Three factors causing the welding defects of circuit board are detailed

- Oct 17, 2018-

1. Solderability of circuit board holes affects welding quality

The poor solderability of the circuit board hole will lead to the defect of virtual welding, which will affect the parameters of components in the circuit and lead to the instability of the multi-layer board components and inner line, leading to the failure of the whole circuit function.Solderability is the wetting property of the molten solder on the surface of a metal, that is, the solder on the surface of the metal form a relatively uniform continuous smooth adhesive film.The main factors affecting the weldability of PCB include :(1) solder composition and solder properties.Solder is an important part of the welding chemical treatment process. It is composed of chemical materials containing flux. The commonly used low melting point co-melting metal is sn-pb or sn-pb-ag.The function of the flux is to help the solder wet the circuit surface of the solder plate by transferring heat and removing rust.Generally use white rosin and isopropanol solvent.(2) welding temperature and surface cleanliness of metal plates also affect weldability.If the temperature is too high, the solder diffusion speed will be accelerated. At this time, the solder has high activity, which will rapidly oxidize the melting surface of the circuit board and solder, resulting in welding defects. If the surface of the circuit board is polluted, weldability will be affected and defects will be produced.

2. Welding defects caused by warping

In the welding process, the circuit board and components warped, and there were defects such as virtual welding and short circuit due to stress deformation.Warping is usually caused by temperature imbalance in the upper and lower parts of the circuit board.For large PCBS, the board also warps as it falls from its own weight.The common PBGA device is about 0.5mm away from the printed circuit board. If the device on the circuit board is large, the welding spot will be under the required force for a long time as the circuit board cools down and returns to its normal shape.

3. The design of circuit board affects welding quality

In the layout, when the board size is too large, the welding is easier to control, but the printing line is long, the impedance increases, the anti-noise ability decreases, and the cost increases.When the temperature is too low, the heat dissipation is reduced, the welding is not easy to control, and the interference between adjacent lines is easy to occur, such as the electromagnetic interference of the circuit board.Therefore, the design of PCB board must be optimized :(1) shorten the connection between high-frequency components and reduce EMI interference.(2) heavy (if more than 20g) components shall be fixed in brackets and then welded.(3) heating elements should be considered heat dissipation problems, prevent components surface has larger Δ T defects and rework, thermal element should be far away from heat source.(4) the arrangement of components should be as parallel as possible, which is not only beautiful but also easy to weld and suitable for mass production.The circuit board is designed to be a 4-3 rectangle.Do not break wire width to avoid discontinuity of wiring.When the circuit board is heated for a long time, copper foil tends to expand and fall off. Therefore, large areas of copper foil should be avoided.