In order to ensure the production quality of PCB, electronic circuit board manufacturers have a variety of inspection methods during the production process, and each inspection method will target different PCB board defects.
Electrical testing usually measures the impedance characteristics between test points to detect all continuity (ie, open and short). Visual testing finds defects by visually inspecting the characteristics of electronic components and the characteristics of printed wiring. The electrical test is more accurate when looking for short-circuit or open-circuit defects. The visual test can more easily detect the problem of incorrect gaps between conductors, and the visual inspection is generally performed in the early stages of the production process to find defects and rework to ensure the highest product qualification rate.
7 Methods For PCB Board Inspection
1. Manual visual inspection of PCB board
Using a magnifying glass or a calibrated microscope, the visual inspection of the operator to determine the failure of the circuit board, and to determine when a correction operation is required, is the most traditional detection method. Its main advantages are low up-front cost and no test fixture. Its main disadvantages are human subjective errors, high long-term costs, discontinuous defect detection, and difficult data collection. At present, due to the increase in the PCB production and the reduction of the lead pitch and component volume on PCBs, this method has become increasingly unfeasible.
2. PCB board online test
The electrical performance is checked to find manufacturing defects and test analog, digital, and mixed-signal components to ensure that they meet specifications. There are several test methods such as needle bed testers and flying probe testers. The main advantages are low test cost of each board, strong digital and functional test capabilities, fast and thorough short and open test, programming firmware, high defect coverage, and easy programming. The main disadvantages are the need for test fixtures, programming and debugging time, the high cost of making fixtures, and the difficulty of using them.
3. PCB board function test
The functional system test is a comprehensive test of the functional modules of the circuit board at the middle stage and the end of the production line using special test equipment to confirm the quality of the circuit board. Functional testing can be said to be the earliest automatic test principle. It is based on a specific board or a specific unit and can be completed with various equipment. There are types of final product testing, the latest solid model, and stacked testing. Functional testing usually does not provide in-depth data such as foot-level and component-level diagnostics for process improvement, and requires specialized equipment and a specially designed test process. Writing functional test programs is complex.
4. Automatic optical inspection
It is also called automatic visual inspection. It is a newer method for confirming manufacturing defects based on the optical principle, using a combination of image analysis, computer, and automatic control technologies to detect and deal with defects encountered in production. AOI is usually used before and after reflow and before electrical testing to improve the qualification rate during electrical processing or functional testing. At this time, the cost of correcting defects is much lower than the cost of the final test, often reaching a dozen times.
5. Auto X-ray inspection
Utilizing the different X-ray absorptivity of different substances, it is necessary to look through the inspected area and find defects. Mainly use to detect defects such as ultra-fine pitch and ultra-high-density circuit boards and bridging, chip loss, and misalignment during the assembly process, can also use the tomography technology to detect IC chips internal defects . It is the only method currently to test the soldering quality of ball grid arrays and shaded solder balls. The main advantage is the ability to inspect BGA soldering quality and embedded components without fixture costs.
6. Laser detection system
It is the latest development in PCB testing technology. It uses a laser beam to scan the printed board, collects all measurement data, and compares the actual measurement value with a preset pass limit value. This technology has been proven on light boards and is being considered for assembly board testing at speeds sufficient for mass production lines. The main advantages are fast output, no fixtures and visual unobstructed access required, high initial costs, and many maintenance and usage issues are its main disadvantages.
7. Size Detection
Use the two-dimensional image measuring instrument to measure the hole position, length, width, position and other dimensions. Since PCB is a small, thin and soft type product, contact-type measurement can easily be deformed to cause inaccurate measurement, and the two-dimensional image measuring instrument has become the best high-precision size measuring instrument. After programing the measuring instrument, it can realize fully automatic measurement, which not only has high measurement accuracy, but also greatly shortens the measurement time and improves the measurement efficiency.
9 Tips for PCB Inspection
When inspecting the PCB Board, pay attention to the following 9 tips to ensure the quality of the printed circuit board product.
DO NOT use a grounded test equipment to contact the live TV, audio, video and other equipment on the bottom plate to detect the PCB without an isolation transformer.
Forbid to directly test the TV, audio, video and other equipment without a power isolation transformer by using grounded instruments and equipment. Although general radio cassette recorders have power transformers, when they come into contact with more special equipment, it is necessary to first ascertain whether the chassis of the unit is charged, otherwise it is very easy to cause short-circuits with power sources. It affects integrated circuits, causing further expansion of faults.
2. When inspecting the PCB, pay attention to the insulation performance of the soldering iron.
Don’t use a soldering iron for welding. To confirm that the soldering iron is not charged, it is better to ground the shell of the soldering iron. Be more careful with MOS circuits, it is safer to use 6 ~ 8V low-voltage circuit iron.
3. Understand the working principle of integrated circuits and related circuits before testing PCB boards
Before inspecting and repairing integrated circuits, you must first be familiar with the functions of the integrated circuits, the internal circuits, the main electrical parameters, the role of each pin, and the normal voltage, waveform, and peripheral components of the pins. If the meet the above conditions, analysis and inspection will be much easier.
4. Don’t cause short circuit between pins when test PCB boards
When measuring voltage or measuring the waveform with an oscilloscope probe, the test leads or probes cannot cause short circuits between the pins of the integrated circuit due to sliding. It is best to measure on the peripheral printed circuit that is directly connected to the pins. Any instantaneous short circuit can easily damage the integrated circuit. Be careful when testing flat-package CMOS integrated circuits.
5. The internal resistance of the test instrument for testing the PCB board should be large
When measuring the integrated circuit pins DC voltage, we should choose a multimeter with an internal resistance greater than 20KΩ / V, otherwise there will be a large measurement error for some pin voltages.
6. Pay attention to the heat dissipation of power integrated circuits when testing PCB boards
The power integrated circuit should dissipate heat well, and can not work under high power without a radiator.
7. Reasonable PCB board lead
you should choose small components to add external components to replace the damaged part of the integrated circuit. And the wiring should be reasonable to avoid unnecessary parasitic coupling, especially the ground between the audio amplifier IC and the preamp circuit end.
8. Check the PCB board to ensure the welding quality
Welding is sure when welding, and the accumulation of solder and pores are likely to cause false soldering. The soldering time is generally less than 3 seconds, the soldering iron power is about 25W.
We should carefully check the soldered integrated circuit. It is best to use an ohmmeter to measure whether has short circuit between the pins, and confirm no solder adhesion, then turn on the power.
9. Do not easily judge the damage of the integrated circuit when testing the PCB
Do not easily judge the damage of integrated circuit, because most integrated circuits are directly coupled. Once a circuit is abnormal, it may cause multiple voltage changes, and integrated circuit damage may not casue these changes. In some cases, when the voltage of each pin is close to the normal value, it may not mean that the integrated circuit is good, some soft faults will not cause the DC voltage to change.