The content of PCB incoming inspection includes PCB size and appearance inspection, PCB warpage and twist inspection, PCB solderability test, PCB solder mask integrality test, PCB internal defect inspection.
PCB size and appearance inspection
PCB size inspection mainly includes: diameter inspection of processing holes, spacing and tolerance inspection, and PCB profile size inspection.
The appearance inspection mainly includes: inspecting the alignment of the solder mask and the pad; whether the solder mask has abnormal conditions such as impurities, peeling, wrinkles, etc; whether the fiducial marks conforms to the standard; whether the circuit conductor width (line width) and spacing meet the requirements; whether the multilayer board has delaminate issue, etc. In practical applications, special equipment for PCB appearance testing is often used for inspection.
Typical equipment is mainly composed of computers, automatic workbenches, and image processing systems. This system can inspect the inner and outer layers of multilayer boards, single-sided/double-sided boards, and base photoimaging films; it can detect problems such as scratches, pinholes, conductor widths, conductor spacing, rough edges, and large-area defects etc.
PCB warpage inspection
Unreasonable design and improper processing in the processing process may cause PCB warpage. The test method is stipulated in the IPC-TM-650 standard. The test principle is: Expose the PCB under testing to a representative thermal environment of the assembly process, and conduct a thermal stress test on it. Typical thermal stress testing methods solder dipping test and 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 testing. The method of manual measuring PCB warpage is to put the 3 corners of the PCB close to the desktop, and then measure the distance from the fourth corner to the desktop. This method can only be used for rough estimation, and more effective methods include the application of ripple imaging technology.
PCB solderability test
The solderability test of PCB focuses on the test of pads and plated through holes. The IPC-S-804 standard specifies the solderability test method of PCB, which includes edge dipping test, spin dipping test, wave dipping test and solder bead test, etc. The edge dipping test is used to test the solderability of surface conductors; the spin dipping test and wave dipping test are used for the solderability test of surface conductors and plated through holes; the solder bead test is only used for the solderability test of plated through holes.
PCB solder mask integrality test
PCBs used in SMT, dry film solder mask and optical imaging solder mask are generally used. These two kinds of solder mask have high resolution and immobility. The dry film solder mask is laminated on the PCB under the effect of pressure and heat. It requires a clean PCB surface and an effective lamination process.
This kind of solder mask has poor viscosity on the surface of tin-lead alloy. Under the impact of thermal stress generated by reflow soldering, the phenomenon of peeling and breaking from the PCB surface often occurs. This kind of solder mask is also relatively fragile, and microcracks may occur under the influence of heat and mechanical force during HASL process. In addition, physical and chemical damage may also occur under the effect of cleaning agents. In order to discovery these potential defects of the dry film solder mask, strict thermal stress tests are supposed to be performed on the PCB during incoming material inspection. This inspection mostly uses the solder float test, the time is about 10-15s, and the solder temperature is about 260-288℃. When the solder mask peeling phenomenon is not observed during the test, the PCB test piece can be immersed in water after the test, and the capillary effect of the water between the solder mask and the PCB surface can be used to observe the solder mask peeling phenomenon. The PCB test sample can also be immersed in the SMA cleaning solvent after the test to observe whether it has physical and chemical action with the solvent.
PCB internal defect inspection
The internal defects of PCBs are generally inspected by microsection technology. PCBs are inspected for microsection after solder floating thermal stress test. The main inspection items are the thickness of copper and tin-lead alloy coatings, the alignment of the internal conductors of the multilayer board, laminate voids and copper cracks, etc.
In summary, incoming material inspection occupies an overwhelming position in the PCB assembly company's product quality control, so the quality control of incoming materials must be raised to a strategic position to deal with.