The factors that constitute the welding defects of circuit boards have the following three reasons:
1, the circuit board hole can be welded to affect the quality of welding
The circuit board hole can welding, weld defects will be generated, parameters of circuit elements, resulting in multilayer board component and the inner line through unstable, cause failure of the entire circuit function. The so-called ability to weld metal surface is molten solder wetting properties, that is, where the metal surface of the solder composition of a layer of a relatively uniform, smooth and continuous film. The main factors that affect the welding performance of printed circuit board are: (1) the composition of the solder and the properties of the solder. Solder is welded chemical treatment process is an important part, it by flux materials containing chemical composition, low melting point eutectic metal Sn Pb or Sn-Pb-Ag. the impurity content to a certain ratio control, to prevent the impurity generated in the oxide is dissolved in the flux. The function of the flux is to remove rust by passing through the heat to help the solder wetting the surface of the circuit board. General use of white rosin and solvents. (2) welding temperature and metal plate surface cleanliness will also affect the ability to weld. Temperature is too high, the solder spread speed, this time with a very high activity, will make the circuit board and solder melting rapid oxidation of the surface, welding defects and circuit board pollution on the surface will influence welding, resulting in defects, these defects including solder ball, solder, open, gloss is not good.
2, warp produced welding defects
Circuit board and components in the process of welding warp, due to the stress deformation and weld defects such as short circuit. Warp is often due to the temperature of the upper and lower parts of the circuit board. On the PCB board, due to its own weight to fall will produce warping. 0.5mm ordinary PBGA away from the printed circuit board, if the circuit board devices is large, with the cooling circuit board after the return to normal shape, solder joints will be out for a long time in stress under, if the device elevation 0.1mm is enough to cause the open circuit cold solder joint.
3, circuit board design affects the quality of welding
In the layout, circuit board size is too large, although welding is easy to control, but the printing long lines, increase impedance and anti noise ability to drop, increase the cost; over an hour, is cooling down, the welding is not easy to control, prone to interference between adjacent lines, such as circuit board of electric magnetic interference. Therefore, it is necessary to optimize the PCB board design: (1) to shorten the connection between the high frequency components and reduce the EMI interference. (2) heavy (such as more than 20g) components, shall be fixed by the bracket, and then welded. (3) the heating element should consider the heat problem, to prevent the element on the surface of the larger delta t produce defects and rework, thermosensitive element should stay away from heat sources. (4) the arrangement of elements is as parallel as possible, so that it is not only beautiful but also easy to weld, so it is suitable for mass production. The circuit board is designed to 4: 3 of the rectangular optimum. Do not change the width of the wire to avoid the discontinuity of the wiring. Long time heating circuit board, copper foil, prone to expansion and loss, therefore, should avoid the use of a large area of copper foil.