Research and Analysis of Anti-overstress Ability of High Power LED Chip

1 Introduction

As a new type of lighting technology, LED has the advantages of low energy consumption, long life, small size, dimmable, flexible control and environmental protection. Its application prospect has attracted worldwide attention. With the decline in LED prices, the market is gradually opening up, and more and more lighting products use LEDs as light sources. Especially in the field of road lighting, high-power LED products have become the protagonist of the market, and LEDs have shined in the field of outdoor lighting [2]. However, with the increase in the application of LED lamps, the number of outdoor LED lamps failed due to lightning surge Increasing. According to the investigation, most of the outdoor LED lamps damaged within the normal service life are due to the over-stress caused by the lightning surge, which has failed the lamp power supply and LED light source. This not only affects the service life of the lamp, but also increases the maintenance cost of the enterprise. In view of this, the anti-lightning surge capability of LED outdoor lamps should attract enough attention.

The anti-lightning surge capability of LED lamps mainly depends on two aspects:

(1) Anti-lightning surge capability and protection mechanism of LED drive power.

(2) Anti-overstress ability of LED chip.

For the LED drive power supply, the anti-lightning surge capability should be judged by two points: (1) The anti-lightning surge capability of its own components to ensure that the power supply still works normally after the lightning surge. (2) The power supply's ability to attenuate the surge current and voltage waveform to ensure that the peak current and voltage that attenuate after the surge passes through the power supply is within the range that the LED chip can withstand. Tianjin University Zhang Jinjian [3] et al. Studied the anti-lightning surge of LED drive power. According to the characteristics of lightning surge, a surge power device such as gas discharge tube, varistor, transient suppression diode and other surge devices was designed to fit a LED power Surge protection circuit, and use lightning surge generator to conduct immunity test to test its anti-lightning performance. The experimental results show that it can withstand the lightning high voltage of 1kV differential mode and 2kV common mode to ensure the normal operation of LED power supply. However, the LED drive power supply's anti-lightning surge requirements are determined by the LED chip's ability to resist over-stress. Therefore, it is very necessary to study the LED chip's ability to resist over-stress.

Based on this, this paper conducts lightning surge experiments on several types of high-power LED chips to explore the ability of different high-power LED chips to resist over-stress, the choice of driving power and LED chips for LED outdoor lamps, and anti-lightning wave The design and R & D of surge capacity provides reference and has important practical significance.

2. Influencing factors of LED chip's ability to resist over-stress

First of all, the current density that the LED chip can bear determines its ability to resist over-stress. The greater the current per unit cross-sectional area that the LED chip can withstand, the stronger its resistance to over-stress. For conventional electrical conductors, the current density must be low enough to prevent the conductor from melting or melting, or the insulating material to be broken down [4]. At high current densities, electromigration can occur inside the LED chip. When the conductive metal material passes through a higher current density, metal atoms will migrate and diffuse along the direction of electron motion. Electromigration in LEDs allows metal atoms to freely diffuse from one lattice to another lattice vacancy. Taking the flip chip as an example, when the electron flow flows from the interconnection lead into the eutectic alloy bump, the geometry of the interconnection lead to the bump is abrupt, so the current density concentration and local Joule heating effect will occur at the interface [5]. The current density concentration makes the current density distribution in the bumps and chip and substrate leads uneven, resulting in a local complex electromigration force at the current density concentration area, which accelerates the electromigration process and accelerates the LED failure. .

Secondly, the current concentration effect affects the chip's ability to resist over-stress. Current concentration is the uneven distribution of current density on the chip, especially near the chip contacts and above the PN junction. The current aggregation phenomenon of the LED chip forms a local overheating on the chip to form a hot spot, which aggravates the electromigration effect and makes the current density locally uneven. Auger recombination of carriers increases [6], affecting the chip's internal quantum efficiency. Leakage of minority carriers through the charge region of the heterojunction will cause the current injection efficiency to decrease, resulting in local uneven light emission and overheating of the LED chip, affecting the light emitting performance and service life of the chip, and eventually causing the LED chip to short circuit or open circuit. This phenomenon is particularly serious when the chip size and injection current are large.

Finally, the current-carrying capacity of the bonding wire of the LED chip is a factor that affects the ability of the LED chip to resist over-stress. Although the failure of the LED due to the fuse of the bonding wire is not common in practical applications, the diameter, length, bonding type, physical material properties and resistance of the bonding wire all have an impact on the current carrying capacity of the gold wire. When the over-stress is large, the conductor fuse to open the LED.

