Production Testing of High Intensity, Visible LEDs using Series 2600
Contents
1. Data amp Data Concatenate data string Loop This will return the characters that are held in the output buffer queue in the order they were written The data return in this case was ASCII This is not the fastest method of data return but it is the easiest to start with Consult the software program and instrument manuals for directions on more expedient data transfer techniques such as binary data transfer and buffered data storage Programming tests for speed TSP Test With many instruments the PC controls all aspects of the test Sequence In each element of a test sequence the instruments must be configured for each test perform the desired action and then return the data to the controlling PC Figure 4 The control ling PC then must evaluate the pass fail criteria and perform the appropriate action for binning the DUT Each command sent and executed consumes precious production time and lowers throughput Communication Communication Obviously a large percentage of this test sequence time is SMU consumed by communicating information to and from the PC Series 2600 instruments offer the unique ability to increase the throughput of complicated test sequences dramatically by decreasing the amount of traffic over the communications bus In these instruments the majority of the test sequence is embed ded in the instrument The Test Script Processor TSP is a full featured test sequence engine that allows control of the t2. Set current source range smua source leveli 5E 6 Set current source level smua source limitv 40 Set source voltage copliance smua source func smua OUTPUT DCAMPS Set source function delay 0 005 Delay Reading 3 smua measure v Perform VR measurement smua source leveli 0 Set source level smua source output smua OUTPUT OFF Disable output end function LEDTest And finally we need to return the data to the computer function ReturnData Data Printing pring 77 print Measurement reading at 10 mA Reading 1 V print Measurement reading at 10 V Reading 2 A print Measurement reading at 5 uA Reading 3 V end function ReturnData These functions can now be called by an external program such as Visual Basic or LabVIEW simply by sending the string of the function name Here is an example for a system using VB6 and a Keithley 488 GPIB card NOTE The single quote denotes a comment in Visual Basic 6 Call Send KeithleyMeter ResetLED status Calls ResetLED Call Send KeithleyMeter LEDTest status Calls LEDTest Calls ReturnData Call Send KeithleyMeter ReturnData status We now need to enter the data to our external program For I 1 4 There are 4 print statements so we need 4 enters Call enter Data 1000 Length KeithleyMeter status Get info back from meter Data
3. ates P S LO LO G HI G G G HI Two wire connections local sense Figure 6 Two wire connections to a 260X SourceMeter channel Model 260X CHANNEL A Fpa E S LO LO G HI G G G HI ra O12 22900090 LO Four wire connections remote sense Figure 7 Four wire connections to a 260X SourceMeter channel Therefore it is important to shorten the test time as much as possible without sacrificing measurement accuracy or stability The Series 260x System SourceMeter family can configure the device soak time before the measurement as well as the amount of time the input signal is acquired The soak time allows any circuit capacitance to settle before the measurement begins The measurement integration time is determined by the number of power line cycles NPLC If the input power were at 60Hz a INPLC measurement would require 1 60th of a second or 16 667ms The integration time defines how long the analog to digital converter ADC acquires the input signal and it repre sents a trade off between speed and accuracy Typical soak times for the V test are from less than one mil lisecond to five milliseconds and from five to 20 milliseconds for the I test By using these short test times errors due to the junction heating are reduced Also the junction heating charac teristics can be determined by performing a series of tests and only varying the test time Lead Resistance A common source of voltage measure
4. The test is performed by sourcing a low level reverse bias current for a specified time then measuring the voltage drop across the LED The measurement result is typically in the range of tens of volts Normally moderate voltage levels volts to tens of volts are used to measure a Leakage Current I The Leakage Current Test measures the low level current that leaks across the LED when a reverse voltage less than breakdown is applied It is a common practice for leakage measurements and more gener ally for isolation measurements to make sure only that a certain threshold is not exceeded in production There are two reasons for this First low current measurements require longer settling times so they take longer to complete Second environmental interference and electrical noise exert greater influence on low level signals so extra care in shielding is required This extra shielding complicates the test fixture and may interfere with automated handlers Test System Description Single LED Test System Figure 2 is a simplified block diagram of an LED test station For automation purposes a PC and a component handler a probe station for on wafer measurements are included Component Handler Test Fixture Bins Digital Lines DIO 2602 2602 Channel A Channel B Figure 2 Block Diagram of a 2602 SourceMeter Based Single LED Test System The main purpose of the PC is to store measurement data in a databa
