Comprehensive testing of high brightness LED production process

High brightness light emitting diodes (HBled) combine the advantages of high output, high efficiency and long life. Manufacturers are developing devices that can achieve higher luminous flux, longer life, richer colors and higher luminosity per unit power. To ensure its performance and reliability, accurate and cost-effective testing must be implemented at every stage of production.

Figure 1 shows the electrical I-V characteristic curve of a typical diode. Although a complete test program can include hundreds of points, probing a limited sample is generally sufficient to provide a figure of merit. Many HBLED tests require driving the device with a known current signal source and measuring its voltage accordingly, or vice versa. Having both synchronizable signal sources and measurement capabilities can speed up system setup and improve throughput. Testing can be performed at the die level (wafers and packages) or at the module/subassembly level. At the module/subassembly level, HBLEDs can be connected in series and/or in parallel; therefore, higher currents are generally required, sometimes up to 50A or more, depending on the actual application. Some die-level tests use currents in the range of 5 to 10A, depending on the size of the die.

Figure 1 Typical HBLED DC I-V curve and test points (not drawn to scale)

1 Forward voltage test

Understanding how new building block materials, such as graphene, carbon nanotubes, silicon nanowires or quantum dots, function in future electronic devices requires the use of metrology techniques that can measure resistance, resistivity, mobility and conductivity over a wide range. This often requires measurements of extremely low currents and voltages. The ability to make precise, repeatable measurements at the nanoscale is extremely important for engineers seeking to develop and commercialize these next-generation materials.

2 Leakage current test

When a reverse voltage below the breakdown voltage is applied, the leakage current (IL) across the HBLED is measured using a moderate voltage value. In production testing, it is common practice to simply ensure that the leakage current does not exceed a certain threshold.

3. Improving the throughput of HBLED production testing

In the past, all aspects of HBLED production testing were controlled by a single PC. In other words, in each element of the test program, the signal source and measurement device must be configured for each test, and after performing the expected actions, the records are returned to the PC. The controlling PC evaluates according to the pass/fail criteria and decides which category the DUT should be classified into. The process of sending instructions from the PC and returning the results to the PC will consume a lot of time.

The latest generation of intelligent instruments, including Keithley's new high-power 2651A system source/measurement instrument (SourceMeter), can greatly improve test throughput by minimizing communication traffic. The main body of the test program is embedded in a Test Processor (TSP) in the instrument, which is a test program engine for controlling test steps, with built-in pass/fail criteria, calculations, and digital I/O control. A TSP can store user-defined test programs in memory and execute the program according to user needs, thereby reducing the time to set up and configure each step in the test program.

4 Single-device LED test system

The component handler transports a single HBLED (or a group of HBLEDs) to a test fixture that is shielded from ambient light and has a built-in photodetector (PD) for optical measurement. Two SMUs are required: SMU #1 provides the test signal to the HBLED and measures its electrical response; SMU #2 detects the photodetector during the optical measurement.

The test program can be programmed to start under the control of a digital signal line from the component handler as a "test start" (SOT). When the instrument detects this signal, the test program starts. Once executed, a digital signal line from the component handler sends a "test completed" mark. In addition, the built-in intelligence of the instrument can perform all pass/fail operations and send digital instructions to the component handler through the instrument's digital I/O port so that the HBLED can classify the HBLED according to the pass/fail criteria. It can then be programmed to perform two actions simultaneously: data is sent to the PC for statistical processing, while a new DUT is transported to the test fixture.

5 Optical Test

Forward current bias is also required for optical measurements, because the current is closely related to the amount of light emitted by the HBLED. The emitted photons can be captured with a photodiode or an integrating sphere, so that the optical power can be measured. The light emission can be converted into a current and measured with an ammeter or a single channel of a source-measure unit.

6 Reverse breakdown voltage test

The reverse bias current applied to the HBLED allows for the reverse breakdown voltage (VR) test. The test current should be set so that the measured voltage value no longer rises significantly with a slight increase in current. At higher voltages, the change in reverse voltage caused by a large increase in reverse bias current is not significant. The VR test method is to output a low reverse bias current for a specific period of time and then measure the voltage drop across the HBLED. The result is generally tens of volts.