1.
Introduction

White light-emitting diodes (WLEDs) are regarded as the fourth generation of light sources, and they are a potential replacement for solid-state lighting in future displays, which has been widely applied in signals, outdoor information displays, liquid crystal display back lighting, landscape lighting, and even the automotive field, since 2010^[1–3]. LEDs have many advantages including energy efficiency, long lifetime, environmental friendliness, wide correlated colour temperatures, and quick start up^{[4, 5]}. A blue LED chip with yttrium aluminium garnet (YAG) yellow phosphors is a conventional light conversion system and has been widely used in industry. Blue chips cannot be adapted into production directly unless they are packaged. Yellow phosphors can emit yellow light under the emission of blue light; the mixture of blue light and yellow light then generates white light. The main source of yellow photons in the WLEDs is mixing the YAG phosphor with silicone or other polymers^[6].



Incorporating flexible substrates in LEDs is a development direction that could further extend their use and has attracted substantial attention in the fields of displays, wearable devices, and lighting^[7–10]. An LED filament is usually constructed as a straight bar and then encapsulated in a bubble. However, this filament shape is unsuitable for a romantic atmosphere or a bulb in which more output performance is required in a limited inner space. Moreover, the luminous power is greatly restricted because of length limitations. To satisfy the needs of different occasions, the shape of the filament must be redesigned.



To easily change the filament, overcome the aforementioned limitations of existing WLED technologies, and enrich the structure of LED filaments, metal should be used as a substrate because of its plasticity and thermal conductivity. In this study, flip chips were attached on a spiral copper substrate through a eutectic welding process, and the copper was covered with a thin layer of thermally conductive plastic. To fabricate a flexible LED filament, two important components must be investigated: the light source and flexible mechanism. A flip chip has excellent stability without a wire. Moreover, heat generated in the flip chip can be dissipated to the packaging board through thermally conductive contact electrodes, rather than the sapphire on the upper level of the chip. Flip chip bonding technology is a potential process in electronic packaging; thus, a flip chip is a more suitable candidate than a dress chip. In addition, the filament shape can easily be changed because of its high flexibility. The flexible spiral substrate can be further shaped to facilitate thermal dissipation. The pitch can be adjusted to achieve different shapes, and then high tensile flexible filaments can be adapted for use.



Therefore, in this study, a spiral filament combining flip chips and a phosphor layer was demonstrated to emit warm white light. A copper substrate was used to host a yellow phosphor because of its high tactility, excellent deformability, and low cost. Furthermore, without the deterioration of total light output, a series of stretches were fulfilled.

2.
Experiments

Fig. 1 displays the manufacturing process of the flexible spiral LED filament with an outside diameter of 26 mm. An average of 94 chips were distributed on the copper substrate; the chip size was 200 × 510 × 150 μm³. In LED chips, the p–n junction converts electricity into light. The luminescence was relatively uniform through the array arrangement. SY-6021 silicone A (Shengzhen Shenghuayang Electronic Material Co. Ltd), SY-6021 silicone B (Shengzhen Shenghuayang Electronic Material Co. Ltd), Lu₃Al₅O₁₂:Ce phosphor (Particle Size: 13 μm, emission band: 530 nm, Yinghe in China, hereafter, phosphor A), (Sr, Ca)AlSiN₃:Eu phosphor (emission band: 640 nm, Yinghe in China, hereafter, phosphor B) and (Sr, Ca)AlSiN₃:Eu phosphor (emission band: 628 nm, Yinghe in China, hereafter, phosphor C) were used in this study. A correlated colour temperature (CCT) of 1800 K, with a weight ratio of silicone A : silicone B : phosphor A : phosphor B of 60 : 15 : 19.5 : 3.0, was used for atmosphere lighting. Moreover, a CCT of 2700 K, with a weight ratio of silicone A : silicone B : phosphor A : phosphor B of 52 : 13 : 12 : 1.12 was used for incandescent lighting. Insulating adhesive and solder paste were used to stick the flip-chip LED on the substrate. The bond head temperature of the paste was 180 °C and the baking time was 40 s. The insulation was located in the middle of the paste to prevent the electrode short circuiting. When these steps were complete, a blue LED arrayed with 94 independent chips was bound around a spiral copper substrate. The rated power output of the blue chips was 0.045 W at 15 mA and the emission wavelength was 460 nm at 150 mA. The maximum emission wavelength was 475 μm under normal operation. The concentration of the mixture substantially influenced the quality of light^[11]. Moreover, different correlated colour temperatures were obtained by controlling the energy ratio of blue and yellow light. The morphology of phosphor gel and the particle distribution also assisted with determining the final optical characteristics of WLEDs^[12–16].






onerror="this.onerror=null;this.src='http://www.jos.ac.cn/fileBDTXB/journal/article/jos/2018/2/PIC/17030029-1.jpg'"
class="figure_img" id="Figure1"/>



Download



Larger image


PowerPoint slide






Figure1.
(Color online) Schematic of the process flow of flexible spiral LED filament.




