Application of electrostatic testing technology in improving LED quality

1. Introduction

LED applications have expanded to various fields, including LCD backlight, mobile phone backlight, signal lights, artistic lighting, architectural lighting, stage lighting control, home lighting, etc. According to DIGITIMES Research, the demand growth from 2010 to 2015 was as high as 30%, which led to a significant increase in LED production capacity. As the LED application environment becomes more diverse and complex, downstream LED manufacturers are increasingly demanding on the quality of upstream die. For example, the voltage value of LED electrostatic discharge (ESD) test has been gradually increased from the original 4kV requirement to 8kV to tolerate the harsh outdoor environment. Therefore, high-voltage LED electrostatic discharge test is a key module that is urgently needed to be developed in the current LED die spot tester.

Static electricity in various modes in the environment, including human body static electricity or mechanical static electricity, can damage LEDs. When static electricity is inductively or directly touched on the two pins of an LED, the potential difference will directly act on both ends of the LED. When the voltage exceeds the tolerance of the LED, the static charge will be discharged between the two electrodes of the LED in a very short time, causing damage to the LED insulation, leakage or short circuit. Therefore, the Solid State Technology Association JEDEC (Joint Electron Device Engineering Council) has formulated the test specifications for the Human Body Model (HBM) and the Machine Model (MM) in JESD22-A114E and JESD22-A115A to ensure the quality of LED products. However, the main disadvantages of purchasing a foreign high-voltage generator with a charge-discharge switching circuit and integrating a prober with an automated mobile platform are slow response speed (0 to 4kV rise time 500ms), and the high-voltage insulation of the probe is not considered, so there are serious problems such as slow grain classification speed and insufficient test waveform stability, which often break down the LED or charge-discharge module, as shown in Figure 1, or the high-voltage testability of the machine is insufficient, and it is still damaged by high-voltage static electricity after shipment, which directly affects the quality of LED finished products. In addition, the time for measuring 20,000 to 40,000 grains on a wafer often takes more than 1 hour, and the detection time needs to be shortened to increase production capacity. Therefore, this article develops a high-speed, large-dynamic-range LED wafer static measurement module, uses a dynamic range control circuit and PID feedback control in the design of a high-speed multi-voltage switching high-voltage generating component, and uses a high voltage dynamic range (250V-8kV) and high-speed static electricity test (80ms), as shown in Figures 2A and 2B, to meet the needs of the domestic LED industry and achieve the purpose of reducing costs and making key modules homemade.

Figure 1 LED damaged by static electricity

Figure 2A High-speed and large dynamic range electrostatic measurement module short-circuit electrostatic test current waveform

Figure 2B High-speed multi-voltage switching high-voltage generating component electrical test output voltage waveform

2. LED wafer electrostatic measurement module system architecture

This paper develops a high-speed, large dynamic range LED wafer electrostatic measurement module as shown in Figure 3. It performs a full inspection test on the electrostatic voltage resistance of the die, and classifies the LED level according to the size of the LED electrostatic voltage resistance. This electrostatic point test full inspection module includes testing high-speed multi-voltage switching high-voltage generating components, probe components, charging and discharging components, and software classification components. The test probe platform moves two probes to contact the positive and negative electrodes of the LED to be tested. The high-speed multi-voltage switching high-voltage generating component generates a human body electrostatic discharge mode or a machine device discharge mode test voltage level according to the software electronic control program setting. The charging and discharging module stores the charge of the high-voltage generator and then performs an electrostatic withstand voltage test on the LED to be tested. Finally, the software classification component displays the electrostatic test results. This technology aims to solve the problems of insufficient dynamic range of existing domestic LED wafer electrostatic measurement modules (500V to 4000V) and slow voltage switching time of foreign modules (0V to 4kV rise time of about 500ms). It is designed into a high-speed and large dynamic range LED wafer electrostatic measurement module, so that the output voltage can cover the large dynamic range of the minimum voltage of 250V to the maximum voltage of 8000V required for standard electrostatic classification; and shorten the rise time from low voltage to high voltage to within 80ms, so as to achieve the purpose of high-speed and large dynamic range LED chip online detection and classification.

Figure 3 High-speed and large dynamic range LED wafer electrostatic measurement module system diagram

The design considerations for each major component are as follows:

1. Test probe assembly design

The test probe assembly is used to transmit voltage and current. The external insulation protection of the probe can prevent leakage current from being generated, thereby improving the accuracy of electrostatic measurement.

