Using Short-Circuit Tracker to Detect PCB Short-Circuit Faults
Scenario 1:
- If the online test of U1 shows pin 4 failure, check impedance to ground. Equal impedance on all output pins indicates internal damage, requiring U1 replacement.
- Measure impedance between pin 4 and ground. High impedance (10 ohms) at low logic state and over 1 kiloohm at high logic state.
- Use multimeter to check resistance. For precise location, employ Qtech’s QT25 or QT50 short-circuit trackers.
Scenario 2:
- Steps for “two-point positioning method” using milliohm meter to locate short circuit.
- If resistance at solder joint is lower than adjacent trace, chip replacement is needed.
- Higher resistance indicates short circuit outside tested chip.
Scenario 3:
- Pin 5 of U3 shorted to ground due to transistor breakdown. Use short-circuit trackers to locate fault.
- Confirm short circuit within U3’s pin 5 and replace U3.
Scenario 4:
- Pin 3 of U2 partially shorted to ground, showing around 10 ohms resistance.
- Normal resistance between input/output pins to ground should not be 10-40 ohms.
It’s essential to detect and address short-circuit faults promptly to ensure optimal PCB functionality and prevent further damage.
PCB Testing: Troubleshooting Pin 4 Short Circuit on U1
During PCB testing of U1, a failure was detected when pin 4 of U1 could not drive a low-resistance input pin. The screen indicated that pin 4 was in a low-resistance state, and measuring its resistance to ground showed approximately 10 ohms.
Traditional milliohmmeters are not sufficient to accurately locate the exact point of this partial short circuit due to the 10-ohm resistance reading. To address this issue, the QT50 short-circuit tracker offers a solution by adjusting the measurement zero position to effectively mask fixed resistances ranging from 10 to 20 ohms.
For optimal operation, it is recommended to set the measurement zero at 10 ohms and the range to 200 milliohms. Follow the outlined steps in the third scenario to proceed with the measurements and pinpoint the source of the short circuit.