5. can be set for the compo nent handler In addition the 2602 s built in intelligence can per form all pass fail operations and send a digital command through the digital I O port on the 2602 to the component handler to bin the LED based on the pass fail criteria Then usually two actions can take place synchronously data transfer to the PC for statisti cal process control SPC and the mechanical placement of a new DUT in the testing fixture LED Test System for Multiple Devices Arrays In addition to single device testing there are also multiple device tests such those that involve a burn in process In these tests multiple parts are measured over a specified time period A con tinuous current flow is usually mandatory to drive the DUTs but multiple light detectors may be multiplexed to a current meter by a switching system The appropriate choices for switching system and meter will be dictated by the dynamic range of electrical cur rents of interest Keithley offers a number of switch options applicable to test ing multiple LEDs Mainframes range from the two slot Model 7001 capable of up to 80 channels of switching to the ten slot Model 7002 mainframe which can handle up to 400 channels Another option is the 7002 HD which allows up to 320 chan nels in one of the world s highest density switch mainframes For low level current measurements Keithley offers the Model 6485 Picoammeter and the Model 6487 Picoammeter Voltage
6. conditions The forward voltage test Vp is performed by sourcing a known current and measuring the resulting voltage drop across the diode Typical test currents are in the milliamps range while the resulting volt age measurement is typically in the range of few volts Forward current biasing is also used for optical tests because electrical current flow is closely related to the amount of light emitted Optical power measurements can be made by placing a photodiode or integrating sphere close to the device under test to capture the emitted photons This light is then converted to a current which can be measured by an ammeter or a channel of a SourceMeter instrument In many test applications the voltage and light output of the diode can be measured simultaneously using a fixed source cur rent value In addition details such as spectral output can be obtained by using the same drive current value and a spectrom eter Reverse Breakdown Voltage Vp and Leakage Current I Tests Applying a negative bias current to the LED will allow probing for the so called Reverse Breakdown Voltage Vp The test cur rent should be set to a level where the measured voltage value no longer increases significantly when the current is increased slightly more At levels higher than this voltage large increases in reverse bias current result in insignificant changes in reverse voltage The specification for this parameter is usually a mini mum value
7. Number 2639 KEITHLEY Application Note Series Production Testing of High Intensity Visible LEDs using Series 2600 System SourceMeter Instruments Introduction Visible light emitting diodes LEDs have gained a reputation for high efficiency and long lifetimes which has led to their use in a growing list of applications including automotive displays and exterior lights street lights outdoor signs and video monitors Extensive research and development efforts by LED manufac turers have led to the creation of LEDs with higher brightness new colors and longer lifetimes which has driven demand and encouraged an even wider array of applications Now more than ever cost effective testing methods are needed to ensure the reli ability and quality of these devices LED testing involves different types of test sequences at vari ous stages of production such as during design research and development on wafer measurements during production and final tests of packaged parts While concrete testing recipes often include a multitude of steps intended to verify product lifetime or extract data on specific performance characteristics they are beyond the scope of this application note This note is intended to provide solid information on the needed ingredi ents for these recipes basic tests that illustrate how to probe for the diodes characteristics and example test setups This note also outlines how to achieve thr
8. Source One of the Model 2602 SourceMeter channels can also be used to measure currents For smaller numbers of LEDs multiple Series 2600 System SourceMeter instruments can be used Figure 3 illustrates a three LED device test system with one PD channel LED 2 LED 3 TSP Link Figure 3 Block Diagram with scalable Model 2602 SourceMeter channels for an LED Array Test System Test Sequence Script Code The following code snippets illustrate a test sequence script for the Model 2602 to perform three electrical tests on an LED The intention of the test steps is to serve as building blocks for creating more specialized applications The first part after the enumeration of tests is a one time only configuration providing a well defined starting condition of the instrument Next the output of the SMU channel is activated and the tests follow sequentially The measurement data is stored in the variable Reading and are sent to a PC via print commands at the end of the listing Note double hyphens indicate comment lines First let s put the instrument into a default setting by sending the following function Example LED Test Sequence 1 Forward Voltage Test VF at 10 mA 2 Leakage Current Test IL at 10 V 3 Reverse Breakdown Voltage Test VR at 5E 6 A function ResetLED One Time Reset amp Setup Reading Create table for readings smua reset reset SMU smua measure nplc 0 01 S
