If the weldability of PCB board is not good, there will be false welding defect, which will affect the parameters of components in the circuit, and lead to the instability of multi-layer board components and inner layer wire conduction, resulting in the function failure of the whole circuit. The so-called weldability is the property that the metal surface is wetted by the molten solder, that is, the metal surface where the solder is located forms a relatively uniform adhesion film which shall be continuous and smooth. The main factors that affect the weldability of PCB board are: (1) the composition of solder and the properties of soldered material. Solder is an important part in the process of welding chemical treatment, which consists of chemical materials containing flux. The commonly used eutectic metals with low melting point are Sn-Pb or Sn-Pb-Ag. The impurity content in it should be controlled in a certain proportion to prevent the oxide produced by impurity from being dissolved by flux. The function of flux is to help the solder wet the circuit surface of soldered board by transferring heat and removing rust. White rosin and isopropanol solvent are generally used. (2) The weldability is also affected by the welding temperature and the cleanliness of the metal board surface. If the temperature is too high, the diffusion speed of the solder will be accelerated. At this time, it has high activity, which will make the circuit board and the molten surface of the solder oxidize rapidly and thus cause welding defects. If the surface of the circuit board is polluted, the weldability will also be affected and thus cause defects. These defects include solder bead, solder ball, open circuit, poor glossiness, etc.
During the welding process, PCB board and components warped, resulting in defects such as false welding and short circuit due to stress deformation. Warpage is often caused by the temperature imbalance of the upper and lower parts of the circuit board. For large PCB board, warpage will also occur due to the falling of the board itself because of the weight. Ordinary PBGA device is about 0.5mm away from PCB board. If the device on the PCB board is large, the welding spot will be under stress for a long time as the PCB board returns to normal shape after cooling down. If the device is raised by 0.1mm, it will cause open circuit of faulty welding.
In view of the layout, if the size of the PCB board is too large, the printing line will be long, the impedance will be increased, the noise resistance will be reduced, and the cost will be increased though the welding process is easier to control; otherwise, the heat dissipation is impaired, the welding process is not easy to control, and adjacent wires tend to interfere with each other, such as electromagnetic interference of circuit board. Therefore, it is necessary to optimize the PCB design: (1) shorten the connection between high frequency components and reduce EMI interference. (2) Heavy (if more than 20g) components shall be fixed with brackets and then welded. (3) The problem of heat dissipation shall be considered for the heating element to prevent defects and rework because of large Δ t on the surface of the element, and the temperature-sensitive element shall be far away from the heating source. (4) The arrangement of components should be parallel as much as possible, so it is not only beautiful but also easy to weld, which is suitable for mass production. The 4:3 rectangle is the best design for the circuit board. Do not change the width of the wire suddenly to avoid the discontinuity of the wiring. When the circuit board is heated for a long time, the copper foil is easy to expand and fall off. Therefore, large amount of copper foil should be avoided.