The factors that cause circuit board soldering defects have the following three reasons:
1. The solderability of the circuit board holes affects the soldering quality.
The poor solderability of the circuit board holes will cause virtual welding defects, which will affect the parameters of the components in the circuit, resulting in unstable conduction of the components and inner layers of the multi-layer board, causing the entire circuit to fail. The so-called solderability is the property of the metal surface being 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 properties of the solder. Solder is an important part of the welding chemical treatment process. It is composed of chemical materials containing flux. The commonly used low-melting eutectic metals are Sn-Pb or Sn-Pb-Ag. The content of impurities must be controlled by a certain proportion, to prevent oxides produced by impurities from being dissolved by the flux. The function of the flux is to help the solder wet the circuit surface of the board being soldered by transferring heat and removing rust. White rosin and isopropyl alcohol 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 diffusion speed of the solder will be accelerated. At this time, it has high activity, which will rapidly oxidize the molten surface of the circuit board and the solder, resulting in welding defects. The contamination of the surface of the circuit board will also affect the solderability and cause defects. These defects Includes tin beads, tin balls, open circuits, poor gloss, etc.
2. Welding Defects Caused by Warpage
The circuit board and components are warped during the welding process, and defects such as virtual welding and short circuit occur due to stress deformation. Warpage is often caused by an unbalanced temperature between the upper and lower parts of the board. For large PCBs, warpage will also occur due to the board weight falling. Ordinary PBGA devices are about 0.5mm away from the printed circuit board. If the device on the circuit board is large, the solder joint will be under stress for a long time as the circuit board cools down and returns to its normal shape. If the device is raised by 0.1mm, it is enough to cause Weld open circuit.
3. The design of the circuit board affects the welding quality.
In terms of 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, the soldering is difficult to control, and adjacent lines are prone to appear. Mutual interference, such as electromagnetic interference of circuit boards, etc.
Therefore, the PCB board design must be optimized:
(1) Shorten the connection 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) The heat dissipation problem should be considered for the heating element to prevent defects and rework caused by large ΔT on the surface of the element, and the thermal element should be kept away from the heat source.
(4) The arrangement of components should be as parallel as possible, which is not only beautiful but also easy to weld, and should be mass-produced. The circuit board design is best with a 4:3 rectangle. Do not change the wire width abruptly 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, the use of large-area copper foil should be avoided.
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