1. Needle bed test method
This method is connected to each detection point on the circuit board by a probe with a spring. The spring makes each probe have a pressure of 100-200g to ensure that each detection point is in good contact. Such probes arranged together are called "needle beds". Under the control of detection software, detection points and detection signals can be programmed. In fact, only the probes of the test points that need to be tested are installed. Although the needle bed test method may be used to detect both sides of the circuit board at the same time, when designing the circuit board, all the detection points should be on the welding surface of the circuit board. The equipment of needle bed tester is expensive and difficult to maintain. The probes are arranged in different ways according to their specific applications.
A basic general-purpose grid processor is composed of a drilled board with the center spacing of pins of 100, 75 or 50mil. The pin acts as a probe and uses the electrical connector or node on the circuit board for direct mechanical connection. If the pad on the circuit board matches the test grid, the polyvinyl acetate film perforated according to the specification will be placed between the grid and the circuit board to facilitate the design of specific detection. Continuity detection is achieved by accessing the end point of the grid (which has been defined as the X-Y coordinates of the pad). Since each network on the circuit board is tested for continuity. In this way, an independent test is completed. However, the proximity of the probe limits the effectiveness of the needle bed test method.
2. Observation of circuit board
The circuit board is small in size and complex in structure, so professional observation instruments must also be used to observe the circuit board. Generally, we use a portable video microscope to observe the structure of the circuit board. Through the video microscope camera, we can clearly see the very intuitive microstructure of the circuit board from the microscope. In this way, it is easier for us to design and test circuit boards. The portable video microscopes msa200 and vt101 used in the factory are more convenient than traditional microscopes because they can realize "observation, detection and discussion at any time"!
3. Double probe flying needle test method
The flying pin tester does not depend on the pin pattern installed on the fixture or bracket. Based on this system, two or more probes are installed on the micro magnetic head that can move freely on the X-Y plane, and the test point is directly controlled by CADI Gerber data. The double probes can move within the range of 4mil from each other. The probes can move independently, and there is no real limit to how close they are to each other. The tester with two movable arms is based on the measurement of capacitance. Place the circuit board tightly on the insulating layer on a metal plate as another metal plate of the capacitor. If there is a short circuit between lines, the capacitance will be greater than at a certain point. If there is an open circuit, the capacitance will decrease.
Test speed is an important criterion for selecting testers. The needle bed tester can accurately test thousands of test points at a time, while the flying needle tester can only test two or four test points of the circuit board at a time. In addition, it may take only 20-30 seconds for the needle bed tester to conduct one-sided test, which depends on the complexity of the board, while the flying needle tester requires IH or more time to complete the same evaluation. Shipley (1991) explained that even though manufacturers of high-yield printed circuit boards think that mobile flying pin testing technology is slow, this method is still a good choice for manufacturers of complex circuit boards with low production.
For bare board testing, there are special testing instruments. A more cost optimized method is to use a general-purpose instrument. Although this kind of instrument is initially more expensive than a special instrument, its initial high cost will be offset by the reduction of individual configuration costs. For general purpose grids, the standard grid for boards with pin components and surface mount devices is 2.5mm. At this time, the test pad should be greater than or equal to 1.3mm. For the grid of IMM, the test pad is designed to be greater than 0.7mm. If the grid is small, the test needle is small and brittle and easy to be damaged. Therefore, it is best to choose grids larger than 2.5mm. The combination of universal tester (standard grid tester) and flying pin tester can make the detection of high-density circuit boards accurate and economical. Another method he suggested is to use a conductive rubber tester, which can be used to detect points that deviate from the grid. However, the height of the pad treated by hot air leveling is different, which will hinder the connection of the test points.
How to quickly detect the fault of circuit board
1. Look at the status of components
Get a faulty circuit board, and first observe whether there is obvious component damage on the circuit board, such as the burning and bulging of electrolytic capacitors, the burning of resistors, and the burning of power devices.
2. See the welding of circuit board
If the printed circuit board is deformed or warped; Whether there are solder joints falling off and obvious weak soldering; Whether the copper coating of the circuit board is warped, burnt and blackened.
3. Plug in of observation element
For example, whether the integrated circuit, diode, circuit board power transformer and other directions are inserted incorrectly.
4. Simple test of resistance, capacitance and inductance
Use a multimeter to simply test the resistance, capacitance, inductance and other suspicious components in the range, and test whether there are phenomena such as larger resistance value, short circuit of capacitance, open circuit and change of capacitance value, short circuit and open circuit of inductance.
5. Power on test
After the above simple observation and test, the fault cannot be eliminated, and the power on test can be carried out. First, test whether the power supply of the circuit board is normal. Such as whether the AC power supply of the circuit board is abnormal, whether the output of the voltage regulator is abnormal, whether the output and waveform of the switching power supply are abnormal, etc
6. Brush program
For programmable components such as single chip microcomputer, DSP, CPLD, etc., we can consider brushing the program again to eliminate the circuit fault caused by abnormal program operation.
7. Repair according to function module
If the circuit board cannot be repaired according to the above steps, it is necessary to determine the circuit module that may have problems according to the circuit fault, and carry out further maintenance according to the design drawings.