When high yield is the goal, careful consideration of available instrumentation helps engineers design an accurate, efficient, and cost-effective test system.
By Dale Cigoy
The two most basic tests for a connector are pin continuity and insulation resistance (otherwise known as isolation). The resistance levels involved depend largely on how crucial they are to the overall performance of the systems in which they are installed.
Isolation is typically tested by applying a voltage across two pins in a connector and measuring the resulting current between them. The resistance is compared to a threshold value, which is usually between 1 MΩ and 1 TΩ. In the electrical equivalent of a connector, isolation resistance is identified as Riso (see Fig. 1). When testing very high resistance, the measured value may change significantly with a change in the applied voltage, an effect known as the voltage coefficient of resistance. This makes it preferable to apply a voltage to the device under test, then measure the resulting current (see Fig. 2).
 FIGURE 1. A connector can be shown in a equivalent electrical diagram with its pin continuity and isolation resistance. |
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 FIGURE 2. The constant-voltage method can be used for measuring high resistance. |
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The test voltage chosen depends on the capabilities of the instrumentation and the degree of current measurement sensitivity available, as well as the ratings of the connector material. Increasing voltage increases the current signal, allowing better measurement resolution. Contributors to error include noise generated by electrically charged objects in the environment, leakage current in the test fixture, and the cable capacitance.
Pin continuity
Continuity (Rpin) is typically tested by sourcing a constant current through the pin and measuring the corresponding voltage drop. Using high currents to test continuity has two advantages. First, it ensures that the resulting voltage signal will be above the noise floor of the test system, which includes the error related to the voltage drop in lead resistances and the voltages due to temperature variations at junctions of dissimilar metals. Second, a higher test current can also serve as a stress test for the connector. A current source and voltmeter are used to measure resistance (see Fig. 3). Most instruments designed to measure low resistance have a built-in current source and voltmeter and can be configured to measure resistance with one instrument bus command or button on the front panel.
 FIGURE 3. The constant-current method can be used for measuring low resistance. |
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The choice of connector testing equipment depends on accuracy and speed requirements, the range of resistances to be measured, the method of measuring resistance, and whether it is necessary to control the value of test current or voltage (see table). Additional features such as handler interfacing and limit testing may also be important.
A combination digital multimeter/switch system measures resistance using the constant-current method. Its range of up to 100 MΩ can be adequate for measuring isolation resistance in many applications. Some models have more specialized features for low-resistance measurement. These include four-wire connections, dry-circuit testing, offset compensation, and low source current to prevent device-heating effects in low-resistance measurements. The switching function of this type of instrument is discussed later in this article.
The next step up in performance is a specialized unit that combines a source-measure unit (SMU) with a set of switching modules. A source-measure unit consists of a voltage source, current source, voltmeter, ammeter, and ohmmeter in a single package, and offers greater measurement sensitivity for pin continuity tests and extended range for isolation resistance tests; its programmability and choice of built-in switching arrangement allow for fast test throughput. A typical instrument of this type can make isolation measurements up to 1 GΩ and continuity measurements down to 10 mΩ.
For some applications, the measurement range of a combined SMU/switch system may be insufficient. In this case, a stand-alone SMU, with or without other instruments, may be appropriate. When sourcing 1.0 A, a good SMU can measure 1 mΩ with 0.3% uncertainty. Because connector pins can overheat at this current level, some SMUs have an auto output-off feature that keeps the source turned on for only a few milliseconds, which reduces heating and also provides cold switching for extended relay life in switch systems.
For extremely low-resistance devices that require a high degree of test accuracy, it may be necessary to pair a nanovoltmeter with the SMU. Using this combination, a 10-mA source can measure 1 mΩ at an uncertainty of 0.45%. This 100x reduction in source current from the previous example leads to only a 0.15% increase in uncertainty.
As discussed previously, it is generally preferable to test high resistances with the source-voltage method. A good SMU voltage source allows it to measure isolation resistances of up to 1 GΩ or more with reasonable accuracy. A high-voltage SMU with a 1100-V source offers the possibility of testing resistances up to 10 GΩ at 500 V with 0.67% uncertainty.
Other instrument choices
Other instruments that can be used for measuring insulation resistance include high-performance digital multimeters (DMMs), which can measure up to 1 GΩ using the constant- current method, and picoammeters that have a programmable source voltage and a V/I resistance mode. For the best high-resistance measurement accuracy of all, an electrometer with a high-voltage programmable source and low ammeter offset can measure insulation resistances of hundreds of gigaohms or teraohms.
Once it is clear what instrumentation option is most appropriate, it is time to look at switching requirements. Answering the following questions will help in designing a switch system: How many devices are to be tested? Is parallel testing needed? Will the system be performing multi-pin-to-pin testing? Other considerations are the maximum voltage and current levels to be sourced and/or measured, and the speed and accuracy requirements.
The multiplexer and the matrix are the two most common switch topologies. Multiplexer cards are used to connect one instrument to many test points or vice versa. In a simple multiplexer configuration, resistors are connected across each relay (see Fig. 4). When only one channel is closed, a device is connected to the inputs of the SMU and can be tested. In a matrix, on the other hand, any one point in the system can be connected to any other point in the system (see Fig. 5). This configuration is useful when more than one instrument is needed to test each device. Although two channels must be closed to perform a measurement, the matrix configuration allows testing any possible combination of connector pins.
 FIGURE 4. A multiplexer configuration allows any device under test to be connected to a test instrument. |
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In addition to the relay configuration, it is very important to consider the specifications of the switch card when choosing switch hardware. When measuring pin continuity (low resistances), choose a switch card with low contact potential and a current rating high enough to withstand test current. When measuring insulation resistances, choose a switch card with low offset current, high isolation resistance, and a voltage rating high enough to withstand the source voltage.
Preprogramming
Many test instruments have source memory list programming, and can run up to 100 complete test sequences without PC intervention, triggering each other directly without using a sometimes-slow GPIB (IEEE-488) link. This also eliminates the delays caused by using a PC to control every step in a test. The role of GPIB is reduced to downloading the test program before the test and uploading the results to the PC afterwards, without interfering with the actual testing.
Many electrical test systems or instruments can involve hazardous voltage and power levels. It is essential to protect operators from any of these hazards at all times. A safe system has test fixtures designed to prevent operator contact with any hazardous circuit, and double-insulated electrical connections. Double insulation ensures the operator is still protected, even if one insulation layer fails.
A safe design uses high-reliability failsafe interlock switches to disconnect power sources when a test fixture cover is opened. Where possible, automated handlers should be used so that operators do not require access to the inside of the test fixture. Finally, proper training is necessary for 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 system designers, integrators, and installers to make sure operator and maintenance personnel protection is in place and effective.
DALE CIGOY is a senior application engineer at Keithley Instruments, 28775 Aurora Road, Cleveland, OH 44139. Tel: (440) 498-2717; email: dcigoy@keithley.com.
