Short circuit inspection method for PCB circuit board

source:Kemaolong electron time:2017-05-15
PCB short circuited circuit board inspection method 1 if manual welding, to develop good habits, first of all, to visually check PCB plates before welding, and use a multimeter to check the key circuit (especially for power and ground) is short circuited; secondly, it use a multimeter to check whether the power supply and ground short-circuit each welding end a chip; in addition, don't throw the welding iron, if the solder thrown to the welding foot on chip (especially the SMD), it is not easy to find.
2. open the PCB diagram on your computer, light up the short circuited network, see where it is closest, and easily connect to one. Particular attention should be paid to the internal short-circuit of the IC.
3. a short circuit is found. Take a plate to cut (especially for single / double plate), after the secant, each part of the function block respectively power, part of a part of the exclusion.
4 the use of short position analyzer, such as: Singapore PROTEQ CB2000 short trackers, Hongkong smart technology QT50 short-circuit trackers, British POLAR ToneOhm950 multilayer board short circuit detector.
5 if there is a BGA chip, because all the solder joints are covered chip is invisible, but it is multilayer (more than 4), so the best in the design of the power of each chip is split and connected by magnetic beads or 0 ohm, such power and short circuit, open magnetic detection, easy to position to a chip. Since the welding of BGA is difficult, if the machine is not automatic welding, and pay little attention, it will short-circuit the adjacent power supply and the ground two solder ball.
6., small size table capacitor, welding must be careful, especially the power filter capacitor (103 or 104), a large number of, it is easy to cause power and ground short circuit. Of course, sometimes, if luck is bad, the capacitor itself is short circuited, so the best way is to check the capacitor before welding. You can also check to see if it is short circuited. The 1 Principle in Figure 1, abcdefgh is a power supply line on the printed circuit board (+5 V), ABCDEFGH is in the ground, under normal circumstances, the two do not communicate directly (without short circuit), normal power can be added. Figure 1 printed circuit board on the negative power supply line short circuit schematic now assume that S point of short circuit, the power can not be added, because the short circuit power supply is too large, may be burned on a printed circuit board line (copper) as shown in figure 1. The 5V power is added to the 50 resistor, and the current of the I=5 / 50=0.1A and 0.1A should be safe for the line due to the shunt action of the resistor. The current flows through the abcd...... The DCBA will form a voltage drop on the conductive line. The sensitive voltmeter is fixed at the A terminal and the positive electrode is BCD from the A point...... Mobile, due to the short circuit point in S, so F until the voltage meter reading will increase, but the move to H point does not increase (voltage readings unchanged), indicating that FH no current flows, so give up H at the direction, changed to find the direction of G, then F to S voltage meter reading is to increase, but at the S point reading is not increased, and in addition to the G-spot outside no other direction lines, thus we can conclude that S is short. 2 since the detection circuit on the printed circuit board by lines of a copper foil, resistance of approximately 0.0005 / mm (depending on the width and thickness of the 0.1A), when electric current flows, each millimeter of lines at both ends of the pressure drop is about 500uV its value is very small. The 20mV three bit semi digital voltmeter has a resolution of 100uV. In order to find the correct voltage, the detection voltage has to be amplified (see Figure 2). Figure 2 Tutu 2 amplifier principle IC1 operational amplifier (LM324) is composed of the inverting amplifier, the amplification of - (R3/R2) =-10/1=-10 (Times) and the 200mV digital voltage meter full input voltage U for 20mV detection, detection resolution of 10uV, and the actual output voltage measurement U (generally less than 15mV the equivalent line length of 300mm on the copper foil), and thus will not drop over range. R1 ensures that when the measuring circuit is open, the op amp is transmitted to zero, and the R3, D1 and D2 protect the digital voltmeter from overload shock. The R4 can balance the offset current of the operational amplifier and reduce the drift of the zero. The op amp zero is not necessary, according to the example, the LM324 offset voltage is less than 1mV, some even only O.1mV (you can choose a small offset voltage from one of 4 independent operational amplifier), magnified 10 times about the number of MV, the offset voltage of the measured temperature coefficient is about 5uV/ DEG C. Since the changes in voltage readings, rather than the absolute values of voltage, are observed when testing, the offset voltage has little effect, but the temperature coefficient of the offset voltage is not too large. In a word, the LM324 with an adjustable zero function can work normally. If the requirement is higher, the automatic zero stable op amp can be used, and the effect will be better. The detection circuit can also be used for non power line short-circuit point, but limiting current of R0 should be changed to 200, the I=25mA, because the 0.1A current may make some small power silicon tube emitter damage, while 25mA is generally safe. I reduces the sensitivity of the test so that the amplification factor should be increased and the R3 is changed to 40K. At this point, the influence of the offset voltage is increased. Therefore, an automatic zero stable OPAMP must be adopted, but the operating principle is the same. 3 test methods in Figure 1 circuit connected, y terminal is fixed at A, with the tip of the X test bar (with a multimeter test bar sharpened restructuring, so as to be able to pierce the solder mask), from the A point, gradually to the distance test, and observe the digital meter readings, if the test rod tip in the printed circuit board line moved to reading to all directions are no longer changes when the point is short, can be eliminated, if there is more than one short, one can find out, one by one to be eliminated. Last need
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