The above factors together affect the LED chip's ability to resist over-stress. Through different chip technology processes, the electromigration and current aggregation effects of the chip can be improved. For example, the optimized interdigitated electrode can improve the current crowding phenomenon; the vertical structure chip allows the current to flow longitudinally in the chip to improve the current aggregation phenomenon. At the same time, the number of flip-chip electrodes and bumps [5], location and ohmic contact processing have a significant effect on the current expansion of the chip. By optimizing the geometry and electrical parameters of the electrodes, bumps, etc., current congestion can be greatly reduced Effect, improve the unevenness of current density distribution, promote current expansion, and reduce the total equivalent resistance of the chip.

It can be seen that LED chips with different structures and different processes have different performances against over-stress under the same surge pulse. The following experiment finds the range of the peak value of single pulse current resistance of common high-power LED chips on the market.

3. Experiments of different high-power LED chips against single pulse current

In order to simulate the impact of actual lightning strikes on LED lamps and chips, Hangzhou Yuanfang EMS61000-5A [7] intelligent lightning surge generator was used to simulate the surge waveform generated in the power grid during the lightning strike.

The output waveform of EMS61000-5A is: standard combined wave of voltage integrated wave 1.2 / 50μs and current integrated wave 8 / 20μs [8], of which voltage integrated wave (as shown in Figure 1) wavefront time: T1 = 1.67T = 1.2 μs ± 0.36μs, half-peak time: T2 = 50μs ± 10μs.

Current composite wave (as shown in Figure 2) wavefront time: T1 = 1.25T = 8μs ± 1.6μs, half-peak time: T2 = 20μs ± 4μs.

When using the LED complete lamp (including lamp beads and driving power supply) as the surge test object, it is found that the output surge current of the standard surge waveform after the LED power supply is uncertain, because the design of the anti-lightning strike of the driving power supply of different manufacturers is different , The shape of the surge waveform and the peak current size at the output end of the power supply, that is, the lamp bead input, are uncontrollable, which increases the uncertainties in the experiment.

To solve the above problems, this experiment uses a DC power supply to directly power a single LED, and adds a surge waveform to the DC circuit. By adjusting the peak value of the pulse voltage output by the device and the resistance value in series with a single LED, the magnitude of the current pulse peak at the input end of the lamp bead is changed. This achieves the determination of the surge waveform and the controllability of the pulse peak current.

During the experiment, the lamp beads are first lit at a working current of 350mA, and the above pulse waveform is applied in a circuit with a resistor and LED lamp beads connected in series. The pulse voltage starts to increase gradually from 250V, and the increase interval is 50V. Each voltage file carries surge surges 5 times, and each waveform interval is 10s. If the lamp beads work normally after the test is completed, the next voltage file is entered to continue the test. At the same time, use an oscilloscope to observe the peak current waveform at both ends of the lamp bead, record the pulse waveform when the lamp bead fails and breakdown, and determine the pulse peak current when it fails (to exclude the influence of the lamp bead protection electrode on the experiment, remove the lamp bead protection electrode before the experiment ).

3.1 Sapphire substrate horizontal structure chip anti-surge test

Surge test was performed on a horizontal structure LED chip with a size of 45mil * 45mil on the market. Starting from the 250V surge, the lamp bead fails when the second surge of 300V surges. The current surge waveform at both ends of the lamp bead is recorded as shown in Figure 3 when it fails.

The experiment tested a total of 5 horizontal structure LED lamp beads, and the peak pulse current when they failed was 15.55A, 15.88A, 15.00A, 15.62A, 15.22A.

The analysis of the lamp beads after the surge revealed that the lamp beads were completely short-circuited, and the chip electrodes were completely broken down and fused, as shown in Figure 4:

Fig. 5 and Fig. 6 show the brightness distribution of the chip surface at 1mA and 150mA respectively. It can be observed from the figure that at 1mA, the uneven current distribution on the surface of the chip causes uneven brightness on the surface of the chip, and as the current increases (150mA), the uneven distribution of current increases.

3.2 SiC flip-chip structure LED chip anti-surge test

A SiC substrate flip-chip with a size of 40mil * 40mil on the market is selected for over-stress test. The lamp bead fails at a surge voltage of 650V, and the surge waveform it bears when it fails is shown in Figure 7.

A total of 5 LED chips with SiC substrate flip-chip structure were tested in the experiment. The peak pulse currents at failure were: 29.22A, 29.68A, 33.57A, 35.68A, 35.39A. By comparison, it was found that the above peak currents were higher than those tested The anti-surge peak current of the horizontal structure chip is doubled.

