The factors contributing to the defects of the circuit board welding are as follows:
1.Weldability of Circuit Board Holes Affects Welding Quality
The poor solderability of the circuit board hole will produce the defect of virtual welding, which will affect the parameters of the components in the circuit, lead to the instability of the multi-layer board components and the inner layer line conduction, and cause the failure of the whole circuit function.
Weldability is the surface of a metal the nature of the wetting of molten solder, i.e., the formation of a relatively uniform continuous smooth adhesive film on the metal surface of the solder.
The main factors affecting the solderability of printed circuit boards are:
1）The composition and properties of the solder. solder is an important part in the process of welding chemical treatment. it consists of chemical materials containing flux. the commonly used low melting point eutectic metal is sn-pb or sn-pb-ag. The impurity content should be controlled by a certain percentage to prevent the oxide from being dissolved by flux. The function of the flux is to help solder wetting the surface of the welded plate by transferring heat and removing rust. Generally use white rosin and isopropanol solvent.
2）Welding temperature and surface cleanliness of metal plates can also affect weldability. If the temperature is too high, the solder diffusion speed will be accelerated, at this time has a very high activity, will make the circuit board and solder melt surface oxidation quickly, resulting in welding defects, circuit board surface contamination will also affect solderability and thus produce defects, these defects include tin beads, tin balls, open circuit, poor gloss, etc.
2.Welding defects resulting from warpage
Circuit boards and components in the welding process to produce warpage, due to stress deformation caused by virtual welding, short circuit and other defects. warpage is often caused by the unbalanced temperature of the upper and lower parts of the circuit board. For large pcb due to the weight of the board fall will also produce warpage. the common pbga device is about 0.5mm away from the printed circuit board, if the device on the circuit board is larger, with the circuit board return to normal shape after cooling down, the solder joint will be under the stress operation for a long time, if the device raises 0.1mm is enough to cause the virtual welding open circuit.
3.Design of Circuit Board Affects Welding Quality
In the layout, when the size of the circuit board is too large, although the welding is easy to control, but the printing line is long, the impedance is increased, the anti-noise ability is reduced, and the cost is increased; too small, the heat dissipation is decreased, 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 board design must therefore be optimized:
1）Shorten the connection between high-frequency components and reduce EMI interference
2）Heavyweight (e.g. over 20g) elements shall be secured with a support and then welded
3）The heating element should consider the problem of heat dissipation to prevent the large ΔT on the surface of the element from producing defects and rework, and the thermosensitive element should be kept away from the heating source.
4）The arrangement of components as parallel as possible, so not only beautiful and easy to weld, suitable for mass production. The circuit board is designed to be 4:3 rectangular best. Do not mutate the wire width to avoid the discontinuity of the wiring. When the circuit board is heated for a long time, the copper foil is prone to expand and fall off, therefore, the use of large area copper foil should be avoided.