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INCREASING TEST THROUGHPUT WITH BETTER INSTRUMENT COORDINATION Andy Armutat Keithley Instruments The built-in intelligence and programmability of today's source-measure units can greatly improve test throughput. Testing speed is important for all electronic components, but it is vital for low-price two- and three-terminal devices like diodes and transistors. Most types of diodes, for example, are tested for at least three basic DC parameters during final inspection: Forward Voltage (VF), Breakdown Voltage (VR), and Leakage Current (IR). These tests must be accurate and quick. Most of these tests require several instruments, such as a DMM, voltage source, and current source. However, using multiple instruments takes up more rack space than a system with all these functions in one unit. Three separate instruments also mean three sets of commands to learn, plus complicating system programming and maintenance. It also makes trigger timing more complex, increases triggering uncertainty, and increases the amount of bus traffic required, which hurts throughput. The first part of the solution is to combine several functions in one instrument. A source-measure unit (SMU) combines a precision voltage source, a precision current source, a voltmeter, and an ammeter in one instrument, saving space and simplifying integration. The second part is to eliminate communication delays between the instruments and the control computer. Using a GPIB (IEEE-488) link to deliver commands to control each step of a test has two drawbacks. First off, GPIB has considerable communications overhead. Secondly, there's generally a PC running WindowsTM at the other end of the line, and Windows has unpredictable timing that it makes unsuitable for close synchronization of multiple instruments. The solution is to let the instruments run themselves. Many of today's instruments have source memory list programming, and can run up to 100 complete test sequences without PC intervention. Each test can include source configurations, measurements, conditional branching, math functions, and pass/fail limit testing with binning capability. Some units can slow down more sensitive measurements and speed up others to optimize overall timing. The role of GPIB is then to download the test program before the test and upload the results to the PC afterwards, without interfering with the actual testing. Instrument triggering Figure 1 shows how a modern instrument (in this case an SMU) handles triggers. In the source- delay-measure (SDM) cycle the source is turned on, a programmable delay is executed, and then the measurement is performed. The user can trigger the beginning of each step, or the instrument can output a trigger after each one. Figure 1. SMU trigger input/output configuration Example: Testing diodes Our first example involves one test instrument