9. currents such as for leakage currents To minimize this problem construct test fixturing with high resistance materials Another way to reduce leakage cur rents is to use the built in guard of the SourceMeter instrument The guard is a low impedance point in the circuit that has nearly the same potential as the high impedance point to be guarded This concept is best illustrated by example Figure 10 In this example the LED to be measured is mounted on two insulated standoffs Guarding is used in this circuit to ensure that all the current flows through the diode and not through the standoffs In general guarding should be used when sourc ing or measuring currents less than 1uA Connecting the Guard terminal of the instrument to the metal guard plate guards this circuit This puts the bottom of the DUT insulator standoffs at almost the same potential as the top Both ends of the insulator are at nearly the same potential so no significant current can SourceMeter IN OUT IN OUT LO A Unguarded SourceMeter GUARD 5 IN OUT HI IN OUT B Guarded Insulator Metal Mounting Plate flow through it All the current will then flow through the LED as desired WARNING Guard is at the same potential as Output HI Therefore if hazardous voltages are present at output HI they are also present at the Guard terminal Electrostatic Interference High resistance measurements can be affected by electrostatic int
10. erference which occurs when an electrically charged object is brought near an uncharged object To reduce the effect of elec trostatic fields a shield can be built to enclose the circuit being measured As shown in Figure 10B a metal shield connected to ground surrounds the LED under test The Output LO terminal of the SourceMeter instrument must be connected to the metal shield to avoid noise due to common mode and other interfer ence Using this type of shield will also help shield operators from contacting the standoff metal plate since the plate is at guard potential Light Interference Testing LEDs involves detecting the amount and intensity of light produced by the LED so the test fixture should be shielded from light Typically the inside of a test fixture is painted black in order to reduce reflection within the fixture Insulator Im Measured current lb DUT current I Leakage current Cable Shield Safety Shield Im lp Metal Mounting Plate Connect to earth safety ground using 18 AWG wire or larger Figure 10 Comparison of unguarded and guarded measurements Equipment List The following equipment is needed to configure the system shown in Figure 2 Model 2602 System SourceMeter instrument Model KPCI 488 IEEE 488 computer interface board with PC or KUSB 488 USB to GPIB Adapter for use on USB ports Light shielded enclosure with calibrated photodetector e Custom digital I O cable for connect
11. est sequence with internal pass fail criteria math calculations and control of digital I O see the Test Sequence with 2602 illustrated in Figure 5 The TSP can store a user defined test sequence in memory and execute it on command This limits the set up and 1 Create the script Figure 4 PC control of standard instruments configuration time for each step in the test sequence and increas es throughput by lessening the amount of communications to and from the instrument and PC Here is a simple step by step process for programming the Model 2602 Test Figure 5 Use of the embedded Test Script Processor TSP in the Model 2602 to store the test sequence Note decreased communications traffic 2 Download the script to the instrument 3 Call the script to run The 2602 script can be written in the Test Script Builder soft ware provided with the instrument or downloaded to the instru ment using another program such as Visual Basic or LabVIEW See Section 2 of the 2602 User s Manual for more information on programming the 2602 Typical Sources of Error Junction Self Heating With increasing test times the semiconductor junction of the LED will tend to heat The two tests susceptible to junction heat ing are the forward voltage and leakage current tests As the junction heats the voltage will drop or more importantly the leakage current will increase during the constant voltage test Model 260X CHANNEL A
12. et measurement aperture smua measure autozero smua AUTOZERO OFF Disable autozero smua sense smua SENSE REMOTE Enable 4 wire measurement GlobalVar 1 end function ResetLED To perform the test sequence we need another function that sets up each test and performs the proper actions function LEDTest configure LED Test Sequence Performs VF IL and VR tests smua source levelv 0 Set source value smua source output smua OUTPUT ON Enable source 1 Forward Voltage Test VF at 10 mA smua measure rangev 6 Set measurement range smua source limiti 0 001 Set source current compliance smua source rangei 0 1 Set source range smua source leveli 0 01 Set source level Select output function smua source func smua OUTPUT_DCAMPS smua source limitv 6 Set source voltage compliance delay 0 001 Delay Reading 1 smua measure v Perform Vf measurement 2 Leakage Current Test IL at 10 V Select current measurement range smua measure rangei 1E 5smua source rangev 40 Select voltage source range smua source levelv 10 Select voltage source value Set source function smua source func smua OUTPUT DCVOLTS smua source limiti 0 1 Set source current compliance delay 0 005 Delay Reading 2 smua measure i Perform IL measurement 3 Reverse Breakdown Voltage Test VR at 5E 6 A smua measure rangev 40 Set voltage measurement range smua source rangei 1E 5
