circuit board welding defects:
1. The weldability of circuit board holes affects the welding quality
Circuit board hole weldability is not good, will produce virtual welding defects, affect the parameters of components in the circuit, lead to the multi-layer board components and inner line conduction instability, cause the whole circuit function failure. The so-called weldability is the property of the metal surface being wetted by molten solder, that is, the metal surface where the solder is formed a relatively uniform layer of continuous smooth adhesion film.
Analysis of the main factors causing the circuit board welding defects
The main factors affecting the solderability of printed circuit board are:
(1) the composition of the solder and the nature of the solder. Solder is an important component in the process of welding chemical treatment. It is composed of chemical materials containing flux. The common eutectic metal with low melting point is sn-pb or sn-pb-ag. The function of flux is to help the solder to wet the circuit surface of the welded plate by transferring heat and removing rust. White rosin and isopropanol are commonly used.
(2) welding temperature and metal plate surface cleaning degree will also affect the weldability. If 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, resulting in welding defects, circuit board surface pollution will affect the solderability of defects, these defects include tin bead, tin ball, open circuit, gloss is not good, etc.
2. Welding defects caused by warping
Circuit board and components in the welding process of warping, due to stress and deformation of virtual welding, short circuit and other defects. Warping is often caused by the temperature imbalance between the upper and lower parts of the circuit board. For large PCB, the board will also warp due to its own weight. Ordinary PBGA device is about 0.5mm away from the printed circuit board. If the device on the circuit board is large, as the circuit board cools down and returns to its normal shape, the solder joint will be under the applied force for a long time. If the device is raised by 0.1mm, it will be enough to lead to virtual open welding.
3. The design of circuit board affects the welding quality
In terms of layout, when the circuit board size is too large, the welding is easier to control, but the printed lines are long, the impedance increases, the anti-noise ability decreases, and the cost increases. Over the hour, the heat dissipation decline, welding is not easy to control, easy to appear adjacent lines mutual interference, such as circuit board electromagnetic interference and so on. Therefore, PCB board design must be optimized:
(1) shorten the connection between high-frequency components and reduce EMI interference.
(2) components with large weight (such as more than 20g) shall be fixed with 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. 4∶3 rectangular circuit board design is the best. Do not change the wire width to avoid discontinuity in the wiring. When the circuit board is heated for a long time, the copper foil is easy to expand and fall off, therefore, large area of copper foil should be avoided.
Responsible editor; zl