Dual-stage testing is a method to isolate in-circuit test (ICT) systems from high-power circuits on a device under test (DUT) during functional tests (FCT).
Dual-stage testing minimizes the risk of having excessive current flowing through the ICT systems and damaging the electronics within the DUT.
In addition, the connections of the test fixtures to the DUT increase capacitive loads on the circuit and interfere with crystals or oscillators’ functions during FCT. Therefore, the ability to disconnect those unnecessary test probes from the DUT before running FCT improves the signal integrity and protects the test systems.
Having both ICT and FCT test methods in a single test station has its risks and considerations. This is especially true with automotive circuit boards. Typical automotive electrical circuits run at 12 V for light vehicles while heavier ones like trucks and buses have circuits running at 48 V.
These operating voltages exceed the range that an ICT system can handle. An ICT system measures passive component values and runs digital pattern tests like flash programming or boundary scan in a low-power state. These tests typically run on 5volts or less.
Although ICT can source and measure higher voltages of up to 12 or 24 V at low current tests, these are nowhere close to the vehicle’s operating power. Using the dual-stage testing method, in this case, creates a protective barrier between the high-power FCT and the low-power ICT, keeping the system and DUT safe from electrical stress damage.
A dual-stage test fixture has two different lengths of test probes; short and long. ICT test uses short probes and FCT uses long probes. Typically, FCT needs only 10~20% of the total probes as it just needs to provide power and communication channels to the DUT.
However, a typical test fixture has test probes on both the top and bottom side. Most of the test probes will be on the bottom and the rest of them on the top. Dual-stage testing operation needs to connect and disconnect test probes on both top and bottom sides.
For bottom side dual stage, the press moves slightly upwards such that the ICT probes are no longer in contact with the DUT because of shorter probe length, leaving the longer FCT probes in contact only. This disconnects the ICT probes at the bottom from the DUT and sets the press into bottom dual stage position.
While it is straightforward to perform dual-stage probing on the bottom side, it is quite different on the top-side probes. Because the direction of movement to engage the DUT is top-down, the top press cannot disconnect the ICT probes on the top by moving itself upwards as it needs to keep pressing down on the DUT to keep contact with the FCT probes at the bottom. Moving the press upwards cannot achieve the top side dual stage position in this case.
To get around this, we install the top side ICT probes on a separate probe plate held by a solenoid. Without having to move the press, we can activate the solenoid and retract the ICT probe plate and disconnect from the DUT on the top side as shown in Figure 2.
Installing electronics like solenoids and sensors or actuators in test fixtures requires added connections into the test fixture which can be troublesome and unreliable. Keysight i7090 automated test handler provides 24 V sources to power solenoids or sensors in the test fixtures through its fixture identification (ID) blocks.
This cuts the dependency on the operator to connect the added cables during setup. You can find these blocks on the rear side of the top and bottom fixture as shown in Figure 3. Small solenoids or sensors installed within the test fixture can tap on the 24 V sources for power so there is no need to connect any external power supply.
A fixture ID block has three clusters of 16 pins each as shown in Figure 4. The middle cluster supplies the 24 V, 100 mA sources on pin 15 and 16 of its cluster. Sensors that need constant power connect to pin 15, while solenoids for dual-stage testing use pin 16. The voltage at pin 15 is always on and gets connected to the sensors once the fixture is inserted into the i7090 handler.
This allows the sensors to be functional even when no test operation is running. The i7090 handler switches 24 V onto pin 16 before going into dual-stage test mode and switches it off once the dual-stage test completes. By doing so, the dual-stage testing solenoids in the test fixture switch into dual-stage testing mode when the test operation enters the dual-stage testing stage. So, there is no need for a testplan to control the switching directly.
Pathwave test executive for manufacturing (PTEM) testplan works with the programmable logic controller (PLC) in tandem to synchronize its tests to the position of the DUT. Testplan executes the ICT tests after the PLC engages the DUT and contacts all the probes in the fixture. This is the all-probe position. When PLC moves the press upwards to disconnect the shorter ICT probes leaving only the long probes in contact, the position is the long-probe position.
For normal test operation that does not require dual stage test, testplan sends the Handler_SetAllProbeDone command to the PLC after ICT completes and PLC releases the DUT to the next downstream system as shown in Figure 5.
Selecting dual-stage testing option changes the process slightly to include added actions of moving the press into the dual-stage test positions before releasing the DUT to the downstream system.
This happens after the ICT operations where the testplan inserts an added step to move the press into the long probe position. This triggers the PLC to move the press to the long-probe position.
As shown in Figure 6, the testplan sends Handler_SetAllProbeDone to show that the ICT operation is complete.
It then sends Handler_MovePressUnitToLongProbePosition to move the press up and disconnect the short probes. With the DUT now having contact to only the longer FCT probes, the testplan can execute the FCT steps without interference from the ICT probes.
Once FCT steps complete, the testplan sends a Handler_SetLongProbeDone command to the PLC showing the completion of all tests. PLC releases the press and transfers the DUT to the downstream system. Figure 6 shows a snapshot of the PTEM testplan for dual-stage test operation.
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