13. ing the 25 pin male D sub connector of the SourceMeter to the component handler e Custom wiring harness for connecting the test equipment to the DUT and photodetector One additional Model 2602 and one TSP Link cable are need ed to configure the system shown in Figure 3 Test System Safety Many electrical test systems or instruments are capable of meas uring or sourcing hazardous voltage and power levels It is also possible under single fault conditions g a programming error or an instrument failure to output hazardous levels even when the system indicates no hazard is present These high voltage and power levels make it essential to pro tect operators from any of these hazards at all times Protection methods include Design test fixtures to prevent operator contact with any haz ardous circuit Make sure the device under test is fully enclosed to protect the operator from any flying debris For example capacitors and semiconductor devices can explode if too much voltage or power is applied Double insulate all electrical connections that an operator could touch Double insulation ensures the operator is still protected even if one insulation layer fails Use high reliability fail safe interlock switches to disconnect power sources when a test fixture cover is opened Where possible use automated handlers so operators do not require access to the inside of the test fixture or have a need to open guards P
14. ment error is the series resistance from the test leads running from the instrument to the LED This series resistance is added into the measurement when making a two wire connection See Figures 6 and 8 The effects of lead resistance are particularly detrimental when long con necting cables and high currents are used because the voltage drop across the lead resistance becomes significant compared to the measured voltage Figure 8 depicts the situation with lead resistances drawn as lumped components The gray rounded rectangle sketches cur rent flow which is nearly unaffected by high impedance voltage meters Voltage drop V I R R Lead resistance l source Too high x reading Voltage drop V I R R Lead resistance Figure 8 Two wire connections to an LED Voltage drop V I R R Lead resistance V meter XXXX l source Correct reading A Voltage drop V I R R Lead resistance Figure 9 Four wire connections to an LED To eliminate this problem use the four wire remote sensing method rather than the two wire technique With the four wire method see Figures 7 and 9 a current is forced through the LED using the Output HI LO test leads and the voltage across the LED is measured using the Sense HI LO set of leads As a result only the voltage drop across the LED is measured Leakage Current Stray leakage in cables and fixtures can be a source of error in measurements involving very low
15. oughput advantages by using new test technologies including instruments enabled with Keithley s Test Script Processor TSP Test Description Testing LEDs typically involves both electrical and optical meas urements This note focuses on electrical characterization including light measurement techniques where appropriate Figure 1 illustrates the electrical I V curve of a typical diode A complete test could include a multitude of voltage values versus current operating points but a limited sample of points is gener ally sufficient to probe for the figures of merit Vi test Figure 1 Typical LED DC I V curve and test points not to scale Some tests require sourcing a known current and measuring a voltage while others require sourcing a voltage and measuring the resulting current A SourceMeter instrument is ideal for these types of tests because it can be configured to source voltages or currents and can also measure each of these signal types Forward Voltage Test V and Optical Tests The V test verifies the forward operating voltage of the visible LED When a forward current is applied to the diode it begins to conduct During the initial low current source values the voltage drop across the diode increases rapidly but the slope begins to level off as drive currents increase The diode normally operates in this region of relatively constant voltage It is also quite useful to test the diode under these operating
16. rovide proper training to all users of the system so they understand all potential hazards and know how to protect themselves from injury It is the responsibility of the test sys tem designers integrators and installers to make sure opera tor and maintenance personnel protection is in place and effective Specifications are subject to change without notice All Keithley trademarks and trade names are the property of Keithley Instruments Inc All other trademarks and trade names are the property of their respective companies KEITHLEY Keithley Instruments Inc 28775 Aurora Road Cleveland Ohio 44139 440 248 0400 Fax 440 248 6168 1 888 KEITHLEY 534 8453 www keithley com Copyright 2005 Keithley Instruments Inc No 2639 Printed in the U S A 0705
17. se for documentation A secondary purpose is to recon figure the test sequence for different parts Series 2600 instruments are unique in terms of their inde pendence from the PC controller Their internal Test Script Processor supports writing a complete test plan that operates on the instrument itself In other words a user can write a com plete PASS FAIL incoming inspection test sequence script and run it from the front panel of the Model 2602 without instru ment reprogramming A more production oriented scenario would look a bit different In production there may be a component handler to transport the individual LEDs to a test fixture where it can be electrically contacted The fixture is shielded from ambient light and houses a photodetector PD for light measurements In this setup a single Model 2602 Dual Channel System SourceMeter instrument can be used for both connections Source Measure Unit A SMUA can be used to supply the test signal to the LED and measure its electrical response while SMUB can be used to monitor the photodiode during optical measurements The test sequence can be programmed to begin using a digi tal line from the component handler that can serve as a start of test SOT signal After the SourceMeter instrument detects the SOT signal the tests for characterization of the LED will begin After all electrical and optical tests are completed a digital line to flag measurement complete
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