White light-emitting diodes (LEDs) have been widely developed and applied in the field of lighting due to their energy saving, environmental protection, high reliability and flexible design. In order to meet the growing demand for lighting, the development and technical improvement of larger output power LEDs has been widely carried out.
1. Defects in the formal packaging structure
At present, many commercial LEDs use a gold wire to connect the PN junction of the chip to the positive and negative package structure of the bracket. However, with the continuous improvement of output power, failure problems such as large light attenuation and light quenching that restrict the development of high-power LEDs have emerged.
The main cause of quenching failure is gold wire breakage. During the gold wire lead connection process, it is affected by multiple factors such as gold purity, bonding temperature, gold wire bending, welding machine precision and bonding process, causing the gold wire to be broken and quenched. Secondly, the silica gel mixed with the phosphor is coated on the surface of the chip to play the dual role of light conversion and protection of the gold wire. When the chip is energized, the temperature rises, and the silicon wire and the solder joint will be impacted due to thermal expansion and contraction of the silica gel. , solder joints are desoldered, causing quenching.
The main reason for the large failure of light decay is the yellowing or transmittance of silica gel. The positive-loading structure LED p, n electrode on the same side of the LED, the current must flow laterally through the n-GaN layer, resulting in current crowding, high local heat, limiting the drive current; second, due to poor thermal conductivity of the sapphire substrate, severely hindered heat Lost. During long-term use, the high temperature caused by poor heat dissipation affects the performance and transmittance of the silica gel, resulting in a large attenuation of light output power.
Therefore, in order to improve the problem that the gold wire of the LED packaged in the package is easy to break and the heat dissipation is not good, researchers in the industry have successively invented the vertical structure LED and the flip-chip structure LED.
Compared with the formal LED, the vertical structure uses a substrate with high thermal conductivity (substrate such as Si, Ge, and Cu) instead of the sapphire substrate, which greatly improves the heat dissipation efficiency; the two electrodes of the vertical structure LED chip are respectively Both sides of the LED epitaxial layer pass through the n-electrode, so that almost all of the current flows vertically through the epitaxial layer of the LED, and the current flowing in the lateral direction is extremely small, and local high temperature can be avoided. However, in the current vertical structure preparation process, the sapphire stripping process is difficult, which restricts the industrialization development process.
The inverted LED of another invention has been widely integrated into the lighting industry because of its integration and mass production, simple preparation process and excellent performance. The flip-chip structure uses the PN junction of the chip directly to the eutectic bonding of the positive and negative electrodes on the substrate, and does not use the gold wire, thereby avoiding the problem of light quenching to the utmost. In addition, the eutectic bonding structure greatly improves the heat dissipation problem. In the process of using high-power LEDs, large current impact is inevitable. Under this circumstance, if the high-current impact stability of the luminaire is not good, it is easy to reduce the service life of the luminaire.
Therefore, this paper compares and studies the variation of light output of vertical structure LED and flip-chip LED with increasing current, and compares it with ordinary formal LED. It is concluded that flip-chip LED has better resistance to large current surge. Sex and light output performance.
2. Sample preparation and test methods
2.1 Sample preparation
The three package structures are shown in Figure 1. The LEDs in the front-mounted LED adopt the sapphire substrate with a peak wavelength of 448 nm, the flip chip adopts the sapphire substrate with a peak wavelength of 447 nm, and the vertical structure chip uses the silicon substrate with a peak wavelength of 446 nm. The three chips are all 1.16 mm x 1.16 mm and have an operating current of 350 mA. The silica gel is manufactured by Priusson's 0967 model and the phosphor is made of Whisper's YAG-4. The positive and negative poles of the positive-loading structure chip are soldered to the positive and negative electrodes of the bracket by gold wire bonding; the positive electrode of the vertical structure chip is soldered to the positive electrode of the bracket by gold wire bonding, and the negative electrode is bonded by gold ball eutectic bonding. On the negative electrode of the stent; the positive and negative electrodes of the flip chip are bonded to the positive and negative electrodes of the stent by gold ball eutectic bonding.
