Technical Library | 2024-08-09 06:34:09.0
Quality control in conformal coating applications is vital for ensuring the reliability and longevity of electronic products. Conformal coatings protect printed circuit boards (PCBs) from environmental factors like moisture, dust, and chemicals. To maintain high standards, various inspection methods are employed throughout the coating process. Visual Inspection This is the first line of defense against defects. Inspectors look for uniform coverage, absence of bubbles, and proper curing. Training is essential to ensure that inspectors can identify subtle issues that may affect performance. UV Inspection Most conformal coatings contain a UV tracer, which makes the coating visible under ultraviolet light. UV inspection allows for easy detection of missed areas, ensuring complete coverage. This step is crucial for verifying that the coating has been applied correctly, especially in hard-to-see areas of the PCB. Automated Optical Inspection (AOI) AOI systems offer a more precise and consistent method for inspecting conformal coatings. They use cameras and specialized software to detect defects that might be missed by the human eye. AOI systems can inspect large volumes of PCBs quickly, making them ideal for high-production environments. Thickness Measurement The thickness of the conformal coating is critical for providing adequate protection without affecting the performance of the PCB. Tools such as micrometers, ultrasonic thickness gauges, and eddy current devices are used to measure the coating thickness. Consistent application is key to preventing issues like cracking or insufficient protection. Functional Testing Beyond visual and automated inspections, functional testing is necessary to ensure that the conformal coating does not interfere with the electrical performance of the PCB. This involves subjecting the coated PCB to environmental stress tests, such as thermal cycling, humidity, and salt spray, to assess its reliability in real-world conditions. Process Control and Documentation Implementing strict process controls is essential to maintaining quality. This includes regularly calibrating equipment, training operators, and documenting every step of the process. Proper documentation helps trace issues back to their source and prevents them from recurring. Conclusion Effective quality control in conformal coating applications ensures that PCBs are protected from environmental damage, thereby extending their lifespan and reliability. By employing a combination of visual, UV, and automated inspections, along with thickness measurement and functional testing, manufacturers can achieve the highest standards in coating quality.
Technical Library | 2021-07-20 20:02:29.0
During the manufacturing of printed circuit boards (PCBs) for a Flight Project, it was found that a European manufacturer was building its boards to a European standard that had no requirement for copper wrap on the vias. The amount of copper wrap that was measured on coupons from the panel containing the boards of interest was less than the amount specified in IPC-6012 Rev B, Class 3. To help determine the reliability and usability of the boards, three sets of tests and a simulation were run. The test results, along with results of simulation and destructive physical analysis, are presented in this paper. The first experiment involved subjecting coupons from the panels supplied by the European manufacturer to thermal cycling. After 17 000 cycles, the test was stopped with no failures. A second set of accelerated tests involved comparing the thermal fatigue life of test samples made from FR4 and polyimide with varying amounts of copper wrap. Again, the testing did not reveal any failures. The third test involved using interconnect stress test coupons with through-hole vias and blind vias that were subjected to elevated temperatures to accelerate fatigue failures. While there were failures, as expected, the failures were at barrel cracks. In addition to the experiments, this paper also discusses the results of finite-element analysis using simulation software that was used to model plated-through holes under thermal stress using a steady-state analysis, also showing the main failure mode was barrel cracking. The tests show that although copper wrap was sought as a better alternative to butt joints between barrel plating and copper foil layers, manufacturability remains challenging and attempts to meet the requirements often result in features that reduce the reliability of the boards. Experimental and simulation work discussed in this paper indicate that the standard requirements for copper wrap are not contributing to the overall board reliability, although it should be added that a design with a butt joint is going to be a higher risk than a reduced copper wrap design. The study further shows that procurement requirements for wrap plating thickness from Class 3 to Class 2 would pose little risk to reliability (minimum 5 μm/0.197 mil for all via types).Experimental results corroborated by modeling indicate that the stress maxima are internal to the barrels rather than at the wrap location. In fact, the existence of Cu wrap was determined to have no appreciable effect on reliability.
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