Figs. 2(a)–2(d) show physical images of the manufacturing process. The optical microscope images in Figs. 2(a) and 2(b) display the substrates without and with welding on the chips. Figs. 2(c) and 2(d) show the substrates without and with phosphor coating after welding on the chips.






onerror="this.onerror=null;this.src='http://www.jos.ac.cn/fileBDTXB/journal/article/jos/2018/2/PIC/17030029-2.jpg'"
class="figure_img" id="Figure2"/>



Download



Larger image


PowerPoint slide






Figure2.
(Color online) (a), (b) Substrates under optical microscope without and with distributing chips. (c), (d) Substrates without and with phosphor coating.




Fig. 3 shows the emission spectra of the flexible spiral WLED filaments. The emission peak of the spectrum was at 623 nm for 1800 K light and 608 nm for 2700 K light. At 450 nm, the emission intensity of 2700 K light was higher than that of 1800 K light. According to the characteristics of the lower CCT light source, the red radiation is more prevalent in the energy distribution. After the CCT was increased, the energy distribution was concentrated and the proportion of blue radiation increased. This demonstrates that the percentage of blue light is the deciding factor for the CCT. Moreover, the CCT increased with the percentage of blue light.






onerror="this.onerror=null;this.src='http://www.jos.ac.cn/fileBDTXB/journal/article/jos/2018/2/PIC/17030029-3.jpg'"
class="figure_img" id="Figure3"/>



Download



Larger image


PowerPoint slide






Figure3.
(Color online) Emission spectra of the phosphor at 1800 and 2700 K.

3.
Results and discussion

First, we did not use a phosphor layer on the flexible spiral substrate. As shown in Fig. 4(a), an array of flip chips attached on the flexible spiral substrate emitted blue light when an electric current was passed through them. Fig. 4(b) indicates that at low currents, the chips and yellow phosphor emitted white light dots. Fig. 4(c) reveals the electroluminescence spectra of the blue LED at an electric current of 15 mA. The filament’s luminous fluxes were measured at 1800 and 2700 K using a calibrated integrating sphere, and the results are shown in Figs. 4(d) and 4(e) as a function of the driving current, which ranged from 8 to 21 mA. The statistical results showed that increasing the input current engendered an increase in the luminous flux; this is because the p–n junction is the emitting part of the LEDs. As the current increased, the number of electrons and holes in the light emitting region increased; thus, the corresponding increase in radiation recombination increased the luminous flux. However, with the same current, the luminous flux at 2700 K was higher than that at 1800 K; however, both fluxes eventually levelled off. Increasing the current also raised the temperature. However, excessive heat accelerates the ageing of the chip and phosphor, consequently harming the optical properties of the filament and finally resulting in a gradually increasing flux. The average luminous efficiency of white light 2700 K is 113 lm/W whereas the white light 1800 K can obtain 81 lm/W. It can be concluded that the luminous efficiency of the spiral filament is high.






onerror="this.onerror=null;this.src='http://www.jos.ac.cn/fileBDTXB/journal/article/jos/2018/2/PIC/17030029-4.jpg'"
class="figure_img" id="Figure4"/>



Download



Larger image


PowerPoint slide






Figure4.
(Color online) (a), (b) Pictures of lighted flexible blue LEDs without and with coating phosphors. (c) Electroluminescence spectra associated with 4–20 mA injection currents. (d), (e) Luminous flux versus injection currents at 1800 and 2700 K (8–21 mA).




To discuss the luminance characteristic of the flexible spiral LED filament in free space, the luminance distribution round spiral LED filament was measured as shown in Figs. 5(a) and 5(b). The filament was measured at 12 positions on average by a BM-7 luminance meter (Hangzhou, Everfine Co. Ltd ) under dark field conditions. Fig. 5(c) shows the changing of luminance with changing of the angle. Moreover, we calculate the uniformity with the formula:






onerror="this.onerror=null;this.src='http://www.jos.ac.cn/fileBDTXB/journal/article/jos/2018/2/PIC/17030029-5.jpg'"
class="figure_img" id="Figure5"/>



Download



Larger image


PowerPoint slide






Figure5.
(Color online) (a) Setup of experiment in dark field operating uniformity test. (b) The sketch of angle adjustment between testing positions. (c) Picture of luminance distribution round the flexible spiral LED filament.

$${ m{Uniformity}} = left( {{L_{min }}/{L_{max }}} ight) times 100% .$$

For a flexible spiral LED filament, its uniformity is 92%. It demonstrates a pleasurable uniform output of luminance.



