Visible light-emitting diodes (LEDs) are renowned for their high efficiency and extended lifespan, making them a popular choice across various industries. Nowadays, manufacturers are pushing the boundaries of innovation by developing LEDs with enhanced luminous flux, longer lifespans, more color options, and improved lumens per watt. Accurate and cost-effective testing is crucial to maintaining the reliability and quality of these devices.
Throughout the production process, different types of test sequences are employed at various stages. For instance, testing occurs during the design development phase, at the wafer level during production, and after the final packaging stage. While LED testing generally involves both electrical and optical evaluations, this discussion will focus primarily on electrical characterization, introducing some optical measurement techniques at relevant points. Figure 1 illustrates the DC IV curve of a typical diode. A comprehensive test should consist of numerous voltage values paired with corresponding current operating points; however, in practice, a limited sample of points is often sufficient to assess the quality factor of the device.
Figure 1. DC IV Curve and Test Points for a Typical LED (Not to Scale)
Many tests require providing a known current and then measuring the voltage, while others necessitate applying a voltage and subsequently measuring the resulting current. Thus, high-speed test instruments with integrated, synchronized source and measurement capabilities are ideal for this kind of testing.
Forward Voltage Test
In the LED test sequence, the forward voltage (VF) test confirms the forward operating voltage on visible LEDs. When a forward current is applied, the diode starts conducting. Initially, at low currents, the voltage drop across the diode rises sharply, but as the drive current increases, the voltage slope flattens. Diodes typically operate in a region where this voltage remains relatively constant. Testing under these operating conditions is also beneficial. The VF test involves providing a known current and then measuring the voltage drop across the diode. Typical test currents range from tens of milliamps to a few amps, with the resulting voltage usually around a few volts. Some manufacturers utilize the results of this test for device sorting since the forward voltage correlates with the chromaticity of the LED (the color quality defined by the primary or complementary color and its purity).
Optical Test
The forward bias current is also utilized for optical testing because the electron current is closely linked to the intensity of luminescence. Optical power can be measured by placing a photodiode near the device under test or capturing the emitted photons. The light is then converted into a current, which is 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 be used to measure detailed parameters like spectral output at the same drive current magnitude.
Reverse Breakdown Voltage Test
The reverse breakdown voltage (VR) can be measured by applying a reverse bias current to the LED. The magnitude of the test current should be set to a point where the measured voltage no longer significantly increases when the current is slightly increased. When the voltage exceeds this value, a substantial increase in the reverse bias current leads to an inconspicuous change in the reverse voltage. This parameter is typically a minimum. 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 size is usually in the tens of volts.
Leakage Current Test
Generally, leakage current (IL) is measured using a medium-sized voltage (from several volts to tens of volts). The leakage current test evaluates the small current leaking from 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 a common practice, especially for isolated measurements. There are two reasons for this. First, low current measurements require longer settling times, prolonging the overall test duration. Second, environmental and electrical noise significantly affect low-value signals, necessitating additional shielding. These additional shielding measures complicate the test fixture and may interfere with robotic operations.
Intelligent Instruments Enhance LED Production Test Efficiency
In the past, many LED production test systems relied heavily on PCs to control all aspects of testing. This meant that for each component in the test sequence, separate configurations were required for the source and test instruments, the necessary operations were performed, and the data was returned to the controlling PC. The PC would then determine pass/fail status and sort the DUT accordingly. Each command sent and executed wasted valuable test time and reduced 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, allow for significantly increased throughput in complex test sequences by reducing communication on the bus. In these instruments, the main part of the test sequence is embedded within the instrument itself. The Test Script Processor (TSP) is a versatile test sequence engine that controls test sequences and digital I/O ports with built-in pass/fail criteria, mathematical functions, and calculation formulas. The TSP can store 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 limiting communication with PCs and instruments. The programming process for these instruments is relatively straightforward: 1) create a script; 2) download the script to the instrument; 3) call the script to execute. For the 2600A Series instruments, users can write or download scripts using the Test Script Builder software provided with the instrument or download them from user applications written in languages like Visual Basic or LabVIEW.
Single LED Device Test System
Figure 2 is a simplified block diagram of a test system for testing a single LED. For automated testing, a PC and a component robot are typically included—wafer probes require a probe station.
Figure 2. Block Diagram of a Single LED Test System Based on Digital Source Meter
In this test structure, the primary function of the PC is to save the measurement data in a database for record-keeping. The secondary role is 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 be run through the instrument panel without reprogramming 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 is used to implement the two connections. Here, the source measurement unit A (SMUA) provides a test signal for the LED and measures its electrical response, while the SMUB is used to monitor the photodiode during the optical measurement process.
The test sequence uses a digit 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. Additionally, 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 according to the pass/fail criterion. Then, two simultaneous operations can be set: transferring the 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, multiple components are measured 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 instance, the Model 3706 Switch/Multimeter has six switch module slots, allowing it to support 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, enabling these instruments to be quickly and easily integrated into a single system. This integration supports tightly synchronized inter-instrument operation and allows 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, but self-heating is one of the most significant sources of error. The two tests most sensitive to junction heating are the forward voltage test and the leakage current test. When the semiconductor junction heats up, the voltage drops, and more importantly, the leakage current increases during constant voltage testing. Therefore, it is crucial 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 of the device and the time it takes to acquire the input signal. All circuit capacitances are stabilized before the 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, the 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, while the incubation time for IL testing ranges from 5 to 20 milliseconds. By utilizing these extremely short test times, errors due to junction heating can be reduced. Additionally, by performing a series of tests and testing only the test time, the characteristics of the junction heating can be analyzed.
To further reduce test time and minimize junction self-heating effects, the 2600A Series instruments support pulse operation. In this mode, they are capable of producing a sophisticated source at the output for a specified period of time. The 1 microsecond pulse width resolution accurately controls the power-up time of the device. These instruments are also capable of outputting current values that greatly exceed their DC capability in pulsed mode of operation. For example, the 2602A is capable of outputting 3A of DC current at 6V. In pulse mode, it can output 10A at 20V.
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