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Factors influencing welding defects of SMT circuit board

Article source:InformationEditor:Dongguan Taijie Electronic Technology Co., Ltd Issuing time:2021-09-28

There are three reasons for the welding defects of circuit board:
1. The solderability of circuit board holes affects the welding quality
Poor solderability of circuit board holes will lead to wrong circuit board welding defects, which will affect the parameters of components in the circuit, resulting in unstable conductivity of multilayer board components and internal wires, resulting in failure of the whole circuit. The so-called solderability is the property that the metal surface is wetted by molten solder, that is, a relatively uniform continuous and smooth film is formed on the metal surface where the solder is located.
The main factors affecting the solderability of printed circuit boards are:
(1) Composition and properties of solder. Solder is an important part of welding chemical treatment process. It consists of chemical materials containing flux. The commonly used low melting point and low eutectic metals are Sn Pb or Sn Pb AG. The impurity content shall be controlled according to a certain proportion to prevent the oxide produced by impurities from being dissolved by the flux. The function of flux is to help solder wet the circuit surface of solder board by transferring heat and removing rust. White rosin and isopropanol solvents are usually used.
(2) The welding temperature and the cleanliness of the metal plate surface will also affect the weldability. If the temperature is too high, the solder diffusion rate will increase. At this time, it will have high activity, which will lead to rapid oxidation of the molten surface of the circuit board and solder, resulting in welding defects. Contamination of the circuit board surface can also affect solderability and lead to defects. These defects include tin beads, tin balls, open circuit, poor gloss, etc.
2. Welding defects of circuit board caused by warpage
Circuit boards and components will warp during welding, as well as defects such as false welding and short circuit caused by stress deformation. Warpage is usually caused by the temperature imbalance between the upper and lower parts of the circuit board. As the weight of the PCB board decreases, the large PCB board will also warp. Ordinary PBGA devices are about 0.5mm away from the printed circuit board. If the equipment on the circuit board is large, as the circuit board cools, the solder joint will be under stress for a long time, and the solder joint will be under stress, resulting in an open weld.
3. The design of circuit board affects the welding quality
In the layout, when the size of the circuit board is too large, although it is easy to control welding, the printed circuit is longer, the impedance increases, the anti noise ability decreases and the cost increases; If it is too small, the heat dissipation will be reduced, the welding is difficult to control, and adjacent lines are prone to mutual interference, such as electromagnetic interference of circuit boards. Therefore, the PCB design must be optimized:
(1) Shorten the connection between high-frequency components and reduce EMI interference.
(2) Components with large weight (e.g. more than 20g) shall be fixed with supports and then welded.
(3) The heat dissipation of heating elements shall be considered to prevent large on the surface of elements Δ T in case of defects and rework, the thermal element shall be far away from the heat source.
(4) The arrangement of components shall be as parallel as possible, which is not only beautiful, but also easy to weld, and is suitable for mass production. The circuit board shall be designed as a 4:3 rectangle. Do not change the lineweight to avoid discontinuous wiring. When the circuit board is heated for a long time, the copper foil is easy to expand and fall off. Therefore, avoid using large-area copper foil.