Three factors of circuit board soldering defect

There are three factors for the circuit board soldering defect:

1. The solderability of the circuit board hole affects the quality of the soldering.

The solderability of the circuit board hole if not good will cause pseudo soldering, will affect the parameters of the components in the circuit, resulting in unstable conduction of the multilayer circuit board component and the inner layer line, causing the entire circuit function to fail. The so-called solderability is the property of the metal surface being wetted by the molten solder, that is, a relatively uniform continuous smooth adhering film is formed on the metal surface of the solder.

The main factors affecting solderability are:

(1) The composition of the solder and the nature of the solder

Solder is an important part of the soldering chemical treatment process, 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 certain ratio, in order to prevent the oxide generated by impurities from being dissolved by the flux. The function of the flux is to help the solder wet the surface of the circuit board being soldered by transferring heat and removing rust. White rosin and isopropanol solvents are generally used.

(2) The soldering temperature and the cleanliness of the surface also affect the solderability. When the temperature is too high, the solder diffusion speed is increased, the activity is very high, and the circuit board and the solder melted surface are rapidly oxidized, causing welding defects, and the surface of the circuit board is contaminated, which also affects the solderability and causes defects. Including tin beads, solder balls, open circuit, poor gloss and so on.

2. Soldering defects caused by bow and twist

Circuit boards and components are bow and twist during soldering, and defects such as pseudo soldering and short circuits are caused by stress deformation. Bow and twist are often caused by temperature imbalances between the upper and lower parts of the circuit board. For large PCBs, bow and twist can also occur due to the weight drop of the circuit board itself. The ordinary PCBA components is about 0.5mm away from the printed circuit board, if the components on the circuit board is large, the normal shape will be restored as the circuit board cools down, and the solder joint will be under stress for a long time, if the component is raised by 0.1mm, it will be enough cause open circuit.

3. The design of the circuit board affects the quality of soldering

In the layout, when the board size is too large, although the soldering is easier to control, but the printed lines are long, the impedance is increased, the noise resistance is reduced, and the cost is increased. When it is too small, the heat dissipation is reduced, the soldering is difficult to control, and adjacent lines are likely to interfere with each other, such as electromagnetic interference of the circuit board. Therefore, the PCB board design must be optimized:

(1) Shorten the wiring between high frequency components and reduce electromagnetic interference.

(2) Components with a large weight (e.g. over 20g) shall be fixed by brackets and then soldered.

(3) The heating element should consider the heat dissipation problem, and the heat sensitive component should be 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 solder, it is suitable for mass production. The circuit board is designed to be a 4:3 rectangle optimal. Do not mutate the wire width to avoid discontinuity of the lines. When the circuit board is heated for a long time, the copper foil is prone to expansion and fall off, therefore, large-area copper foil should be avoided.