Visible light-emitting diodes (LEDs) are highly efficient and durable, making them popular across various industries. Recent advancements in manufacturing have led to devices that emit more luminous flux, last longer, offer more color options, and achieve higher lumens per watt. Testing the reliability and quality of these devices is crucial, requiring precise and cost-effective methods.
Throughout the production process, there are different stages where various tests are conducted. For instance, there’s testing during the design development phase, wafer-level testing during production, and final testing post-packaging. While LED testing generally covers both electrical and optical aspects, this discussion will focus on electrical characterization, with some optical measurement techniques briefly mentioned. Figure 1 illustrates the DC IV curve of a typical diode. A comprehensive test should include numerous voltage values and corresponding current points, but often, a limited number of sample points suffice to evaluate the device's quality.
Figure 1. DC IV Curve and Test Points for a Typical LED (Not to Scale)
Many tests involve providing a known current and measuring the voltage, or applying a voltage and then measuring the resultant current. Thus, high-speed test instruments with integrated, synchronized source and measurement capabilities are ideal for these tasks.
Forward Voltage Test
In the LED testing sequence, the forward voltage (VF) test confirms the forward operating voltage of visible LEDs. When a forward current is applied, the diode starts conducting. Initially, at low currents, the voltage drop across the diode increases rapidly, but as the current grows, the voltage slope flattens out. Diodes typically operate in regions where the voltage remains relatively constant. Testing under these conditions is beneficial. The VF test involves providing a known current and measuring the voltage drop across the diode. Test currents usually range from tens of milliamps to a few amps, with resulting voltages typically in the few-volt range. Some manufacturers use VF test results for device sorting since forward voltage correlates with the chromaticity of the LED.
Optical Test
The forward bias current is also used in optical testing because the electron current is closely tied to the intensity of luminescence. Optical power can be measured by placing a photodiode close to the device under test or capturing emitted photons. The light is then converted to current and measured using an ammeter or a channel of the source measuring instrument.
In many test applications, the voltage of the diode and the emitted light can be measured simultaneously using a fixed current source. Additionally, spectrometers can measure detailed parameters like spectral output at the same drive current magnitude.
Reverse Breakdown Voltage Test
The reverse breakdown voltage (VR) can be determined by applying a reverse bias current to the LED. The test current should be set at a level where the measured voltage doesn't significantly increase even if the current is slightly raised. When the voltage exceeds this value, a substantial rise in reverse bias current results in minimal changes in reverse voltage. This parameter is usually a minimum requirement. During VR testing, a small reverse bias current is loaded for a specific duration, followed by measuring the voltage drop across the LED. The measurement typically falls within the tens-of-volts range.
Leakage Current Test
Leakage current (IL) is generally measured using a medium-sized voltage (a few volts to tens of volts). The IL test gauges the small current leaking through the LED when the applied reverse voltage is below the breakdown voltage. Ensuring that the leakage does not surpass a certain threshold during production is standard practice, often done in isolated measurements. There are two main reasons for this. First, low current measurements require longer settling times, thus taking more time to complete. Second, environmental and electrical noise significantly affect low-value signals, necessitating additional shielding. These extra shielding measures complicate the test fixture and may disrupt robotic operations.
Intelligent Instruments Boost LED Production Testing Efficiency
In the past, many LED production test systems relied on PCs to control every aspect of testing. This meant configuring each test step individually for the source and test instruments, executing the necessary operations, and returning data to the controlling PC. The PC would then perform pass/fail judgments and sort the DUTs accordingly. Each command sent and executed wasted valuable test time, reducing productivity. Clearly, in this PC-centric test structure, much of the test sequence time was consumed by communication between the PC and the test instruments.
In contrast, many modern smart instruments, such as the 2600A Series SourceMeter, enable significantly increased throughput for complex test sequences by reducing communication on the bus. These instruments embed the main part of the test sequence within themselves. The Test Script Processor (TSP) is a versatile test sequence engine that controls test sequences and digital I/O ports, incorporating built-in pass/fail criteria, math, and calculation formulas. The TSP can save a user-defined test sequence in memory and execute it upon command. This minimizes the setup and configuration time for each step in the test sequence and boosts test throughput by reducing communication with PCs and instruments. Programming this type of instrument is relatively straightforward: 1) create a script; 2) download the script to the instrument; 3) call the script to execute. Users of the 2600A Series instruments can write or download scripts using the Test Script Builder software provided with the instrument itself, or download them from user applications written in languages such as Visual Basic or LabVIEW.
