PCB Fabrication

Debug circuit board

by:A-TECH      2020-04-30
When hardware engineers debug the circuit board, they directly contact the bare board, which is easy to cause damage to the circuit board. We have summarized several tips to effectively avoid accidental damage to the circuit board. 1. Touch the metal first, then take the circuit board. The human body has static electricity. Especially in dry areas, touch the metal and it will often be electrified. Static electricity is very harmful to the circuit board, and it is easy to break the components when hitting the bare board. Factory workers wear electrostatic bracelets and wear anti-static clothing. However, hardware development engineers generally do not have such good Electrostatic protection measures in the office. Therefore, we must develop a good habit of anti-static: Before taking the circuit board, first find a metal object to touch, put the static electricity on the body, and then take the circuit board. 2, take the board of the board, do not pinch the chip, or because of static electricity. Although it is possible to discharge static electricity by touching the metal, it is impossible to touch the metal all the time when you take the circuit board to the laboratory. At this time, the body will also generate static electricity. Therefore, hold the board edge of the circuit board instead of directly holding the chip part. In this way, static electricity can be conducted to the ground line of the circuit board instead of directly hitting the chip. ( The board side of the circuit board is generally designed with a circle of GND lines) 3. Use DC power supply instead of adapter or battery. When hardware engineers debug circuit boards, they need power supply. Direct use of DC adapter or battery, can not directly see the voltage and current, once the power supply or circuit board voltage is too high or the current is too large, can not be directly found. When I smell the smell, it is already late. Therefore, hardware engineers are recommended to use digital DC power supply, which can see voltage and current at any time. Once abnormal current and voltage are found, power can be cut off at the fastest speed to protect the circuit board. By watching the change of working current, we can also find out whether the circuit board is faulty. For example, the working current of some most circuit boards is 100mA, but one is 150mA, which indicates that this circuit board has a local short circuit. ( If you don't look at the numbers and only supply power through the adapter or battery, you won't find this problem. ) 4. Touch the power cord first, and then fix the line. If DC power supply is used for power supply, the positive and negative power cords are usually welded. Don't connect the wires at once, but touch the power cord first and hold the data of DC power supply to see if there is any short circuit or large current. If you are sure that there is no problem, then fix the line connection. For example, if the positive and negative poles of the power cord are welded backwards, or the DC power supply voltage is adjusted too high, the problem can be found in half a second. In both cases, once the short circuit or high voltage time exceeds 1 second, the chip is likely to be burned. 5. Cables and circuit boards. When fixing the circuit board with adhesive tape for debugging, there will be more wires soldered or Plugged in: power cord, serial port cable, USB cable, JTAG cable, peripheral cable, etc. These lines, and the connection point of the circuit board, are often just a test point. Once the force is stressed, the test point is torn off and the board is scrapped. Therefore, it is necessary to fix the circuit board with adhesive tape. You can stick the circuit board and cable on the table, or you can find a piece of foam or cardboard. This avoids the direct stress of the solder joints and is almost impossible to be torn off. I believe that every hardware engineer has encountered such situations as 'turning the circuit board and falling to the ground' and 'hanging a few wires when standing up. 6. The exposed part of the circuit board is protected by insulating glue. When the circuit board is debugged on the bare board, there is no protection of the shell, and many test lines are welded around, which is easy to short circuit! Therefore, the components of the circuit board should be pasted with insulating adhesive or masking tape so that they will not be touched or short-circuited. Don't rush to power on the newly welded circuit board. Check it before power on and make sure there is no problem before power on. Here are some steps I need to do to debug the new circuit board myself. I. Visual inspection before power-on 1. Visual inspection of whether there are welding slag and tin balls on the circuit board. It is better to use a magnifying glass to observe. The presence of welding slag and tin balls may cause short circuit of some pins, resulting in abnormal chip function. 2. Observe whether there is a virtual welding problem in the welding of components. It is better to use a magnifying glass. 3. Check whether directional components are welded correctly against the welding drawing. Some components may cover the direction mark of component packaging after welding, so the direction should be determined against the welding drawing. 2. Check whether there is a short circuit between the power supply terminal and the ground before power-on. 1. Use the beep file to measure whether the power input and output terminals are conductive to the ground (It is best to reserve test points during circuit design) If the beep is called, it means that there is a short circuit between the power supply and the ground. The short circuit between the power supply and the ground is a very serious problem. If the circuit board with short circuit is powered on, the circuit may burn. 2. In the circuit design, a 0 ohm resistor can be used to separate the power supply from the rear stage circuit. Do not weld the 0 ohm resistor before power-on, measure whether there is short circuit to ground in the power supply part and the back-stage circuit power supply respectively. If there is short circuit, it can also be determined whether the power supply is short circuit or the back-stage circuit is short circuit. 3. Observation after power-on 1. I believe that the power supply for debugging circuit boards is more DC power supply. When we use DC power supply, if we power on, it is found that the current output by the DC power supply is much larger than the normal working current of the circuit. At this time, the circuit must have a short circuit phenomenon. The power supply should be turned off immediately and powered on after the fault is eliminated. 2. Do not rush to measure electrical indexes such as voltage and current after power-on. First observe whether the circuit has any abnormal phenomena, such as whether there is smoke, whether there is abnormal smell, whether the integrated circuit package is touched by hand, whether it is hot, etc. If there is an abnormal phenomenon, turn off the power supply immediately and turn it on after troubleshooting. It is added here that the temperature of the general power tube and the power chip may also reach 60 ℃ under normal operation, but this is not an abnormal scalding phenomenon, and the hand touch chip can last for more than 3 seconds, explain that the temperature is below 60 °c. 3. If there is no such abnormal situation, the electrical indexes of each part can be measured after power-on. If they are all normal, the circuit function can be debugged.
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