2.2 Test methods
The luminous flux, luminous efficiency and color temperature are measured by the STC4000 fast spectrometer produced by Hangzhou Yuanfang Co., Ltd. The test principle is shown in Figure 2. The LED to be tested is placed in the center of the integrating sphere by a fixed fixture. The LED is emitted through the white diffuse reflection layer inside the integrating sphere. The diffuse reflection part of the light is transmitted to the micro multi-channel spectrometer through the narrow-pass optical aperture fiber on the surface of the integrating sphere. The data collected by the spectrometer passes through the USB. The interface is sent to the computer for processing and display. The light source is powered by a constant current source.
3. Results and discussion
3.1 The relationship between luminous flux and current
Figure 3 shows the change in luminous flux as a function of current for flip-chip LEDs, vertical package LEDs, and package LEDs with drive currents from 50 mA to 2 000 mA. It can be seen from Fig. 3 that as the current increases gradually, the luminous flux of the three structural LEDs increases with the increase of the current, but the growth rate gradually decreases.
When the drive current reaches 1 200 mA, the vertical structure LED first reaches the luminous flux saturation point, and the luminous flux of the flip-chip LED under this current condition is 14.7% higher than that of the formal LED, which is 25.9% higher than that of the vertical structure LED. As the current continues to increase, the change in luminous flux of the vertical structure LED indicates that it is near failure. The luminous flux of the flip-chip LED is saturated at a current of 1 550 mA, which is 350 mA more than the saturation current of the vertical structure LED. The luminous flux test results show that the flip-chip structure has a low PN junction temperature and good heat dissipation.
Therefore, it is concluded that flip-chip LEDs have higher reliability than other two types of LEDs, especially high resistance to high current surges. This performance is beneficial to improve the service life of LEDs in practical applications.
3.2 Luminous efficiency as a function of current
Figure 4 shows the relationship between three LED structure currents and luminous efficiency. It can be seen from Fig. 4 that when the current is increased from 50 mA to 2 000 mA, the luminous efficiency of the three LEDs is declining, and the luminous efficiency of the flip-chip LED is higher than the other two LEDs in the entire current variation range. Luminous efficiency. The vertical structure LED has a current of more than 1 200 mA, and the luminous efficiency drops rapidly, indicating that the light output is abnormal, which is in agreement with the luminous flux test result. When the operating current of the three LEDs is 350 mA, the luminous efficiency of the flip-chip LED is 8 lm/W higher than that of the vertical structure LED, which is 31 lm/W higher than the LED of the formal structure.
The increase in luminous flux and luminous efficiency of flip-chip LEDs may be due to:
(1) The external quantum efficiency of the flip-chip LED is high. The refractive index profiles of the three package structures are shown in Figure 5. Figure 5a shows the refractive index profile of the flip-chip package structure; Figure 5b shows the refractive index profile of the vertical package structure and the package structure. According to Snell's law, the critical angle of total reflection of flip-chip LED light from GaN to sapphire is θ=sin-1 (n sapphire/n GaN)=44.5°, and the critical angle of sapphire to packaged silica gel is θ=sin-1 (n-silica/ n sapphire) = 57.4 °; while the vertical structure and the LED of the formal LED are directly transmitted from the GaN to the encapsulated silica layer, the critical angle of total reflection is θ = sin-1 (n silica gel / n GaN) = 36.2 °, less than flip-chip The critical angle of the light transmission interface. A larger critical angle allows for more light output. Therefore, the flip-chip structure has higher external quantum efficiency than the positive- and vertical-structure LEDs, resulting in higher white light-emitting efficiency.
