is only for reducingslightly larger the spectral overlappingthan an of LED LED chip, filaments with an area by usingratio of a 1.5:1 CSP [23]. layer CSP and hasa the layer advantages of dispensing of having phosphor. a small package size and high assembly density [21]. Thus, in this work, we report a new phosphor design for 2. Experimentalreducing the Section spectral overlapping of LED filaments by using a CSP layer and a layer of dispensing phosphor. The fluorescent film used for packaging is made by TECORE SYNCHEM, Inc., Tianjin, China, our partner2. Experimental company, Section using their self-developed preparing technology. A red-emitting phosphor CASN: Eu2+The, withfluorescent a wavelength film used for of packaging 618 nm, is was made used by TECORE to make SYNCHEM, the fluorescent Inc., Tianjin, film China, with a mass fractionour of partner 22.2 wt.%. company, The manufacturingusing their self-developed process ofpreparing CSP LEDs technology. is shown A red-emitting in Figure1 .phosphor Firstly, under a CASN: Eu2+, with a wavelength of 618 nm, was used to make the fluorescent film with a mass certain pressure, rearranged chips were pressed into a softened film under vacuum conditions. Then, fraction of 22.2 wt.%. The manufacturing process of CSP LEDs is shown◦ in Figure 1. Firstly, under a after beingcertain baked pressure, in an rearranged oven for threechips hourswere pressed at a temperature into a softened of 300 film C,under the pressedvacuum conditions. chips coated with film wereThen, cut after into being pieces, baked which in an weoven called for three CSP hours LEDs. at a After temperature that, the of red300 °C, CSP the LEDs pressed were chips welded to a 135-mmcoated long with flexible film were substrate cut into pieces, using which a solid we crystal called CSP machine. LEDs. After Each that, flexible the red filament CSP LEDs substrate were was weldedwelded with 51to CSPa 135-mm chips, long which flexible were substrate connected using ina solid series. crystal The machine. uniform Each mixture flexible of filament green-emitting phosphorsubstrate LuAG:Ce was welded3+ and with package 51 CSP resin chips, was which used were as connected a second in layer series. package The uniform for themixture CSP of filament. green-emitting phosphor LuAG:Ce3+ and package resin was used as a second layer package for the Compared with the traditional dispensing packaging method, this packaging method is more complex CSP filament. Compared with the traditional dispensing packaging method, this packaging method but hasis a more lower complex cost. but has a lower cost. FigureFigure 1. Schematic 1. Schematic stacked stacked type type chip-scale chip-scale package package (CSP) (CSP) filament filament design design and fabrication. and fabrication. Appl. Sci. 2018, 8, 1940 3 of 10 Appl. Sci. 2018, 8, x FOR PEER REVIEW 3 of 10 As mentioned above, stacked packaging structure can improve the optical efficiency of remote As mentioned above, stacked packaging structure can improve the optical efficiency of remote encapsulation of an LED. However, due to the air gap between the interfaces of the two packages, encapsulation of an LED. However, due to the air gap between the interfaces of the two packages, there are many optical phenomena, such as refraction, reflection and scattering at the interfaces [24].

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