High-density, visible LED's production test

Test Description

High-performance LED is usually 5 to conduct the test. Including the three tests on the DC spectrum (forward voltage, reverse breakdown and leakage current test) and two tests on the spectrum (light intensity and wavelength test). In a production environment, usually involves testing only the DC testing capacity problems. Although useful for optical testing, but usually slower, generally reserved for engineering or quality control laboratory. The following details the various tests and test requirements.

DC test

Figure 1 shows the DC test described in this article three test points.



Figure 1: Typical DC I-V curve of LED and test point

Forward voltage test (VF)

VF test test visible light LED's forward voltage. Over the operating voltage, circuit current will result in a significant increase was significantly higher forward voltage, shown in Figure 1. A certain period of time (eg 1ms) the LED forward bias on the specific load current (eg 10mA), measuring the voltage drop across the LED. Measurement results are usually the order of a few hundred millivolts.

Reverse breakdown voltage test (VR)

VR test test LED's reverse breakdown voltage, and diodes similar. When the voltage is higher than the voltage, the reverse bias current of the significant increase in the reverse voltage changes little. This parameter is usually a minimum of technical indicators. When tested in a specific period of time to provide a low reverse bias current while measuring the voltage drop across the LED. Measurement results are usually a few volts to tens of volts in magnitude.

Leakage Current Test (IL)

IL test test LED leakage current, which is lower than when the reverse voltage breakdown voltage, LED in the small leakage current. Test load specified reverse voltage, was measured after a certain period of time corresponding current flowing through the LED. Testing process to test whether the measured leakage current is below a certain threshold. The current measurement results are usually in the nano-amp to the order of milliamperes.

Optical Testing

Test light intensity and radiant intensity

Light (ie light) is usually the size of the strength of lumens / steradian or candela to represent. It usually ranges in size from a few milli-candela Candela to the order. We can use this parameter to calculate the radiation intensity, in watts / steradian to represent. LED radiation intensity measurement is the total output (power), and measures the luminous intensity within the range of visible light output (power). The size range of radiation intensity is less than 1μW/sr omitted to tens mW / sr range. With the total light output (lumens / W) divided by input power can be calculated at the light efficiency. Light intensity usually photodetector (PD) were measured. PD flow through the reverse leakage current size and light intensity on top of it is proportional. Therefore, if the LED light with the corresponding PD PD simultaneous measurement of the leakage current, one can calculate light intensity. Light intensity measured using this method, simply use the high-speed DC tester can be constructed to test the DC and optical test systems. If you do not want to use this DC test method, you must use the accumulated balls, paper will not discuss this in detail.



Figure 2: Table based on the digital source / electrometer visible LED production test system block diagram of a typical test

Note: Before luminous and radiant intensity measurements need to use the cumulative test fixture ball.

Wavelength and color test

We usually use the spectrometer to measure the wavelength, which measures the output of the primary and LED peak wavelength. LED output spectrum is called the far field diagram (farfiELd pattern, FFP), similar to the LED peak wavelength of a center of normal curve. Full width at half maximum (Full width at halfmaximum, FWHM) calculated as the half-light intensity of the spectrum bandwidth is used to represent the working wavelength range of LED. With ISO / CIE standard colorimetric system measured LED output color information, the system can measure the three primary colors based on (red, blue and green) the size of the output color.

Test System Description

Single LED Test System

LED was placed in the test fixture, connected to a digital input of the source table. Place the device and connecting the device under test (DUT) operations are often completed by the robot components, and to automate the production process. Test fixture is usually dark, and to prevent the impact of ambient light causes the test data error. Photodetector (PD) integrated in the test fixture, when the robot into the test fixture to LED when to test them. Figure 2 shows typical DC characteristics of the system structure.

LED digital source table can be three types of DC testing. Since this instrument can provide any polarity current source or voltage source, so no need to flip or move placed in the position of the first test to complete all the LED on the DC characteristics of the test. Digital source and electrometer in combination, to measure the size of light intensity. LED's light intensity to be on the feature analysis, digital source table need to work in the LED (voltage) range of multi-point current scan (as shown in Figure 3a), using a 6517A can be measured at the light through the PD (such as shown in Figure 3b). For high throughput applications, it is generally only one or a few test points on the measurement of light intensity.



Figure 3a: LED measurement circuit



Figure 3b: photodetector power supply circuit

Multi / Array LED System

For the LED array, multi-die package or aging test application, we often need to simultaneously test a lot of LED. A test multiple devices in the most cost effective way to integrate test system switch. Aging test is usually to delay the need for power LED, which requires no dedicated power supply with a switch function. PD aging test system of measurement commonly used way of monitoring multiplexed LED performance at different times. Figure 4 shows an LED switch test system configuration example. Actual system can be configured any number of diodes, support a variety of electrical indicators.



