PCB Technical - Some common causes of PCB circuit board failure

Since the printed circuit board is not a general end product, the definition of the name is a bit confusing. For example, the motherboard of a personal computer is called a motherboard and cannot be called a circuit board directly. Although there is a circuit board on the motherboard, it is not the same, so when you evaluate the industry, you can't say the same thing. For example, because the integrated circuit components are mounted on the circuit board, the news media call it an IC board, but in essence it is not equivalent to a printed circuit board. We usually say that a printed circuit board refers to a bare board-that is, a circuit board without upper components. In the process of PCB board design and circuit board production, engineers not only need to prevent the PCB board from being accidentally encountered during the manufacturing process, but also need to avoid design errors.

1. Circuit board short circuit: For this type of problem, one of the common faults that directly cause the circuit board to work, the biggest reason is that the PCB short circuit is the incorrect design of the pad. At this time, the circular pad can become an oval shape. Shape, increase the distance between points to prevent short circuits. Improper design of the direction of the PCB proofing components will also cause the circuit board to short-circuit and fail to work. If the pin of the SOIC is parallel to the tin wave, it is easy to cause a short circuit accident. In this case, the direction of the part can be modified to be perpendicular to the tin wave. The PCB may also be short-circuited, that is, automatic plug-in bending. Since the IPC stipulates that the length of the wire is less than 2mm, when the bending angle is too large, this part may drop, so it is easy to cause a short circuit, and the welding point needs to be greater than 2mm offline.

2. PCB solder joints become golden yellow: Generally speaking, the solder of PCB circuit board is silver-gray, but occasionally there are golden solder joints. The main reason for this problem is that the temperature is too high, only need to reduce the temperature of the tin furnace.

3. Black and granular contacts on the circuit board: The dark or small particle contacts on the PCB are mainly due to solder contamination and excessive oxides in the tin, which form an overly fragile solder joint structure. Care must be taken not to confuse the dark color and low tin content caused by the use of solder. Another reason for this problem is that the composition of the solder itself used in the manufacturing process has changed, and the impurity content is too high. It is necessary to add pure tin or replace the solder. The tinted glass acts as a physical change in the fiber layer, such as the separation between the layers. However, this situation is not a bad solder joint. The reason is that the substrate heating is too high, and it is necessary to reduce the preheating and soldering temperature or increase the substrate travel speed.

4. PCB components are loose or misaligned: During the reflow soldering process, small parts may float on the molten solder and eventually fall off from the target solder joints. Possible reasons for the displacement or tilt include the vibration or rebound of the components on the soldered PCB due to insufficient circuit board support, reflow oven settings, solder paste problems, and human error.

5. Open circuit board: When the trace is broken or the solder is only on the pad and not on the component lead, an open circuit will occur. In this case, there is no glue or connection between the component and the PCB. Just like short circuits, these can also occur during production or during welding and other operations. Shaking or stretching the circuit board, dropping the circuit board or other mechanical deformation factors can damage the circuit board or solder joints. In addition, chemicals or moisture can cause the solder or metal parts to wear, causing component wires to break.

6. Welding problems: The following are some of the problems caused by poor welding: the solder joints are disturbed: the solder moves due to external interference before solidification. This is similar to a cold solder joint, but for different reasons, it can be corrected by reheating, and the solder joint is cooled without external interference. Cold welding: This happens when the solder cannot melt properly, resulting in rough surfaces and unreliable connections. Cold solder joints can also occur because excessive solder will prevent complete melting. The solution is to reheat the joint and remove the excess solder. Solder bridge: This situation occurs when the solder crosses and physically connects the two leads together. These may cause accidental connections and short circuits, and when the current is too high, it may cause the components to burn out or the wires to burn out. The pads, pins or lead are not wet enough. Too much or too little solder. Lands raised due to overheating or rough soldering.

7. The badness of the PCB board is also affected by the environment: due to the structure of the PCB itself, when it is in an unfavorable environment, it is easy to cause damage to the circuit board. Extreme temperature or temperature changes, other conditions such as high humidity and high-intensity vibration are the factors that lead to reduced circuit board performance or even scrap. For example, changes in ambient temperature can cause circuit boards to deform. This can damage the solder joints, bend the shape of the circuit board, or may also cause the copper wires on the circuit board to break. On the other hand, moisture in the air can cause oxidation, corrosion and rust on metal surfaces, such as exposed copper traces, solder joints, pads and component leads. Dirt, debris or debris that accumulates on the surface of components and circuit boards can also reduce air flow and component cooling, which can lead to PCB overheating and performance degradation. The vibration, drop, impact or bending of the PCB will cause the PCB to deform and cause cracks, while high current or overvoltage can cause damage to the PCB or cause rapid aging of components and paths.

8. Human error: Most defects in PCB manufacturing are caused by human error. In most cases, wrong production processes, incorrect component placement and unprofessional manufacturing specifications result in up to 64% avoidable. A product defect has occurred.