PCB is an indispensable part of modern electronics and the carrier of electrical connection of electronic components. With the continuous development of electronic technology, the density of PCB is getting higher and higher, so there are more and more requirements for welding process. Therefore, it is necessary to analyze and judge the factors affecting PCB welding quality and find out the causes of welding defects, so as to make targeted improvement and improve the overall quality of PCB board. Let’s take a look at the causes of welding defects on PCB board.

Why do PCB welds have defects?

Causes of circuit board welding defects are:

1. The weldability of the circuit board hole affects the welding quality

The hole weldability of circuit board is not good, it will produce virtual welding defects, affect the parameters of components in the circuit, lead to the instability of the multilayer board components and inner line conduction, cause the failure of the whole circuit function.

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The main factors affecting 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 welding chemical treatment process, it is composed of chemical materials containing flux, commonly used low melting point eutectic metal is Sn-Pb or Sn-Pb-Ag. The content of impurities should be controlled to prevent the oxide produced by impurities from being dissolved by flux. The function of flux is to help the solder wet the circuit surface of the soldered plate by transferring heat and removing rust. White rosin and isopropyl alcohol are generally used.

(2) Welding temperature and metal plate surface cleanliness will also affect weldability. The temperature is too high, the solder diffusion speed is accelerated, at this time has a very 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, including tin beads, tin balls, open circuit, gloss is not good.

2. Welding defects caused by warping

Circuit boards and components warped during welding, resulting in defects such as virtual welding and short circuit due to stress deformation. Warping is usually caused by temperature imbalance between the upper and lower parts of the circuit board. For large PCBS, warping also occurs when the board falls under its own weight. Ordinary PBGA devices are about 0.5mm away from the printed circuit board. If the components on the circuit board are large, the solder joint will be under the stress for a long time as the circuit board returns to its normal shape after cooling. If the component is raised by 0.1mm, it will be enough to cause the virtual welding open circuit.

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

In the layout, the size of the circuit board is too large, although the welding is easier to control, but the printing line is long, the impedance increases, the anti-noise ability decreases, the cost increases; Too small, the heat dissipation decreases, welding is not easy to control, easy to appear adjacent lines interfere with each other, such as the electromagnetic interference of the circuit board. Therefore, PCB board design must be optimized:

(1) Shorten the connection between high-frequency components and reduce EMI interference.

(2) Components with large weight (such as more than 20g) should be fixed with support and then welded.

(3) Heat dissipation of heating elements should be considered to prevent large δ T surface defects and rework, and heat sensitive elements should be kept away from heating sources.

(4) The arrangement of components as parallel as possible, so that not only beautiful and easy to weld, suitable for mass production. 4∶3 rectangular circuit board design is best. Do not mutate wire widths to avoid discontinuities in wiring. When the circuit board is heated for a long time, copper foil is easy to expand and fall off. Therefore, large copper foil should be avoided.

In summary, in order to ensure the overall quality of PCB board, it is necessary to use excellent solder, improve the solderability of PCB board, and prevent warping to prevent defects in the production process.