Use short circuit tracker to find PCB short circuit fault

Use short circuit tracker to find PCB short circuit fault
The following describes how to use the short circuit tracker to find PCB short circuit fault
The first case:
If the online function test result of U1 is that pin 4 is not flipped and the test fails, the user should switch the pin status window to impedance display mode and compare the grounding impedance between U1 pin 4 and other output pins. If the impedance of each output pin is equal, the internal function of U1 is damaged, and U1 should be replaced.
Measure the impedance of U1 pin 4 to ground. If the logic state of the pin is completely low, its impedance will not be very low (greater than 10 ohms). If the pin is at the low high level, its impedance will be greater than 1 kiloohm.
Use a 3 meter to measure the resistance of the fourth contact of U1 to the ground. If it is close to zero, you can use a milliohmmeter to find the real short-circuit point through the "two-point positioning method" pioneered by Qtech. QT25 and QT50 short circuit trackers in Qtech system products are ideal instruments for finding short circuit faults in circuit boards.
The second case:
Operation steps of "two-point positioning method":
First, place the measuring probe of the milliohmmeter on the solder joint near the root of the pin (as shown in the figure below), set the measuring range of the milliohmmeter to 200 milliohms, and measure the resistance; Then place the probe on the connected copper wire (as shown in the figure below), which is 3-4 mm from the solder joint of the test pin, and read the resistance value. If the former resistance value is less than the latter, it means that the short circuit point is located on the internal drive circuit of the chip under test.
Circuit board

There is no doubt that the chip should be replaced at this time If the previous resistance is greater than the latter, it means that the short circuit point is not in the chip under test, but on other chips connected to the outside of the chip, or the wire between copper and PCB
The short-circuit resistance measured with a milliammeter depends on the degree of short-circuit, but as long as the two resistance values measured with the "two-point positioning method" are different, the real short-circuit point can be determined.
Case 3:
The real PCB fault point is that the fifth pin of U3 is short circuited to ground (the transistor inside the input pin is broken down), but when the function test of U1 is executed, the test fails because the fourth pin cannot be flipped. Switch the fixture window to impedance display mode, compare the impedance of the fourth pin of U1 with other output pins, and find that the impedance of the fourth pin is significantly lower than that of other pins (about zero). At this time, the user is not sure that the real fault is U1, so the QT25 or QT50 short circuit tracker should be used for further measurement. First, use the "two-point positioning method" to determine whether the short circuit point is inside or outside U1. After measurement, it is found that the short circuit point is outside U1. At this time, the user can find all chips connected to the fourth pin of U1 through the circuit diagram of the board under test, or use the circuit tracking function of Qtech. Then use a short circuit tracker to measure the grounding resistance of the pin connected to the fourth pin of U1 on these wafers. It is found that the grounding resistance of the fifth pin of U3 is the lowest. At this time, the possibility of short circuit between U1 and U2 will be eliminated, and the problem will focus on the fifth pin of U3. Use the "two-point positioning method" again to determine that the short circuit point is in the fifth pin of U3. At this time, the U3 chip should be replaced.
However, if U3 is tested at this time for greater certainty, the test results may still pass. This is because although the chip is short circuited to ground at pin 5, there is still a certain resistance value. As long as the resistance value is greater than the minimum drive resistance value of the tester and the logic function in U3 is not damaged, the functional test will pass the chip. For example, as with the 7400 reverse switch, if one of its input pins is short circuited to ground, its function test can still pass. In this case, the user can only determine the fault point by comparing it with the result of learning a good circuit board. However, in this example, as long as the internal short circuit of U3 pin 5 is detected, it can be determined that U3 is damaged. Because PCB circuit designers cannot short-circuit the output pin of the chip to ground.
The fourth case:
The third pin of U2 is not completely short circuited to ground. Use a 3 meter to measure the ground resistance of the contact pin, which is about 10 ohms.
The user must remember that the resistance of the output and input pins of a normal chip to ground will not be between 10-40 ohms.
At this time, the PCB test of U1 will fail (because the 4th pin of U1 cannot normally drive such a low resistance input pin), and the screen displays that the 4th pin of U1 is in a low resistance state Use a 3 meter to test that the ground resistance of this pin is also about 10 ohms In order to find the exact location of the incomplete short circuit, the ordinary milliohmmeter is powerless, because the resistance of the measured leg to the ground is about 10 ohms To solve this problem, the QT50 short circuit tracker is used to adjust the measurement zero position, which can mask a 10-20 ohm fixed resistance The specific operation steps are: set the measurement value 0 to 10 ohms and the range to 200 milliohms, and then measure according to the steps in case 3 above