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PCBA processing has different service methods. There are OEM materials and processing materials. We need to do PCB incoming inspection for customer-supplied PCBs. Next, I will introduce how to detect PCB incoming materials.
1. PCB size and appearance inspection
The content of PCB size inspection mainly includes the diameter, spacing and tolerance of the processing hole, and the small edge size of the PCB. Appearance defect detection mainly includes solder mask and pad alignment, whether the solder mask has abnormalities such as impurities, peeling and wrinkles, whether the reference mark is qualified, whether the circuit conductor width (line width) and spacing meet the requirements, and more Whether the laminate has any remaining layers, etc. In practical applications, special equipment for PCB appearance testing is often used for testing.
Typical equipment is mainly composed of computers, automatic workbench image processing systems and other parts. This system can detect the inner and outer layers of the multilayer board, single/dual panels, and base map films, and can detect broken lines, overlapping lines, scratches, pinholes, line widths, rough edges, and large Area defects, etc.
2. PCB warpage and distortion detection
Unreasonable design and improper processing of the process may cause the warpage and bending of the PCB. The test method is stipulated in standards such as IPC-TM650.
The principle of the test is basically: Expose the tested PCB to a representative thermal environment of the assembly process, and conduct a thermal stress test on it.
Typical thermal stress test methods are spin dipping test and solder float test. In this test method, the PCB is immersed in molten solder for a certain period of time, and then taken out for warpage and distortion testing.
The method of manually measuring PCB warpage is to close the three corners of the PCB to the desktop, and then measure the distance between the fourth corner and the desktop. This method can only be used for rough estimation, and more effective methods include the ripple camera method.
The corrugated image method is: place a piece of light with 100 lines per inch on the PCB to be tested, and set up a standard light source at an angle of 45” above to pass through the light shed to the PCB. The light produces the light shed image on the PCB, and then a CCD is used. The camera observes the light studio image directly above the PCB (0”).
At this time, the collective interference fringes generated between the two light booths can be seen on the entire PCB. This fringe shows the offset in the Z-axis direction. The number of fringes can be counted to calculate the offset height of the PCB, and then pass The calculation is converted into a degree of warpage.
Three, PCB solderability test
The solderability test of PCB focuses on the test of pads and plated through holes. Standards such as IPCS-804 stipulate the solderability test method of PCB, which includes edge dip test, rotary dip test and solder bead test.
The edge dip test is used to test the solderability of surface conductors, the rotation dip test and the wave test are used to test the solderability of surface conductors and electrical through holes, and the solder bead test is only used for solderability testing of electrical through holes.
Four, PCB solder mask integrity test
The PCB used in SMT generally uses dry film solder mask and optical imaging solder mask. These two kinds of solder mask have high fraction and non-fluidity. The dry film solder mask is laminated on the PCB under pressure and heat. It requires a clean PCB surface and an effective lamination process.
This kind of solder mask has poor adhesion on the surface of tin-lead alloy. Under the thermal stress impact of reflow soldering, the phenomenon of peeling and breaking from the PCB surface often occurs. This kind of solder mask is also relatively brittle. Microcracks may occur under the influence of heat and mechanical force during leveling.
In addition, physical and chemical damage may also occur under the action of cleaning agents. In order to prevent these potential defects of dry film solder mask, a strict thermal stress test should be performed on the PCB during incoming material inspection. This detection mostly uses the solder float test, the time is about 10-15, and the solder temperature is about 260-288°C.
When the solder mask peeling phenomenon is not observed during the test, the PCB sample can be immersed in water after the test, and the capillary action of the water between the solder mask and the PCB surface can be used to observe the solder mask peeling phenomenon. The PCB specimen can also be immersed in the SMA cleaning solvent after the test to observe whether it has physical and chemical effects with the solvent.
Five, PCB internal defect detection
The detection of internal defects of PCB generally adopts microsection technology, and the specific detection method is clearly stipulated in related standards such as IPC-TM-650. After the solder float thermal stress test, the PCB is subjected to microsection inspection. The main inspection items include the thickness of the copper and tin-lead alloy coating, the alignment of the internal conductors of the multilayer board, the interlayer voids and the copper cracks.
