PCB Tech - Causes of PCB soldering defects

The factors that cause PCB soldering defects have the following three reasons:

1. The solderability of PCB holes affects soldering quality

The poor solderability of PCB holes will cause false soldering defects, which will affect the parameters of the components in the circuit, resulting in unstable conduction of the multi-layer board components and the inner line, causing the entire circuit to fail. The so-called solderability is the property that the metal surface is wetted by molten solder, that is, a relatively uniform continuous smooth adhesion film is formed on the metal surface where the solder is located. The main factors that affect the solderability of printed PCBs are: (1) The composition of the solder and the nature of the solder. Solder is an important part of the welding chemical treatment process. It is composed of chemical materials containing flux. Commonly used low melting point eutectic metals are Sn-Pb or Sn-Pb-Ag. The impurity content must be controlled by a certain proportion, In order to prevent the oxides generated by impurities from being dissolved by the flux. The function of the flux is to help the solder wetting the surface of the circuit to be soldered by transferring heat and removing rust. White rosin and isopropanol solvents are generally used. (2) The welding temperature and the cleanliness of the metal plate surface will also affect the weldability. If the temperature is too high, the solder diffusion speed will increase. At this time, it will have high activity, which will quickly oxidize the PCB and the molten surface of the solder, resulting in soldering defects. Contamination of the circuit board surface will also affect the solderability and cause defects. These defects include Tin beads, tin balls, open circuits, poor gloss, etc.

2. Welding defects caused by warpage

Circuit boards and components warp during the welding process, and defects such as virtual welding and short circuit due to stress deformation. Warpage is often caused by the temperature imbalance of the upper and lower parts of the PCB. For large circuit boards, warping will also occur due to the drop of the board's own weight. Ordinary PBGA devices are about 0.5mm away from the printed PCB. If the components on the circuit board are large, the solder joints will be under stress for a long time as the circuit board cools down and return to normal shape. If the device is raised by 0.1mm, it will be enough to cause false Welding is open.

3. PCB design affects welding quality

In the layout, when the PCB size is too large, although the soldering is easier to control, the printed lines are long, the impedance increases, the anti-noise ability is reduced, and the cost increases; Interference, such as electromagnetic interference of circuit boards. Therefore, the PCB board design must be optimized: (1) Shorten the wiring between high-frequency components and reduce EMI interference. (2) Components with heavy weight (such as more than 20g) should be fixed with brackets and then welded. (3) Heat dissipation issues should be considered for heating components to prevent defects and rework caused by large ΔT on the surface of the components, and the thermal components should be far away from the heating source. (4) The arrangement of the components is as parallel as possible, which is not only beautiful but also easy to weld, and is suitable for mass production. The best PCB design is a 4:3 rectangle. Do not change the wire width to avoid wiring discontinuities. When the PCB is heated for a long time, the copper foil is easy to expand and fall off. Therefore, avoid using large-area copper foil.