(2) Flip-chip PN junction to low ambient thermal resistance. As the current increases, the temperature of the chip increases due to thermal resistance, which increases the non-radiative recombination probability of carriers, reduces the probability of radiation recombination, and causes the luminous efficiency to decrease. The higher the thermal resistance, the higher the temperature rise of the chip, and the faster the luminous efficiency decreases. The flip-chip PN junction and the positive and negative poles of the bracket adopt eutectic soldering, the heat transmission distance is short, and the heat dissipation area is large, which is more conducive to heat conduction, so that a lower thermal resistance value can be obtained, and the PN junction temperature is lowered, thereby slowing down the light effect. speed. This is in agreement with the experimental results of luminous flux as a function of current.
3.3 Color temperature test
Color temperature is the most common indicator of the spectral quality of the source. The color temperature requirements for LED light sources are mostly low, and for the same batch of products, the smaller the color temperature deviation, the better the quality. The research on the control of color temperature has always been a key parameter for enterprises to meet customer needs.
Figure 6 is a comparison of current color temperature curves of three package structure LEDs. Through experimental tests, as the drive current increases, the color temperature of the LEDs of the three package structures increases with the increase of the current, while the slope of the color temperature of the flip-chip LED rises to a minimum of about 0.40, and the color temperature of the LEDs of the package is increased. The slope is about 0.67, while the vertical structure LED has a color temperature increase slope of about 0.84 when the current is less than 1 200 mA (the luminous flux saturation point). When the current exceeds 1 200 mA, the color temperature parameter approaches failure, which is consistent with the luminous flux test and luminous efficiency test results. The color temperature saturation point of the flip-chip LED is approximately 1 600 mA, which is 400 mA higher than the color temperature saturation point of the vertical structure LED. It shows that the flip-chip LED is stable under the condition of large current impact, and the light output characteristics are more stable than the vertical structure LED.
4 Conclusion
Flip-chip LEDs and vertical structure LEDs were fabricated using the same size 1.16 mm GaN-based blue chip. The luminous flux, luminous efficiency and color temperature of the two LEDs under different driving current conditions were tested by STC4000 fast spectrometer and constant current power supply. It was found that the vertical structure LED failed to emit light at a current exceeding 1 200 mA, and the current value of the flip-chip LED failed to be 1 550 mA. The increase in the failure current value of the flip-chip LED makes the reliability of the LED increase and improves the service life of the LED.
1. Defects in the formal packaging structure
At present, many commercial LEDs use a gold wire to connect the PN junction of the chip to the positive and negative package structure of the bracket. However, with the continuous improvement of output power, failure problems such as large light attenuation and light quenching that restrict the development of high-power LEDs have emerged.
The main cause of quenching failure is gold wire breakage. During the gold wire lead connection process, it is affected by multiple factors such as gold purity, bonding temperature, gold wire bending, welding machine precision and bonding process, causing the gold wire to be broken and quenched. Secondly, the silica gel mixed with the phosphor is coated on the surface of the chip to play the dual role of light conversion and protection of the gold wire. When the chip is energized, the temperature rises, and the silicon wire and the solder joint will be impacted due to thermal expansion and contraction of the silica gel. , solder joints are desoldered, causing quenching.
The main reason for the large failure of light decay is the yellowing or transmittance of silica gel. The positive-loading structure LED p, n electrode on the same side of the LED, the current must flow laterally through the n-GaN layer, resulting in current crowding, high local heat, limiting the drive current; second, due to poor thermal conductivity of the sapphire substrate, severely hindered heat Lost. During long-term use, the high temperature caused by poor heat dissipation affects the performance and transmittance of the silica gel, resulting in a large attenuation of light output power.
Therefore, in order to improve the problem that the gold wire of the LED packaged in the package is easy to break and the heat dissipation is not good, researchers in the industry have successively invented the vertical structure LED and the flip-chip structure LED.
Compared with the formal LED, the vertical structure uses a substrate with high thermal conductivity (substrate such as Si, Ge, and Cu) instead of the sapphire substrate, which greatly improves the heat dissipation efficiency; the two electrodes of the vertical structure LED chip are respectively Both sides of the LED epitaxial layer pass through the n-electrode, so that almost all of the current flows vertically through the epitaxial layer of the LED, and the current flowing in the lateral direction is extremely small, and local high temperature can be avoided. However, in the current vertical structure preparation process, the sapphire stripping process is difficult, which restricts the industrialization development process.