Single LED Device Test System
Figure 2 shows a simplified block diagram of a test system for a single LED. For automated testing, a PC and a component robot are typically included—a probe station is required for wafer probes.
Figure 2. Block Diagram of a Single LED Test System Based on Digital Source Meter
In this test setup, the PC’s main functions are to save measurement data in a database for record-keeping and to reconfigure the test sequence for different components. The 2600A Series is unique in that it can operate independently of the PC controller. The embedded TSP on each instrument allows users to write a complete test plan that can be executed on the instrument itself. In other words, users can write a complete PASS/FAIL test sequence script that can run directly through the instrument panel without needing to reprogram the instrument.
The production test system can use a component robot to transfer a single LED to the test fixture for electrical contact. The fixture shields ambient light and is equipped with a photodetector (PD) for optical measurements. In the configuration shown in Figure 2, a 2602A dual-channel source meter implements the two connections. Here, the source measurement unit A (SMUA) provides a test signal for the LED and measures its electrical response, while SMUB monitors the photodiode during the optical measurement process.
The test sequence uses a digital line of the component robot at the start of programming as a "test enable (SOT)" signal. When the digital source meter detects this SOT signal, the LED feature analysis test begins.
After all electrical and optical tests are completed, the system sets a digital line labeled "Measurement Complete" for the component robot. Moreover, the instrument's intelligent function performs all pass/fail operations, sending a digital command to the component robot via the digital I/O port on the instrument, sorting the LEDs based on the pass/fail criterion. You can then set two operations to occur simultaneously: transferring data to the PC for statistical process control and transferring a new DUT to the test fixture.
Multi-Device/Array LED Test System
In multi-device testing, such as aging tests, we measure multiple components simultaneously within a specified timeframe. Driving a DUT typically requires continuous current, but multiple optical detectors can multiplex an ammeter through a switching system. Users can select the appropriate switching system and ammeter based on the dynamic range of the measured current.
Multiple LED device tests can choose from many types of switches. For example, the Model 3706 Switch/Multimeter has six switch module slots, supporting up to 576 multiplexed channels or 2,688 matrix intersections. Similar to the 2600A Series instruments, it also includes an onboard TSP and TSP-Link® inter-device communication/trigger bus, allowing these instruments to integrate quickly and easily into a single system. This integration supports tightly synchronized inter-instrument operation and enables them to operate under the control of a test script. Figure 3 shows the structure of a three-LED device test system with a photodiode (PD) channel.
Figure 3. Block Diagram of an LED Array Test System Built with a Scalable 2602A Source Meter Channel
Minimizing LED Test Errors
Common sources of measurement error in LED production testing include lead resistance, leakage current, electrostatic interference, and optical interference. However, self-heating is one of the most significant sources of error. The forward voltage test and the leakage current test are particularly sensitive to junction heating. As the semiconductor junction heats up, the voltage drops, and more critically, the leakage current increases during constant voltage testing. Therefore, it is vital to minimize test time as much as possible without compromising measurement accuracy or stability.
A smart instrument with an onboard test script engine simplifies the pre-measurement soak time and the time needed to acquire the input signal. All circuit capacitances stabilize before measurement begins during the hold time. Measuring the integration time depends on the number of power line cycles (NPLC). If the input power is 60 Hz, a 1NPLC measurement takes 1/60 second, or 16.667 ms. The integration time determines the time at which the A/D converter acquires the input signal, balancing between measurement speed and accuracy.
The typical hold time for VF testing ranges from less than a few hundred microseconds to 5 milliseconds, and the incubation time for IL testing ranges from 5 to 20 milliseconds. Utilizing these extremely short test times helps reduce errors caused by junction heating. Additionally, conducting a series of tests and focusing solely on the test time allows for analyzing the characteristics of junction heating.
To further reduce test time and minimize junction self-heating effects, the 2600A Series instruments support pulse operation. In this mode, they can produce a sophisticated source at the output for a specified period. The 1-microsecond pulse width resolution accurately controls the power-up time of the device. These instruments are also capable of outputting current values far exceeding their DC capability in pulse mode. For example, the 2602A can output 3A of DC current at 6V. In pulse mode, it can output 10A at 20V.
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