1. After receiving the faulty PCB, it’s crucial to follow a clear sequence of steps, knowing exactly what to do first and what to do next. It’s important not to rush into the repair process, dismantling the board haphazardly, as this could lead to further damage and prevent a successful repair. Avoid performing unnecessary tasks, as this would only waste time and energy without resolving the issue.
2. The first step is to gather as much information as possible about the fault, as reported by the PCB user. Ask detailed questions about when the fault occurred and the conditions surrounding it. Ideally, you should learn about the issue directly from the on-site “witness” who observed the failure. Find out whether the fault occurred suddenly or gradually, if it happens under specific conditions, and whether there are any alarm signals associated with the failure. This information is vital for a targeted approach to troubleshooting. Keep in mind that users are often in different roles and may not always provide accurate descriptions of the fault. As a maintenance technician, it’s important to differentiate between useful and irrelevant information.
3. The next step is the “kanban,” or visual inspection of the circuit board. Avoid relying on improper inspection techniques. A wealth of practical experience tells us that many circuit board failures are visible to the eye! The faults you can see are often the simplest, most direct, and most obvious. By comparing the visible issues with the reported symptoms, you can gain a clearer understanding of the problem. Maintenance, like diagnosing a disease, involves “watching, smelling, and asking.” Therefore, the “kanban” inspection should be treated as a critical part of the diagnostic process.
**What to look for in a “Kanban”?**
1: Inspect the PCB components and traces for any signs of damage. Check if any fuses have blown, if any components are visibly burnt or scorched, if component leads are rusted, if connectors are poorly connected, or if capacitors, batteries, and similar components show any signs of leakage. Look for corrosion damage to the copper traces, mechanical damage to components or circuits, and open SMD pins. To get a clearer view, use a magnifying glass and proper lighting to ensure you don’t miss any potential issues. Different components have varying probabilities of failure. For example, aluminum electrolytic capacitors tend to age and degrade over time. The likelihood of damage also depends on the operational environment and the age of the components. Keep these factors in mind when inspecting the board.
2: Determine whether the “Kanban” has been previously repaired. Many boards that are repaired are not original, and if they have undergone repairs by others, the situation can vary. If the repairs were performed by skilled technicians, many potential failure points may have been addressed, but you may face difficulties if additional failures arise. However, if the repairs were done poorly or by unqualified individuals, the initial failure might still exist, or even new issues could have been introduced. For instance, improper soldering, poor connections, or incorrect handling can result in further damage. Pay close attention to signs of previous repairs when examining the board and look for any secondary faults caused by improper soldering, such as reversed soldering or incomplete joints.
3: Consider the age of the “Kanban.” Most ICs on the board will have a production date marked on them. For example, “9622” indicates that the product was made in the 22nd week of 1996. Equipment manufacturers typically order ICs directly from manufacturers, so the production date of the ICs may be slightly ahead of the equipment’s assembly date. By checking the IC date codes, you can estimate when the device was likely used, which will help identify potential failure types. If the circuit board has been in service for more than seven or eight years, the components may have deteriorated due to aging. For boards used within three to four years, the failure is more likely due to poor environmental conditions, operator errors, or random malfunctions.
“Kanban” inspection involves more than just visually inspecting the board. While observing, one must also analyze and think critically. It’s important to note that most circuit boards we work with lack schematics. Experienced technicians can “visualize” the circuit—by simply looking at the board, they can deduce its design, its component layout, and the overall functionality. With this mental map, a repair strategy can be formed quickly and accurately. For example, when repairing a switching power supply, you would expect to find a PWM controller, a switching transistor, and a transformer. In a microcontroller circuit, you’d be looking for components like crystal oscillators, reset circuits, power supply filters, and program memory. Similarly, when dealing with an analog circuit, expect to see passive components like resistors and capacitors around the op-amp ICs.
By mentally mapping the layout of components and their functions, you create a mental schematic that guides troubleshooting. If no obvious faults are found during this visual inspection, further testing should be done using tools like multimeters, oscilloscopes, and in-circuit testers. For powered and unpowered conditions, multimeters can be used to check continuity, diodes, and resistance, while in-circuit testers can be used to measure component characteristics like the VI curve. Based on user-reported symptoms, focus your testing on the likely fault areas. Some components can’t be fully evaluated without removing them from the board, so disassembly may be necessary. However, always try to test components while they are still on the board to avoid secondary damage, particularly for components with fine-pitch leads that can be difficult to rework.
The power supply is often the root cause of many failures. To ensure proper operation of the PCB, the power supply must be in good condition. This includes checking parameters like voltage levels, current capacity, and ripple. Power-on testing is the most straightforward way to verify this. You should have a 250W transformer available to isolate and convert 220V AC into various common voltages, such as 12V, 24V, 110V, or 380V, depending on the board’s requirements. A three-channel adjustable DC power supply (with at least 3A output) can be used to power the DC side of the board, simulating normal operating conditions. After powering the board, some faults may become immediately apparent, while others may require the load and peripheral circuits to be connected to fully observe. Whenever possible, simulate the external load and peripherals. Pay attention to indicator status, alarm messages, and compare these with any available documentation to narrow down the scope of your investigation.
Many PCB failures are interrelated, meaning that a failure in one component can lead to failures in others. For example, a short circuit in one component may cause excessive current to flow through adjacent components, leading to further damage. Similarly, a component exposed to excessive voltage could cause neighboring components to fail as well. Thorough investigation is essential when dealing with such issues.
Finally, be meticulous during the post-repair process. Ensure that components with specific orientation requirements are installed correctly, referring to the silkscreen markings or notes made during disassembly. During power-on testing, verify that the correct voltages are applied with proper polarity to avoid reversing the repair. Taking these steps will help avoid costly mistakes and ensure the repair is completed successfully.
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