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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, a device handler, and a PC. The diodes arrive
with unknown polarity, but the component handler can rotate them if necessary (Figure 2).
Figure 2. Diode testing setup with component handler
The test steps are as follows:
1. The operator tells the PC that a diode production lot is in place and ready for test.
2. The PC preconfigures the tests that the SMU will perform on each diode via GPIB.
3. The SMU waits for the Start of Test trigger from the handler.
4. When the first diode is in position, the handler sends a Start of Test trigger signal to the
SMU, indicating the first diode is ready for testing.
5. The SMU executes a polarity test. If the diode is in forward polarity, the SMU proceeds
with functional tests (Step 6). If it's reversed, a signal is sent to the handler to turn the
device and return to step 4.
6. Once the diode is in forward polarity, the SMU runs diode functional tests in the order
stored in source memory, makes pass/fail determinations, and saves data for each test.
7. The SMU sends an overall pass/fail code and End of Test signal to the handler and
simultaneously sends test data to the PC via GPIB.
8. Steps 3