The inverted LED of another invention has been widely integrated into the lighting industry because of its integration and mass production, simple preparation process and excellent performance. The flip-chip structure uses the PN junction of the chip directly to the eutectic bonding of the positive and negative electrodes on the substrate, and does not use the gold wire, thereby avoiding the problem of light quenching to the utmost. In addition, the eutectic bonding structure greatly improves the heat dissipation problem. In the process of using high-power LEDs, large current impact is inevitable. Under this circumstance, if the high-current impact stability of the luminaire is not good, it is easy to reduce the service life of the luminaire.
Therefore, this paper compares and studies the variation of light output of vertical structure LED and flip-chip LED with increasing current, and compares it with ordinary formal LED. It is concluded that flip-chip LED has better resistance to large current surge. Sex and light output performance.
2. Sample preparation and test methods
2.1 Sample preparation
The three package structures are shown in Figure 1. The LEDs in the front-mounted LED adopt the sapphire substrate with a peak wavelength of 448 nm, the flip chip adopts the sapphire substrate with a peak wavelength of 447 nm, and the vertical structure chip uses the silicon substrate with a peak wavelength of 446 nm. The three chips are all 1.16 mm x 1.16 mm and have an operating current of 350 mA. The silica gel is manufactured by Priusson's 0967 model and the phosphor is made of Whisper's YAG-4. The positive and negative poles of the positive-loading structure chip are soldered to the positive and negative electrodes of the bracket by gold wire bonding; the positive electrode of the vertical structure chip is soldered to the positive electrode of the bracket by gold wire bonding, and the negative electrode is bonded by gold ball eutectic bonding. On the negative electrode of the stent; the positive and negative electrodes of the flip chip are bonded to the positive and negative electrodes of the stent by gold ball eutectic bonding.
2.2 Test methods
The luminous flux, luminous efficiency and color temperature are measured by the STC4000 fast spectrometer produced by Hangzhou Yuanfang Co., Ltd. The test principle is shown in Figure 2. The LED to be tested is placed in the center of the integrating sphere by a fixed fixture. The LED is emitted through the white diffuse reflection layer inside the integrating sphere. The diffuse reflection part of the light is transmitted to the micro multi-channel spectrometer through the narrow-pass optical aperture fiber on the surface of the integrating sphere. The data collected by the spectrometer passes through the USB. The interface is sent to the computer for processing and display. The light source is powered by a constant current source.
3. Results and discussion
3.1 The relationship between luminous flux and current
Figure 3 shows the change in luminous flux as a function of current for flip-chip LEDs, vertical package LEDs, and package LEDs with drive currents from 50 mA to 2 000 mA. It can be seen from Fig. 3 that as the current increases gradually, the luminous flux of the three structural LEDs increases with the increase of the current, but the growth rate gradually decreases.
When the drive current reaches 1 200 mA, the vertical structure LED first reaches the luminous flux saturation point, and the luminous flux of the flip-chip LED under this current condition is 14.7% higher than that of the formal LED, which is 25.9% higher than that of the vertical structure LED. As the current continues to increase, the change in luminous flux of the vertical structure LED indicates that it is near failure. The luminous flux of the flip-chip LED is saturated at a current of 1 550 mA, which is 350 mA more than the saturation current of the vertical structure LED. The luminous flux test results show that the flip-chip structure has a low PN junction temperature and good heat dissipation.
Therefore, it is concluded that flip-chip LEDs have higher reliability than other two types of LEDs, especially high resistance to high current surges. This performance is beneficial to improve the service life of LEDs in practical applications.
