Three Causes of Welding Defects in PCB
Effect of Weldability of Plate Hole on Welding Quality
Weldability is that the metal surface is wetted by the molten solder, that is, the metal surface where the solder is located forms a continuous, relatively uniform, smooth adhesion film. Poor solderability of PCB holes will result in virtual soldering defects, which will affect the parameters of components in the circuit. Unstable multi-layer board components and inner wires will lead to the failure of the whole circuit in serious cases.
The solderability of PCB is affected by the following factors:
(1) The composition of the welding material and the properties of the material to be welded. As an important part of the welding chemical treatment process, welding materials contain Flux-assisted chemical materials. Generally, low melting point eutectic metals are Sn-Pb or Sn-Pb-Ag, in which the impurity content can not exceed a certain proportion to prevent impurities from producing oxides and being dissolved by flux. Solders usually use white rosin and isopropanol solvents to remove the rust on the surface of printed circuit boards by heat transfer, so as to help solders wet the surface of soldered printed circuit boards better.
(2) The solderability of PCB is also affected by the welding temperature and the cleanliness of the metal plate surface. High temperature will accelerate the diffusion of solder and improve the activity of solder, resulting in rapid oxidation of the melted surface of PCB and solder, forming welding defects; contaminated PCB surface will also affect the solderability of PCB, resulting in tin beads, tin balls, open circuit, poor gloss and other defects.
Welding Defects Caused by Warpage
In the welding process of circuit boards and components, there are some defects, such as virtual welding and short circuit, due to the warping of stress and deformation. The temperature imbalance between the upper and lower parts of the circuit board is the main reason for the warpage of the circuit board. For large circuit boards, falling weight can also cause warping. The distance between the ordinary PBGA device and the circuit board is about 0.5mm. If the device on the circuit board is large, the circuit board will gradually return to its normal shape after cooling, and the stress will act on the solder joint for a long time. At this time, if the device is raised by 0.1mm, it may lead to virtual soldering open circuit.
Welding quality is affected by circuit board design
Although welding on board with large layout size is easier to control, too large a plate size will lead to long printed lines, increased impedance, reduced noise resistance and increased cost of the board; too small a plate size, reduced heat dissipation capacity, the welding process is not easy to control, and adjacent line phases will occur. The situation of mutual interference. Therefore, it is necessary to optimize the design of printed circuit boards:
(1) To shorten the lines between high-frequency components and reduce electromagnetic interference as far as possible.
(2) Large weight components over 20g should be fixed by brackets and then welded.
(3) Heat-generating elements should consider the issue of heat dissipation, to prevent defects caused by higher surface temperature of components, and heat-sensitive elements should be as far away as possible from the source of heat.
(4) In order to make the circuit board beautiful and easy to weld, the components are arranged in parallel as far as possible, which is also conducive to mass production. The optimum proportion of circuit board design should be 4:3 rectangle. Width of conductor should be balanced as far as possible to prevent discontinuous wiring. Large area copper foil should be avoided to prevent expansion and shedding of PCB when heated for a long time.
