There are three reasons for the welding defects of pcb:
1, the solderability of the orifice of the circuit board affects the quality of welding
Circuit board hole welding is not good, will cause weld defects, affect the parameters of the circuit components, resulting in multilayer board components and inner line conduction instability caused by the failure of the circuit function. Solderability is the property of a metal surface being wetted by a molten solder, i.e., a relatively uniform, continuous, smooth, attached film on the metal surface of the solder. The main factors that affect the solderability of printed circuit boards are:
(1) the composition of solder and the properties of the solder. Solder is an important component of the chemical process of welding. It consists of chemical material containing flux. The impurity content should be controlled by a certain ratio to prevent the oxide produced by the impurity from being dissolved by the flux. The function of the flux is to transfer the heat to remove the corrosion to help the solder to damp the surface of the soldered circuit. Generally use white rosin and isopropyl alcohol solvents.
(2) the welding temperature and the cleanness of the surface of the metal plate will also affect the weldability. The temperature is too high, then the spreading speed, this has high activity, will make the circuit board and solder melting surface oxidized, welding defects, the surface of the circuit board by the pollution will also affect the weldability of the resulting defects, these defects include solder ball, solder, open, gloss is not good.
2. Welding defects caused by warping
Circuit board and components in the process of welding warp, due to the stress deformation and weld defects such as short circuit. Warpage is often due to the uneven temperature of the upper and lower parts of the circuit board. For large PCB, the warp will also warp due to the weight of the plate itself. PBGA device from printed circuit board common about 0.5mm, if the circuit board device is larger, with normal shape recovery circuit board after cooling, the solder joint will be a long time in the stress under the action, if the device is enough to cause the weld open up 0.1mm.
3, the design of the circuit board affects the quality of welding
In the layout, the size of the circuit board is too large, although the welding is easy to control, but the printing line length, impedance increased, decreased, anti noise ability increased cost; too small, is cooling down, welding is not easy to control, easy to appear adjacent lines of mutual interference, such as circuit board electromagnetic interference etc.. Therefore, the PCB board design must be optimized:
(1) shorten the connection between the high frequency components and reduce the interference of EMI.
(2) heavy components (such as more than 20g) shall be fixed with a bracket and welded.
(3) heating element should consider the heat dissipation problem, prevent the component surface from having bigger Delta T to produce the defect and rework, the heat sensitive element should keep away from the heat source.
(4) the arrangement of the components is as parallel as possible, so that it is not only beautiful but also easy to weld, and should be produced in large quantities. The circuit board is designed with a rectangular optimum of 4: 3. The width of the wire should not be mutated to avoid discontinuities in the wiring. When the circuit board is heated for a long time, the copper foil tends to expand and fall off. Therefore, a large area of copper foil should be avoided.