3.2 Luminous efficiency as a function of current
Figure 4 shows the relationship between three LED structure currents and luminous efficiency. It can be seen from Fig. 4 that when the current is increased from 50 mA to 2 000 mA, the luminous efficiency of the three LEDs is declining, and the luminous efficiency of the flip-chip LED is higher than the other two LEDs in the entire current variation range. Luminous efficiency. The vertical structure LED has a current of more than 1 200 mA, and the luminous efficiency drops rapidly, indicating that the light output is abnormal, which is in agreement with the luminous flux test result. When the operating current of the three LEDs is 350 mA, the luminous efficiency of the flip-chip LED is 8 lm/W higher than that of the vertical structure LED, which is 31 lm/W higher than the LED of the formal structure.
The increase in luminous flux and luminous efficiency of flip-chip LEDs may be due to:
(1) The external quantum efficiency of the flip-chip LED is high. The refractive index profiles of the three package structures are shown in Figure 5. Figure 5a shows the refractive index profile of the flip-chip package structure; Figure 5b shows the refractive index profile of the vertical package structure and the package structure. According to Snell's law, the critical angle of total reflection of flip-chip LED light from GaN to sapphire is θ=sin-1 (n sapphire/n GaN)=44.5°, and the critical angle of sapphire to packaged silica gel is θ=sin-1 (n-silica/ n sapphire) = 57.4 °; while the vertical structure and the LED of the formal LED are directly transmitted from the GaN to the encapsulated silica layer, the critical angle of total reflection is θ = sin-1 (n silica gel / n GaN) = 36.2 °, less than flip-chip The critical angle of the light transmission interface. A larger critical angle allows for more light output. Therefore, the flip-chip structure has higher external quantum efficiency than the positive- and vertical-structure LEDs, resulting in higher white light-emitting efficiency.
(2) Flip-chip PN junction to low ambient thermal resistance. As the current increases, the temperature of the chip increases due to thermal resistance, which increases the non-radiative recombination probability of carriers, reduces the probability of radiation recombination, and causes the luminous efficiency to decrease. The higher the thermal resistance, the higher the temperature rise of the chip, and the faster the luminous efficiency decreases. The flip-chip PN junction and the positive and negative poles of the bracket adopt eutectic soldering, the heat transmission distance is short, and the heat dissipation area is large, which is more conducive to heat conduction, so that a lower thermal resistance value can be obtained, and the PN junction temperature is lowered, thereby slowing down the light effect. speed. This is in agreement with the experimental results of luminous flux as a function of current.
3.3 Color temperature test
Color temperature is the most common indicator of the spectral quality of the source. The color temperature requirements for LED light sources are mostly low, and for the same batch of products, the smaller the color temperature deviation, the better the quality. The research on the control of color temperature has always been a key parameter for enterprises to meet customer needs.
Figure 6 is a comparison of current color temperature curves of three package structure LEDs. Through experimental tests, as the drive current increases, the color temperature of the LEDs of the three package structures increases with the increase of the current, while the slope of the color temperature of the flip-chip LED rises to a minimum of about 0.40, and the color temperature of the LEDs of the package is increased. The slope is about 0.67, while the vertical structure LED has a color temperature increase slope of about 0.84 when the current is less than 1 200 mA (the luminous flux saturation point). When the current exceeds 1 200 mA, the color temperature parameter approaches failure, which is consistent with the luminous flux test and luminous efficiency test results. The color temperature saturation point of the flip-chip LED is approximately 1 600 mA, which is 400 mA higher than the color temperature saturation point of the vertical structure LED. It shows that the flip-chip LED is stable under the condition of large current impact, and the light output characteristics are more stable than the vertical structure LED.
4 Conclusion
Flip-chip LEDs and vertical structure LEDs were fabricated using the same size 1.16 mm GaN-based blue chip. The luminous flux, luminous efficiency and color temperature of the two LEDs under different driving current conditions were tested by STC4000 fast spectrometer and constant current power supply. It was found that the vertical structure LED failed to emit light at a current exceeding 1 200 mA, and the current value of the flip-chip LED failed to be 1 550 mA. The increase in the failure current value of the flip-chip LED makes the reliability of the LED increase and improves the service life of the LED.
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