Poor weldability of circuit board holes will lead to virtual welding defects, which will affect the parameters of components in the circuit and lead to unstable conduction of multilayer board components and inner line, resulting in failure of the entire circuit function. 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 continuous smooth adhesion film.
(1) the composition of solder and the nature of solder.
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 eutectic metal is Sn-Pb or Sn-Pb-Ag, and the content of impurities should be controlled to a certain extent to prevent the oxide produced by impurities from being dissolved by flux. The function of flux is to help the solder moisten the circuit surface of the welded plate by transferring heat and removing rust. Generally use white rosin and isopropanol solvents.
(2) Welding temperature and the surface cleanliness of the metal plate will also affect the weldability.
The temperature is too high, the solder diffusion speed is accelerated, at this point has a high activity, will make the circuit board and solder melt surface quickly oxidation, welding defects, circuit board surface pollution will also affect the weldability to produce defects, these defects include tin ball, tin ball, open, gloss is not good, etc.
Circuit boards and components warp in the welding process, resulting in virtual welding, short circuit, and other defects due to stress deformation. Warping is often caused by temperature imbalance in the upper and lower parts of the circuit board. For large PCBS, the weight of the board itself can cause warping. The normal 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 stress for a long time. If the device is raised by 0.1mm, it will be enough to cause open virtual welding.
In the layout, the board size is too large, although welding is easier to control, the printing line is long, impedance increases, anti-noise ability decreases, the cost increases; When it is too small, the heat dissipation drops, welding is not easy to be controlled, and adjacent lines are easy to interfere with each other, such as electromagnetic interference of circuit board.
(1) shorten the connection between high-frequency components and reduce EMI interference.
(2) Components of large weight (in excess of 20g) shall be secured by brackets and then welded.
(3) heating elements should be considered heat dissipation problems, prevent components surface has larger ΔT defects and rework, the thermal element should be far away from heat source.
(4) The arrangement of components as parallel as possible, so that not only beautiful and easy to welding, suitable for mass production.
The rectangular circuit board with a 4:3 design is the best. Do not change the wire width to avoid discontinuity of the wiring. Copper foil tends to swell and fall off when the circuit board is heated for a long time. Therefore, a large area of copper foil should be avoided.
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