PCB Fabrication

Three factors of circuit board welding defects

by:A-TECH      2021-03-20
There are many reasons that affect circuit board welding defects, but the most important ones are the following three. 1. The solderability of the circuit board holes affects the quality of welding. The 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 the inner wires. Cause the entire circuit function 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 in a certain proportion to prevent the oxides generated by the 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, resulting in defects such as virtual welding and short circuit due to stress and deformation. Warpage is often caused by the temperature imbalance of the upper and lower parts of the circuit board. For large PCBs, 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 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 return to normal shape. If the device is raised by 0.1mm, it will Enough to cause a virtual open circuit. 3. The design of the circuit board affects the quality of soldering. In the 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 decreases, and the cost increases; if it is too small, the heat dissipation decreases. Welding is not easy to control, and it is easy for adjacent lines to interfere with each other, 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 elements to prevent defects and rework caused by large ΔT on the surface of the elements, and heat sensitive elements should be kept away from heat sources. (4) The arrangement of components should be as parallel as possible, so that it is not only beautiful but also easy to weld, and is suitable for mass production. The circuit board is best designed as 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 prone to swell and fall off. Therefore, avoid using large-area copper foil.
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