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Do you know the three main causes of PCB solder defects?

2022-08-16

1. The solderability of the circuit board hole is not good, which will cause welding defects, affect the parameters of the components in the circuit, cause the components of the multi-layer board and the inner conductive instability, resulting in the failure of the entire circuit function. The so-called solderability, is the metal surface is the nature of molten solder wetting, that is, on the metal surface where the solder is formed a relatively uniform, continuous, smooth adhesion film. The main factors affecting the solderability of printed circuit boards are: (1) solder composition and solder performance. Solder is an important part of welding chemical treatment process. It consists of a chemical material containing flux. The commonly used eutectic metals with low melting points are Sn-Pb or Sn-PB-Ag. The content of impurities must be controlled in a certain proportion. To prevent oxides produced by impurities from being dissolved by flux. The purpose of the flux is to help the solder moisten the circuit surface of the circuit board being soldered by transferring heat and removing rust. White rosin and isopropyl alcohol solvents are commonly used. (2) Welding temperature and surface cleanliness of the metal plate will also affect weldability. If the temperature is too high, the solder will spread faster. At this point it has a high activity, will quickly oxidize the circuit board and solder molten surface, causing welding defects. Contamination of the circuit board surface can also affect solderability and cause defects. These defects include tin beads, tin balls, circuit breaks, poor gloss, etc.

 

2. Welding defects caused by warping
The warping of circuit boards and components occurs in the welding process, resulting in defects such as 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 can also occur due to the decrease of the board's own weight. The normal PBGA device is about 0.5mm from the printed circuit board. If the device on the board is large, the solder joints will be stressed for a long time as the board cools and returns to its normal shape. If the device is elevated by 0.1mm, it is sufficient to cause an open weld.

 

3. the design of the circuit board affects the welding quality
In terms of layout, when the circuit board size is too large, although the welding is easier to control, the printed line is longer, the impedance increases, the noise resistance decreases, and the cost increases; If it is too small, heat dissipation will be reduced, welding is difficult to control, and adjacent lines are easy to appear. Mutual interference, such as electromagnetic interference of circuit boards. Therefore, PCB board design must be optimized as follows: (1) shorten the connection between high-frequency components and reduce EMI interference. (2) Heavy parts (such as more than 20g) should be fixed with brackets before welding. (3) The heating body should take heat dissipation into account to prevent defects and rework caused by large δT on the surface of the heating body, and the heating body should be far away from the heat source. (4) The arrangement of components should be as parallel as possible, both beautiful and easy to weld, should be mass production. The circuit board is designed as a 4:3 rectangle. Do not change the wire width suddenly to avoid wiring discontinuity. 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 areas of copper foil should be avoided.

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