Insulation resistance test of printed circuit boards-EEWORLD

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Printed circuit boards ( PCBs ) are used in most electrical products . If a PCB has low insulation resistance (IR), the performance of the circuits on the PCB will be greatly reduced. Factors that affect the surface resistance of a PCB include the PCB material, the presence of coatings (such as solder mask or conformal coating), and the cleanliness of the board. Temperature and relative humidity also affect the surface resistance.

To test insulation resistance, a special test template is made on the printed circuit board, which is called a template. The typical resistance range is 107Ω to 1016Ω, which is measured using a picoammeter and a voltage source. The test board usually has several terminals, so the test system includes a scanning card to switch the picoammeter and voltage source to the terminals.

Figure 1 shows a simplified system diagram for testing a 5-terminal (W) test board. In this circuit, a Keithley 7011-S 1×10 Multiplexer Card is used to connect the voltage source to terminals 1 and 5 or terminal 3. A 7158 Low Current Switch Card is used to connect the voltmeter to terminals 2 or 4.

At the start of the measurement, close channel 1. After the specified “dwell” time, activate channel 41 and measure the current. The ratio of the applied test voltage to the measured current is the resistance between terminals 1 and 2. Next, release channel 41 and activate channel 42 to measure the insulation resistance between terminals 5 and 4. Note that the path from 5 to 4 is already biased. Since no additional dwell time is required, the measurement is immediate. Release channel 42, then open channel 1 and close channel 2 to apply the test voltage to terminal 3. The 1 MΩ resistor on channel 1 will discharge the voltage between terminals 1 and 5 and will not interfere with the measurement. After the specified dwell time, activate channel 41 again and measure the insulation resistance between terminals 3 and 2. Release channel 41 and activate channel 42 to measure the resistance between terminals 3 and 4, which is already biased.

Test a 5-terminal

Figure 1. System used to measure the insulation resistance of a test specimen.

When no channels are energized on the 7158 switch card, terminals 2 and 4 are connected to the LO terminals of the picoammeter (pA), which are electrically guard terminals. The guard connection prevents leakage current from the de-energized channels from interfering with the measurement. [page]

Five "W" test boards can be tested in the above manner using one Model 7011 Switch Card and one Model 7158 Switch Card. The Model 7111 Form C Switch Card can be used in place of the Model 7011 Switch Card. If this card is used, de-energizing a channel will connect the terminal to the circuit LO instead of the voltage source. This eliminates
the need for the 1 MΩ resistor. When using the Model 7011 or Model 7111 Switch Card, the test voltage must not exceed 110 V.

The 100 kΩ resistor is used to limit the available current in the event of a short circuit on the test board. The current limiting resistor should be selected to be no more than 10% of the lowest resistance being measured.

The circuit in Figure 2 is used to test a prototype with 10 two-terminal leakage paths. Each prototype can be thought of as a set of isolated resistors (R1, R2, R3, R3, and so on). Each resistor requires a voltage channel and a current channel. The Model 7158 Switch Card connects an electrometer or picoammeter to one end of each resistor, while the Model 7169A 20-Channel Form C Switch Card connects a voltage source to the other end of the resistor.

Testing the circuit of a single 10-resistor prototype

Figure 2. Circuit for testing a single 10-resistor prototype

To test resistor R1, first energize channel 1 to apply voltage to the test sample. After the specified "hold" period, energize channel 21 and measure the resulting current. Repeat this process for the other resistors.

When the channel is de-energized, the corresponding resistor terminals are connected to the circuit LO, allowing the charge across the resistor to dissipate when no measurement is being made.
The current limiting resistor (R) protects the scanner from damage when the test sample is short-circuited.

Low noise coaxial cable should be used between the test board and the 7158 switch card to avoid noise currents.

Reference address：Insulation resistance test of printed circuit boards

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