Figure 4: Multiple LED to a source table 2400 digital switch and a number of PD to the 6517A-type switching electrometer (Click image to enlarge)

In multi-device test system, select a single LED for each test, and the LED and PD light intensity for the detection of the corresponding relay is closed. The required number of DC source table test, and then load sufficient current to light LED, when LED is lit, 6517A PD increases the leakage current measurement. When this testing process is complete, then select a device for the next switch channels.

7011 multiplexer card type offset current indicator is <100pA, this value may exceed the tolerance of the test system. Thus, 7158 (or 7058) Low Current Scanner Card instead of 7011, the offset can be reduced to <1pA (<30fA typical value). Note that it is important to reduce the use of low current cards will be available to the system channel density. Low Current card only 10 channels can be used for scanning, which means that when they are replaced with 7011, the need to use the card number will quadruple.

Integrated Devices

At present, many sensors are in the same package with integrated LED and PD, constitute a reflective target for the sensor, light switch or some other hybrid devices. Need to simultaneously test the integrated device the performance of LED and PD. PD which you want to use an external monitor or other photosensitive device separate LED output. DC test done using digital source table, but when measuring light intensity, the internal and external PD should be measured. Not only must pass all required LED DC and optical testing, and must also be within the PD data from the "standard" data obtained from external devices to associate. Figure 5 shows a table with three digital sources such test system instance.



Figure 5: Using the 2400 digital source meter to measure the integrated LED / PD device (Click image to enlarge)

According to the current sensitivity of the test specification requirements, we can use 2400 or 6517A measuring PD. 2400 for the accurate measurement of current as low as 10nA is useful, and 6517A can be reliably measured than 10fA current.

Test System Safety

Many electronic test system or equipment capable of measuring or providing dangerous voltage or power. There may, in a single fault conditions (such as programming errors or equipment failure), there is no danger even if the system will display the output risk level.

For these high-voltage and high power, it is necessary to protect the operator has to avoid these dangers.

Protection methods include:

● Design of test fixtures to prevent operator contact with any danger circuit.

● Make sure that the device under test completely sealed to protect the operator from flying debris damage. For example, capacitors and semiconductor devices under high voltage explode.

● The operator may be exposed to all the electrical connections for double insulation. Double layer of insulation to ensure that the insulation failure even still be able to protect the operator.

● When the test fixture shell is open, highly reliable, fail-safe interlock switch to disconnect the power.

● If possible, use automated robots, so that operators do not have access to the internal or open test fixture protection.

● detector on all users of the system for hands-on training so that they understand all the potential dangers and know how to protect themselves from harm.

● test system designers, integrators and installers responsible for ensuring that operators and maintenance personnel on the protection measures are in place and effective.

Methods and Techniques

Based on the synchronous Trigger Link

Trigger Link is a test instrument used in the system bus, a hardware handshake to ensure proper testing sequence. Keithley Instruments is the standard all the latest features, including all the instruments mentioned in this article. When the meter and switch hosts connected through the Trigger Link cable, they can trigger each other to achieve faster test operation. The built-in PC does not need direct control of most of the bus system functions. When using the Trigger Link function properly, PC needs to be done only function is to initialize the test and retrieve data from the system.

To learn more about how to configure the synchronous Trigger Link method test system details, please refer to Keithley Application Note No. 2217 "multi-trigger digital sync source table."

Contact Check

Digital access to checking the source table to help users eliminate the contact fatigue, damage or contamination, loose or broken connection, relay failures and other problems caused by measurement errors and false product failures. At the beginning of the implementation of automated test sequences for each prior to testing the device under test (DUT) in the exposure, which helps reduce false failures caused by processing and cost.

Contact check function test HI / LO test whether the resistance between the leads of less than a certain threshold. Contact Check to Output HI / LO, Sense HI / LO, and Guard / Guard Sense of between. By using the pulse converter and the reference resistor, we can very quickly check the contact (usually within 350μs). Reference resistor can be set to three different values ​​(5Ω, 15Ω, 50Ω). Contact check function is not in the DUT to transmit the signal - only three pairs in the HI / LO between the leads. If you find access to inspection failure, the test operation will be ignored and the panel, IEEE-488 interface bus and the digital I / O port failure indication is given.

