- Dec 28, 2017 -
The three factors that cause the welding defect of circuit board
1. The weldability of circuit board holes affects welding quality
The weldability of circuit board pcb holes is not good,will produce pseudo soldering defects, and will affect the parameters of the components in the circuit, it leads to the instability of the multi-layer board pcb components and inner line conduction, which causes the failure of the whole circuit function.
The so-called weldability is the wetting property of the metal surface by the molten solder. That is, the metal surface of the solder is formed by a relatively uniform, continuous, smooth attached film.
The main factors affecting the weldability of PCB are: (1) The composition of the solder and the properties of the solder. Solder is an important part of the process of welding chemical treatment, it is made up of chemical materials containing flux,commonly used low melting point eutectic metals are Sn-Pb or Sn-Pb-Ag.The impurity content should be controlled by a certain ratio, in case that the oxide produced by the impurity is dissolved by the flux.The function of the flux is through the transfer of heat,removal of corrosion to help solder wetting the surface of the welded board.White rosin and isopropanol are generally used. (2)Weldability is also affected by the welding temperature and the surface cleanliness of the metal board.When the temperature is too high, the diffusion speed of the solder is accelerated,it has high activity at this time, which will quickly oxidize the melting surface of the circuit board and solder, to produce welding defect; Contamination on the surface of a circuit board will also affect weldability to produce defects,these defects include tin sweat, solder ball,open - circuit, poor glossiness and so on.
2, welding defects caused by warpage
During the welding process of the circuit board and components will produce warpage,because of the stress deformation produce defects such as pseudo soldering ,short circuit, etc. Warping is often caused by the temperature imbalance between the upper and lower parts of the circuit board.The big PCB will also be warped because of the weight of the board itself.The common PBGA device is about 0.5mm from the printed circuit board,if the device is larger on the circuit board,return to normal shape as the PCB is cooled,the solder joint will be under stress for a long time,if the device 0.1mm elevation is enough to cause pseudo soldering and open - circuit.
3, The design of the circuit board affects the quality of the welding
In the layout, the size of the circuit board pcb is too large, although the welding is easy to control, but the printing line is long, impedance increased, decrease in anti noise ability and increase in cost; the size of the circuit board pcb is too small, the heat dissipation is reduced, the welding is not easy to control, and the adjacent lines are easy to interfere with each other, such as the electromagnetic interference of the circuit board pcb. Therefore, the design of the PCB plate must be optimized:
(1) shortening the connection between high frequency components to reduce EMI interference.
(2) The large weight (such as more than 20g) component should be fixed with the support and then welded.
(3) The heating element should consider the heat dissipation, prevent the surface of the element to have a larger Delta T to produce the defect and rework, and the heat sensitive element should be far away from the heat source.
(4) Components are arranged as parallel as possible, it is not only beautiful and easy welding, suitable for mass production. The design of the circuit board is the best rectangle of 4: 3. Do not mutate the width of the wire to avoid the discontinuity of the wiring. When the circuit board is heated for a long time, the copper foil can easily expand and fall off. Therefore, the use of large area copper foil should be avoided.
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