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LED Production Test Using the B2900A Series of SMUs

Technical Overviews

Introduction

Numerous emerging light emitting diode (LED) applications have created a demand for LEDs with higher quality and reliability. Testing that is both more accurate and faster and that permits quicker sorting is therefore required for both on-wafer devices during fabrication and for packaged devices during final test.

Both electrical and optical tests are typically performed on LEDs. The Keysight Technologies, Inc. B2901/02/11/12A Precision Source/Measure Unit is the best solution for all electric tests such as a forward voltage test (VF), a leakage current test (IR) and a breakdown voltage test (VBR). In addition, it can be used to provide forward current bias during optical test.

The B2900A Series of SMUs is a compact and cost-effective benchtop Source/Measure Unit (SMU) with the capability to source and measure both voltage and current. They cover currents from 10 fA to 3 A (DC)/10.5 A (pulse) and voltages from 100 nV to 210 V.

In addition to these comprehensive measurement capabilities, the B2900A Series of SMUs possesses high throughput that reduces test times. The B2900A Series of SMUs also has many features that make it well-adapted for production test, such as pass/fail binning, a digital I/O interface for handler control, and code compatibility with standard single and dual channel SMU products.

This technical overview focuses on the electrical testing of LEDs in production. If you need to perform optical tests in addition to electric tests, then dual-channel models of the B2900A Series of SMUs can be used. The dual-channel models of the B2900A Series of SMUs permit one channel to measure photo diode current while the other channel drives the LED.

Easy production test system configuration

As will be discussed later, in most production testing measurement results are compared with pre-defined limits and pass/fail judgments are made. Output signals from the GPIO port of the B2900A Series of SMUs can be used to communicate with the component handler to sort devices based on the pass/fail criteria.

The B2900A Series of SMUs supports several communication protocols, GPIB, USB and LAN, and these can be used with both SCPI and IVI-COM drivers. SCPI is an industry-standard command set for basic instruments with a uniform structure that supports a common set of commands.

The SCPI command set of the B2900A Series of SMUs not only supports its advanced features but also general-purpose SMU commands (such as those used by the Keithley 2400) to simplify test program migration. In addition, the IVI-COM drivers for the B2900A Series of SMUs work in a variety of programming environments and languages, so you can develop programs without having to use low-level commands.

Program memory improves throughput

Fast test times are essential to maximize throughput and maintain high levels of factory productivity. Besides possessing fast intrinsic measurement speed, the B2900A Series of SMUs has a program memory function that can be used to improve production test throughput.

Program memory allows you to store long strings of SCPI command lines once into the volatile memory of the B2900A Series of SMUs and then recall those strings multiple times while the program is executing using a single SCPI command.

By storing the command strings in memory, the time that would have been spent sending those same commands over a communication bus is eliminated. For tests that utilize lots of repeated code (such as subroutines), program memory can dramatically reduce test times. Of course, programs can be saved to or loaded from any attached USB flash memory device.

The B2900A Series of SMUs also has a data buffer on each SMU channel that can hold up to 100,000 data points. This enables you to transfer all the data in the buffer at once after a series of measurements have completed instead of having to transfer data after every measurement. One way to use this to improve throughput would be to have the B2900A Series of SMUs send measurement data to a PC while a component handler places a new device on a DUT interface. 

Multiple pass/fail judgment modes 

In production test, a limit test function is generally used to eliminate defective devices through a pass/fail judgment based on pre-defined limits. Recognizing that there are a variety of pass/fail limit scenarios, the B2900A Series of SMUs supports two modes: Compliance mode and Limit mode (up to 12 binning limits possible). 

Compliance mode utilizes the intrinsic compliance feature of the B2900A Series of SMUs that allows a limit to be placed on voltage or current output to prevent device damage. 

When the SMU’s output reaches the limit value during a measurement it is in compliance status. If Compliance mode is enabled, then the test fails when the SMU reaches compliance status. One possible use of this feature is to determine the polarity of LEDs.

Limit mode is usually used to determine if a device parameter is within specified low and high limits. When Limit mode is enabled the B2900A Series of SMUs makes a Pass/Fail judgment based on whether or not the measured value is within specified low and high limits.

A typical use of this mode is to perform grading and sorting. For example, using two binning limits in Limit mode it is possible to sort devices into five classes.

After performing pass/fail testing with these modes, you can view the results on the wide QVGA LCD display of the B2900A Series of SMUs. In addition, you can program the B2900A Series of SMUs to output specified Pass/Fail bit patterns through the GPIO port to other equipment such as handlers for component binning. 

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