Causes of welding defects on PCB boards

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Causes of welding defects on PCB boards [Copy link]

Causes of welding defects on PCB boards

PCB is an indispensable component of modern electronics and is the carrier of electrical connection of electronic components. With the continuous development of electronic technology, the density of PCB is getting higher and higher, and the requirements for welding process are also increasing. Therefore, it is necessary to analyze and judge the factors affecting the quality of PCB welding and find out the causes of its welding defects, so that targeted improvements can be made to improve the overall quality of PCB boards. Let's take a look at the causes of welding defects on PCB boards! 1. The solderability of circuit board holes affects the welding quality If the solderability of circuit board holes is not good, it will cause cold soldering defects, affect the parameters of components in the circuit, cause unstable conduction of multilayer board components and inner layer lines, and cause the entire circuit function to fail. The factors that affect the solderability of printed circuit boards are mainly: (1) The composition of the solder and the properties of the solder. Solder is an important component of the welding chemical treatment process. It is composed of chemical materials containing flux. The commonly used low-melting point eutectic metals are Sn-Pb or Sn-Pb-Ag. The impurity content must be controlled to a certain ratio to prevent the oxides produced by the impurities from being dissolved by the flux. The function of the flux is to help the solder wet the surface of the circuit board to be welded by transferring heat and removing rust. White rosin and isopropyl alcohol solvents are generally used. (2) The welding temperature and the cleanliness of the metal plate surface will also affect the solderability. If the temperature is too high, the solder diffusion speed will accelerate. At this time, it has a high activity, which will cause the circuit board and the solder melting surface to oxidize rapidly, resulting in welding defects. The contamination of the circuit board surface will also affect the solderability and cause defects. These defects include tin beads, tin balls, open circuits, poor gloss, etc. 2. Welding defects caused by warping Circuit boards and components will warp during the welding process, and defects such as cold solder joints and short circuits will occur due to stress deformation. Warping is often caused by temperature imbalance between the upper and lower parts of the circuit board. For large PCBs, warping will also occur due to the weight of the board itself. Ordinary PBGA devices are about 0.5mm away from the printed circuit board. If the device on the circuit board is large, as the circuit board returns to its normal shape after cooling, the solder joints will be under stress for a long time. If the device is raised by 0.1mm, it is enough to cause a cold solder joint and an open circuit. 3. The design of the circuit board affects the welding quality In terms of layout, when the size of the circuit board is too large, although welding is easier to control, the printed lines are long, the impedance increases, the anti-noise ability decreases, and the cost increases; if it is too small, the heat dissipation decreases, the welding is difficult to control, and adjacent lines are prone to mutual interference, such as electromagnetic interference of the circuit board. Therefore, the PCB board design must be optimized: (1) Shorten the connection between high-frequency components and reduce EMI interference. (2) Heavy components (such as those exceeding 20g) should be fixed with a bracket and then welded. (3) Heat dissipation issues should be considered for heating components to prevent defects and rework caused by large ΔT on the component surface. Thermistors should be kept away from heat sources. (4) The components should be arranged as parallel as possible, which is not only beautiful but also easy to solder and suitable for mass production. The best circuit board design is a 4:3 rectangle. The wire width should not change suddenly to avoid wiring discontinuity. When the circuit board is heated for a long time, the copper foil is prone to expansion and falling off. Therefore, large-area copper foil should be avoided. In summary, in order to ensure the overall quality of the PCB board, during the production process, it is necessary to use excellent solder, improve the solderability of the PCB board, and prevent warping and defects.