The welding defects of circuit boards are caused by the following three factors:

1. The weldability of circuit board holes affects the welding quality

Poor weldability of circuit board holes will produce virtual welding defects, affect the parameters of components in the circuit, lead to the instability of multilayer components and inner wire conduction, and cause the functional failure of the entire circuit. The so-called welderability is the property of the metal surface being wetted by molten solder, that is, the metal surface where the solder forms a layer of relatively uniform continuous smooth adhesion film.

The main factors affecting the weldability of printed circuit board are:

(1) the composition of the solder and the nature of the solder. Solder is an important part of the chemical process of welding. It is composed of chemical materials containing flux. The common low melting point eutectic metal is Sn-Pb or Sn-Pb-Ag. The content of impurities should be controlled by a certain proportion to prevent the oxides produced by impurities from being dissolved by flux. The function of flux is to help solder wet the circuit surface of the plate by transferring heat and removing rust. White rosin and isopropyl alcohol solvents are commonly used.

(2) Welding temperature and metal surface cleanliness will also affect the weldability. The temperature is too high, the solder diffusion speed is accelerated, at this time has a high activity, will make the circuit board and solder melt surface rapid oxidation, welding defects, circuit board surface pollution will also affect the weldability resulting in defects, these defects include tin bead, tin ball, open circuit, glossiness is bad.

2. Welding defects caused by warping

Circuit boards and components in the process of welding produce warping, due to stress deformation and produce defects such as welding, short circuit. Warping is often caused by an imbalance in temperature between the upper and lower parts of the board. For large PCBS, warping will occur when the board falls under its own weight. A common PBGA device is about 0.5mm away from the PCB. If the device on the PCB is large, the solder joint will remain under stress for a long time as the PCB is cooled and returns to its normal shape. If the device is raised 0.1mm, it will be enough to cause a virtual weld to open.

3. The design of circuit board affects the welding quality

In the layout, the size of the circuit board is too large, although welding is easier to control, but the printing line is long, the impedance increases, the anti-noise ability decreases, the cost increases; After hours, the heat dissipation decreases, welding is not easy to control, easy to appear adjacent lines interference with each other, such as the electromagnetic interference of the circuit board. Therefore, PCB board design must be optimized:

(1) Shorten the connection between high-frequency components and reduce EMI interference.

(2) Large weight (such as more than 20g) components, should be fixed with a support, and then welded.

(3) Heat dissipation of the heating element should be considered to prevent defects and rework caused by large ΔT on the surface of the element. The thermal sensitive element should be far away from the heating source.

(4) the arrangement of components as parallel as possible, so not only beautiful and easy to weld, suitable for mass production. Circuit board design is the best 4∶3 rectangle. Do not mutate the wire width to avoid discontinuity in the wiring. When the circuit board is heated for a long time, copper foil is easy to expand and fall off. Therefore, large area copper foil should be avoided.