PCB Tech - Analysis of the factors that cause PCB welding defects

There are mainly three factors that cause defects in PCB circuit board welding

1. The solderability of PCB holes affects soldering quality

Poor solderability of the circuit board holes will result in 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 inner wires, 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 circuit boards 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 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 a high activity, which will cause the circuit board and the molten surface of the solder to oxidize rapidly, resulting in soldering defects. Contamination on the surface of the circuit board will also affect the solderability and cause defects. These defects Including 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 circuit board. The large PCB will also warp due to the drop of the board's own weight. Ordinary PBGA devices are about 0.5mm away from the printed circuit board. If the device on the circuit board is large, the solder joints will be under stress for a long time as the circuit board cools down, and the solder joints will be under stress for a long time. Weld open circuit.

3. PCB design affects welding quality

In the PCB layout, when the size of the circuit board 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; The lines interfere with each other, such as the electromagnetic interference of the circuit board. 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 components should be as parallel as possible, so that it is not only beautiful but also easy to weld, and it 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 circuit board is heated for a long time, the copper foil is easy to expand and fall off. Therefore, avoid using large-area copper foil.