Verify LED Polarity

We can add a polarity in the test set test. Polarity test before the test can be safe in the completion and quickly determine the direction of LED. We can use two ways to determine breakdown characteristics of LED LED polarity. One is by being in the LED current, measure voltage. If the voltage is lower than 1V (typically) that the diode is positive polarity, said the high breakdown voltage and reverse polarity. Another method is through the negative current in the LED, if the measured voltage is less than 1V, said reverse polarity, high voltage breakdown, and that positive polarity. Which method to test the specific choice depends on the polarity of the overall structure of the test program.

To learn more about how to use the polarity of the test results and a variety of optional digital source table and robotic components, please refer to Keithley Application Note No. 1805, "the source tables based on Series 2400 Diode Production Testing."

Common error sources

Results from the heat

With the increase in test time, LED semiconductor junction is often fever. Results fever of the most sensitive test of two tests is positive voltage and leakage current tests. When the junction heating, the voltage will drop, but more importantly, in the regulator during the test leakage current will increase. Therefore, without compromising accuracy or stability in the case of test time as short as possible is very important.

Table Series digital source devices can be configured soak time before the measurement (soak time) and access to the input signal time. Soak time to make the circuit capacitance to stabilize before the measurement. Measurement integration time depends on the power of linear cycles (NPLC). If the input power frequency is 60Hz, then the measurement will be required 1NPLC 1 / 60 seconds, that 16.667ms. Determines the integration time mode - digital converter (ADC) for input signal of the time, it reflects the speed and accuracy trade-off between the two.

Typical VF test soak time from 1 ms to 5 ms range, IL test is 5 milliseconds to 20 milliseconds. Use shorter test time, you can reduce the heat caused the error statement. In addition, a series of tests and test time can be determined only change the characteristics of junction heating.

Lead Resistance

A common source of measurement error voltage to the LED from the instrument to bring the test leads connected to the series resistance. When using 2-wire connection mode (as shown in Figure 6a), which will join the series resistance measurement. When a long cable and high current, the impact of lead resistance is particularly harmful, since the voltage on the lead resistance compared to the measured voltage drop on the immensely great.



Figure 6a: wire connection



Figure 6a: wire connection

To solve this problem, the best detection method using four lines instead of a remote two-lane technology. Four-wire connection method (as shown in Figure 6b), the current through the Output HI / LO test leads to flow through the LED, use Sense HI / LO LED on the measurement of the voltage leads. This is just the measured voltage drop on the LED.

Leakage current

In measuring very low current, such as leakage current, cable and fixtures in the leakage current is a spurious source of error. In order to minimize the impact of such problems, should use high resistance material test fixture. Another approach is to reduce the leakage current table with digital built-in protection circuit source. This protection circuit is a circuit in a low impedance point, it is protected with a high impedance point to be nearly the same potential. Figure 7 is a good example of this principle.



Figure 7:2400 protection technology series (click image to enlarge)

In this example, the LED under test installed in the two insulating bracket (RL1 and RL2). The circuit protection circuit can be used to ensure that all of the current to flow through the diode, will not flow through the stent. Generally, when measuring the resulting current source or when the current is less than 1μA cable protection measures should be adopted. Guard end of the instrument is connected with metal plates to protect this circuit. This makes insulation RL1 and RL2 of the bottom and top is almost the same potential. Both ends of the insulators is almost the same potential, so there is no significant current flows through them. All the currents are as we hoped it would flow through the LED.

Warning: Guard Output HI terminal end and has the same potential. Therefore, if the Output HI dangerous voltage, then the Guard side there are the dangerous voltage.

Static

When a charged object close to another uncharged object, susceptible to high-resistance measurement of static. To reduce the impact of electrostatic field, a shield can be used to encapsulate circuit under test. Figure 7. A grounded metal shield wrapped tested the LED. Digital Output LO source end of the table must be connected to the metal shield to prevent common mode interference and other interference caused by noise. The use of such shields also help to prevent operator contact with metal stents, because the metal plate also has a protection potential.

Optical interference

LED LED light test needs to detect the size and strength, so the test fixture should be dark. In general, we can test the fixture's internal jet black, in order to reduce the internal reflection of light fixture.

Keithley has developed a program sample Microsoft ® Visual Basic sample procedures, the test system in Figure 4 multi-LED / PD test. Users can download sample programs from the ftp://ftp.keithley.com/pub/instr/SourceMeter/VisibleLED.zip. In addition, you can download from the same directory 3x2400.zip documents, the description given in Figure 5, the trigger mechanism and test systems. Examples of these programs shows how effective the test parameters for each configuration 2400 and 6517A, and how to conduct a quick test using Trigger Link sequenced.

Note: The procedures provided by these tests are designed to help explain the concepts described in this article. To comply with the required test parameters and timing requirements may need to modify some of these procedures.