What are the inspection categories of PCB?

1. PCB dimension and appearance inspection

The content of PCB dimension inspection mainly includes the direct machining hole, spacing and tolerance, PCB edge dimension, etc. The content of appearance defect detection mainly includes: welding resistance film and welding pad alignment; Whether there are impurities, peeling, wrinkling and other abnormal conditions of welding resistance film; Whether the benchmark mark is qualified; Circuit conductor width (line width) and spacing meet the requirements; Whether or not the multilayer strip, etc. In practical application, special equipment for PCB appearance test is often used to detect it. Typical equipment is mainly composed of computer, automatic workbench, image processing system and so on. The system can detect the inner and outer layers of the multilayer board, single/double panels, and the bottom drawing film. Can detect broken wire, wiring, scratch, pinhole, line width, line distance, edge rough and large areas of defects.

Improper design and improper handling of the process may cause warping and distortion of the PCB. The test method is to expose the tested PCB to the representative thermal environment of the assembly process and conduct a thermal stress test. Typical thermal stress testing methods are the rotary dip test and solder float test, in which the PCB is impregnated in the molten solder for a certain period of time and then removed for warping and twisting detection. PCB circuit defect AOI detection device and detection principle are shown in the figure.

The method of manually measuring PCB is: press the three corners of the PCB against the table top, and then measure the distance from the fourth corner to the table top. This method can only be used for rough estimation. More effective methods include the application of ripple imaging technology. The method of corrugated image is: place a grating of 100 lines in inch on the tested PCB, and set a standard light source above to shoot the grating onto the PCB at an incidence Angle of 45℃. The grating generates a grating image on the PCB, and then observe the grating image directly above the PCB with a CCD camera. At this point, the geometric interference fringes generated between the two gratings can be seen on the entire PCB. The fringes show the offset in the Z direction, and the number of fringes can be counted to calculate the offset height of the PCB, which is then converted to warp by calculation.

2. Test the solderability of PCB

PCB weldability test focuses on the test of pad and electroplate through hole. Ipc-s-804 and other standards provide PCB weldability test method, which includes edge impregnation test, rotary impregnation test, wave crest impregnation test and solder bead test.

3. PCB resistance welding film integrity test

The dry film solder mask and optical imaging solder mask are generally used on PCB, which have high resolution and illiquidity. Dry film resistance welding is pressed on the PCB under pressure and heat. It requires clean PCB surface and effective laminating process. The surface viscosity of the solder film is poor, and under the impact of the thermal stress caused by reflow welding, the surface of PCB is often peeled and fractured. This welding resistance film is also relatively brittle, the normal temperature and mechanical force may produce micro-cracks, in addition, in the action of cleaning agents may also produce physical and chemical damage. In order to find out the potential defects of dry film resistance welding film, strict thermal stress test should be carried out on PCB in incoming material inspection. When the phenomenon of welding resistance film stripping cannot be observed during the test, PCB specimen can be immersed in water after the test and observe the phenomenon of welding resistance film stripping by capillary action of water between the welding resistance film and the surface of PCB. PCB specimens can also be immersed in SMA cleaner after test to observe whether there are physical and chemical effects with the solvent.

4. PCB internal defect detection

The internal defects of PCB are generally detected by microsection technology, whose specific detection method is specified in relevant standards such as ipc-tm-650. The main items of microsection detection include the thickness of copper and tin-lead alloy coatings, alignment between conductor layers in multilayer plates, laminate gap and copper cracks, etc.