Analysis of the lamp beads after the surge revealed that the lamp beads were completely short-circuited, as shown in Figure 8:

Fig. 9 and Fig. 10 show the brightness distribution of the chip surface at 1mA and 150mA, respectively, for the SiC substrate full-mount structure chip.

Observe the brightness distribution of the surface of the chip under the 1mA and 150mA of the flip chip structure of the SiC substrate. It is found that the brightness distribution of the chip is relatively uniform under low current, indicating that the chip current distribution is uniform. At the same time, with the increase of current (150mA), there is no obvious uneven distribution of current in the chip.

For GaN-based blue LEDs, the lattice mismatch rate between SiC and GaN is only 3.4%, which is much smaller than the 17% lattice mismatch between bluestone substrate and GaN. The epitaxially grown GaN film on SiC substrate has more The low dislocation defect density means that GaN LEDs with SiC substrates have higher internal quantum efficiency and are suitable for operation at high current densities. In addition, the thermal conductivity of SiC is very high (420W / mK), which is more than fifteen times that of sapphire (23-25W / mK) [9], which is conducive to the heat dissipation of LED devices and improves the reliability of LEDs.

3.3 Sapphire peeling substrate flip chip LED chip over-current stress test

An over-stress test using a flip-chip lamp bead with a size of 55 mil * 55 mil and a sapphire stripped substrate on the market was selected. The chip fails at a surge voltage of 350V. The waveform of the failed surge is shown in Figure 11.

The peak values ​​of the pulse currents that the experimentally tested 5 sapphire peeled substrate flip-chip lamp beads failed were: 16.2A, 16.59A, 12.23A, 14.49A, 14.53A.

Figure 12 The surface of the chip after the surge of the sapphire flip-chip LED light source fails. The chip surface can clearly observe the breakdown caused by the excessive local temperature caused by the current crowding.

3.4 SiC substrate vertical structure LED chip anti-overstress test

A vertical structure chip with a size of 55 mil * 55 mil and SiC as the substrate is selected for lightning surge test on the market. The chip fails at a surge voltage of 600V. The failed surge waveform is shown in Figure 13.

The experimental test of the peak current of the 5 SiC substrate vertical structure lamp beads when they failed was: 24.4A, 28A, 25.2A, 24.6A, 26.0A.

An analysis of the failed lamp beads revealed that the failure area was concentrated near the electrodes, as shown in Figure 14.

3.5 Si substrate transfer vertical structure LED chip anti-overstress test

Select a chip on the market with a size of 45mil * 45mil, Si substrate transfer vertical structure chip for over-stress test, the chip fails under 350V surge voltage. The surge waveform it bears when it fails is shown in Figure 15.

The peak value of the pulse current that the 5 Si substrate vertical structure chips tested in the experiment failed was: 16.6A, 16.6A, 16.4A, 16.2A, 16.5A.

By analyzing the failed chip, the breakdown of the metalized electrode near the N electrode can be clearly observed, as shown in Figure 16.

Record the above test results as shown in Table 1:

4 Conclusion

Through testing the over-stress resistance of common high-power LED chips on the market, it is found that the over-stress resistance of LED chips with different structures and different processes varies greatly. The peak range of the single pulse current that it withstands when it fails is between 12A and 35A. For the LED drive power supply, under the premise of ensuring its normal operation when it is subjected to a lightning surge, it also needs to ensure that the peak current of the surge waveform at its output is less than 12A, so as to protect the LED lamp beads and avoid their immediate failure. Of course, another situation needs to be considered, that is, when the LED lamp bead is subjected to over-stress, the initial failure only manifests as a leakage of the lamp bead. It takes a certain period of time before the luminous flux decreases significantly or the lamp is dead. This situation has higher requirements on the anti-lightning ability of the LED drive power. In the later stage, we will increase the experimental content of the lamp bead leakage detection and accelerated aging after the surge impact, and improve this part of the research work.

Another aspect of improving the ability of LED lamps to resist lightning strikes is to increase the resistance to over-stress of LED light sources used in lamps. In Table 1, you can see the flip-chip structure SiC substrate (patterned substrate, without completely peeling the substrate) The peak value of the pulse current that can withstand reaches 32A, and it has a good performance in resisting over-stress. It can be used in combination with a driving power supply with strong anti-lightning surge ability to improve the overall anti-lightning performance of the lamp. Of course, the manufacturing process of high-power LED chips (such as chip structure, epitaxial manufacturing, etc.) has a decisive role in the chip's ability to resist over-stress, which is not fully reflected in Table 1. This is also the direction of our next stage of the project.