Insulation design is divided into two categories: internal insulation and external insulation. Internal insulation is the insulation inside the module. It includes solid dielectric insulation and combined insulation composed of different dielectrics. Although external atmospheric conditions have little effect on internal insulation, factors such as material aging, high temperature, continuous heating and moisture have an adverse effect on the insulation strength of internal insulation. At the same time, if the internal insulation is broken down, its insulation strength cannot recover by itself. External insulation refers to various forms of insulation formed under conditions of direct contact with the atmosphere, including air gaps and the exposed surface of the module solid insulation. The outstanding feature of external insulation is that after the discharge stops, its insulation strength can usually be quickly and completely restored, and it has nothing to do with the number of repeated discharges. The insulation strength of external insulation is closely related to external atmospheric conditions and will be affected by many factors such as atmospheric temperature, pressure, humidity, etc. Taking the atmosphere as an example, the insulation strength in the atmosphere is generally about 30kV/cm, and it is about 10kV/cm when there are water droplets. When the temperature rises from room temperature to 100 degrees Celsius, the insulation strength drops to 80%. Therefore, the design will estimate the range of material insulation strength changes caused by temperature and humidity, and use this to design the safety distance required for withstand voltage.

The insulation test of the probe can be determined by the insulation strength test. The test includes two types: withstand voltage test and breakdown test. The withstand voltage test is to apply a certain voltage to the test piece, and after a period of time, whether a breakdown occurs is used as the standard for judging whether the test is qualified or not. The breakdown test is to gradually increase the voltage applied to the test piece under certain conditions until the test piece breaks down.

2. High-speed multi-voltage switching high-voltage generating components and charging and discharging components

The design of this component includes control loop stability design and interference prevention. The control loop stability work includes component model establishment, stability condition analysis, loop stability test, etc. The interference prevention method is to reduce parasitic capacitance. The value of the parasitic capacitance of the circuit board is related to the geometric position of the circuit board wiring line, the line width, and the insulation material of the circuit board. In order to reduce the parasitic capacitance of the line, the interference-susceptible points are marked first during the design. When routing, the marked point position is given priority for routing, and the lines that are not susceptible to interference are routed last.

3. Software components

Control the probe's lower needle position, trigger the high voltage generator's charge and discharge module, control the input voltage to discharge the LED to be tested after charging is completed, and display the measurement results.

3. LED Wafer Electrostatic Measurement Module System Assembly and Test Results

The design and production of the high voltage generator AC voltage modulation circuit is completed as shown in Figure 4. The AC voltage amplitude is actually measured using a high voltage probe, peak to peak, 6.26kV - maximum AC amplitude: 3.13kV. The test results show that the maximum DC voltage value is 8.08kV. At 8kV voltage, the short-circuit current test through the short-circuit output terminal is conducted under the condition of discharge resistance: 1500 Ω +/- 1%, and the peak current reaches 5.46A (theoretical value: 8000/1500=5.33). The LED electrostatic point test module specification verification is completed at an electrostatic voltage of 4Kv and under the following test conditions:

(1) Under normal temperature, humidity and atmospheric conditions

(2) Test probe: Current probe with bandwidth greater than 1 GHz

(3) Charging capacitance: 100 pF +/- 10% (effective capacitance)

(4) Discharge resistance: 1500 Ω +/- 1%

Repeatedly measure the HBM short-circuit peak current 5 times and the results are as follows:

The peak current measurement theoretical value is 2.66A. After one hour, the peak current is 2.70A with an offset of 1.5%, which meets the test specifications of peak current 2.40~2.96A@4kV and HBM load short-circuit rise time 2.0~10ns@4kV.

Figure 4 High voltage generator complete circuit module

IV. Conclusion

This article develops a high-speed, large dynamic range LED wafer electrostatic measurement module, which allows the output voltage to cover the large dynamic range of the standard electrostatic classification, from the minimum voltage of 250V to the maximum voltage of 8000V; and shortens the rise time from low voltage to high voltage to less than 80ms. In the future, small-scale trial production and endurance testing will be carried out on the client side, and modifications will be made according to commercialization needs to achieve the purpose of high-speed and large dynamic range LED grain online detection and classification. In terms of application, in addition to being used for LED electrostatic testing, it can be applied to semiconductor BGA, CSP (Chip Scale Package), FC (Flip Chip) micro-component wafer electrostatic testing with probe point testing technology. Further applications include X-ray Tubes, Photomultiplier Tubes, Electron Beam Focusing, etc.