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Three major factors that cause board soldering defects

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Published by: Keyou September 30,2019

1. The solderability of the board hole affects the quality of the soldering

 

The solderability of the circuit board hole is not good, and the solder joint defect will be generated, which will affect the parameters of the components in the circuit, resulting in unstable conduction of the multilayer board component and the inner layer line, causing the entire circuit function to fail. The so-called solderability is the property of the metal surface being wetted by the molten solder, that is, a relatively uniform continuous smooth adhering film is formed on the metal surface of the solder.

 

            Three major factors that cause board soldering defects

 


The main factors affecting 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 consists of chemical materials containing flux. The commonly used low melting point eutectic metal is Sn-Pb or Sn-Pb-Ag. The impurity content should be controlled by certain ratio. In order to prevent the oxide generated by impurities from being dissolved by the flux. The function of the flux is to help the solder wet the surface of the board being soldered by transferring heat and removing rust. White rosin and isopropanol solvents are generally used.

    (2) The soldering temperature and the cleanliness of the metal plate surface also affect the solderability. When the temperature is too high, the solder diffusion speed is increased. At this time, the activity is very high, and the circuit board and the solder melted surface are rapidly oxidized, causing welding defects, and the surface of the circuit board is contaminated, which also affects the solderability and causes defects. Including tin beads, solder balls, open circuit, poor gloss and so on.

 

2. Welding defects caused by warpage

Circuit boards and components are warped during soldering, and defects such as solder joints and short circuits are caused by stress deformation. Warpage is often caused by temperature imbalances in the upper and lower parts of the board. For large PCBs, warpage can also occur due to the weight drop of the board itself. The ordinary PBGA device is about 0.5mm away from the printed circuit board. If the device on the circuit board is large, the normal shape will be restored as the circuit board cools down, and the solder joint will be under stress for a long time. If the device is raised by 0.1mm, it will be enough. Open welding.

 

3, the design of the circuit board affects the quality of welding

In the layout, when the board size is too large, although the welding is easier to control, the printed lines are long, the impedance is increased, the noise resistance is reduced, and the cost is increased; when the temperature is too small, the heat dissipation is lowered, the welding is difficult to control, and adjacent lines are prone to occur. Mutual interference, such as electromagnetic interference on the board. Therefore, the PCB board design must be optimized:

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

    (2) Components with a large weight (eg over 20g) shall be fixed by brackets and then welded.

    (3) The heating element should consider the heat dissipation problem, prevent the surface of the component from having a large ΔT defect and rework, and the heat sensitive component should be away from the heat source.

    (4) The arrangement of the components is as parallel as possible, so that it is not only beautiful but also easy to weld, and should be mass-produced. The board is designed to be a 4:3 rectangle optimal. Do not mutate the wire width to avoid discontinuity of the wiring. When the board is heated for a long time, the copper foil is prone to expansion and fall off. Therefore, large-area copper foil should be avoided.

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