Following successful use under flat conditions, the device performance was tested under tensile conditions. If the tensile conditions severely change the light output, the filament would be unsuitable for application in related cases, and the design should be revised. Stretching the 2D metal substrate produced a 3D conical spiral structure. The filament achieved 360 degrees of luminous effect, and the metal substrate was able to cool rapidly. At a height of 0 cm, the device was not stretched, and as the height increased, the device was stretched further. Figs. 6(f)–6(i) shows that the flexible spiral WLED filaments worked sufficiently at various stretching heights. The spiral comprised 3.5 circles.






onerror="this.onerror=null;this.src='http://www.jos.ac.cn/fileBDTXB/journal/article/jos/2018/2/PIC/17030029-6.jpg'"
class="figure_img" id="Figure6"/>



Download



Larger image


PowerPoint slide






Figure6.
(Color online) Filament at different stretching heights.




To study the thermal characteristic of the surface at different heights, the thermal temperature was measured using an infrared temperature instrument. On the basis of the colour of the filament in Fig. 7(a), the highest point of the temperature field was in the inner circle of the spiral WLEDs. The temperature gradually decreased as the radius increased.






onerror="this.onerror=null;this.src='http://www.jos.ac.cn/fileBDTXB/journal/article/jos/2018/2/PIC/17030029-7.jpg'"
class="figure_img" id="Figure7"/>



Download



Larger image


PowerPoint slide






Figure7.
(Color online) Temperature field distribution at different stretching heights.




The current power conversion efficiency for blue LED chips is approximately 55%; thus, 45% of the input electrical energy is directly converted to heat^[17]. Fig. 8(a) shows that for different stretching heights, as the current increased, the highest temperature of the filament gradually increased. For the spiral WLEDs, the temperature was the highest with no stretching. The temperature decreased as the stretching height increased from 0 to 2 cm, and it increased when the stretching heights increased from 2 to 3 cm. This indicates that the stretching height of 2 cm is the inflection point. Fig. 8(b) shows that as the current increased, the luminous flux of the flexible spiral WLED gradually increased. The slope of the curve gradually decreased, with the line finally becoming flat. The luminous flux was the lowest without any stretching, and it increased as the stretching height increased from 0 to 2 cm. As the stretching height continued to increase, the luminous flux became stable. This indicates that the stretching height of 2 cm is the inflection point for the luminous flux. As illustrated in Figs. 8(a) and 8(b), the temperature and luminous flux exhibited an inverse relationship. Without appropriate heat dissipation, the temperature of a p–n junction rises rapidly, which would affect the properties of luminescence. LED chips generate both light and heat, which are then transmitted through the packaging materials. The power of the input electrical energy is certain; according to the law of the conservation of energy, the sum of the converted light and heat energy is constant. Therefore, the accumulation of heat implies a decrease in light output. Moreover, the higher the tension of the filament is, the larger the pitch is, and the wider the unit space volume for cooling becomes.






onerror="this.onerror=null;this.src='http://www.jos.ac.cn/fileBDTXB/journal/article/jos/2018/2/PIC/17030029-8.jpg'"
class="figure_img" id="Figure8"/>



Download



Larger image


PowerPoint slide






Figure8.
(Color online) (a) Highest temperature at different stretching heights and injection currents ranging from 8 to 20 mA. (b) Luminous fluxes associated with different stretching heights at injection currents ranging from 8 to 20 mA.




The accumulated heat negatively influences the reliability and longevity of LEDs; thus, ageing tests should be performed. As presented in Fig. 9(b), the flexible spiral WLED filament was sealed in the glass bulb and lit up for 1000 h without interruption at room temperature (26 °C). The current was 15 mA and the stretching height was 2 cm. The luminous flux of the flexible spiral WLED decayed by only 0.85%.






onerror="this.onerror=null;this.src='http://www.jos.ac.cn/fileBDTXB/journal/article/jos/2018/2/PIC/17030029-9.jpg'"
class="figure_img" id="Figure9"/>



Download



Larger image


PowerPoint slide






Figure9.
(Color online) (a) Packaged light bulb with flexible spiral WLED filament. (b) Ageing test of spiral WLED device over 1000 h.

4.
Conclusion

This study presents a WLED filament with a new shape and favourable tensile properties. In this report, a spiral copper substrate, face-up chips, and phosphors were applied to produce the flexible spiral WLED filament. CCTs of 1800 and 2700 K were realised by controlling the weight ratio of red and yellow phosphors. The luminous flux changed according to the injected current. The filament showed optimal optical characteristics at a stretching height of 2 cm. Moreover, according to the ageing test, the filament exhibited high reliability. We believe that the flexible spiral WLED filament has numerous applications.