Three reasons cause circuit board soldering defects
1. the solder ability of the circuit board hole affect the quality of welding
The solder ability of the circuit board hole is not good, and it will cause the false weld defect, which will affect the parameters of the components in the circuit, leading to the instability of the multilayer component and the inner layer wire, causing the entire circuit function to fail. The so-called solder ability is the property that the metal surface is wetted by the molten solder, that is, the surface of the metal on which the solder is formed forms a relatively uniform, continuous, smooth and adherent film.
The factors influencing the solder ability 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 process. It is composed of flux-containing chemical materials. The commonly used low-melting eutectic metal is Sn-Pb or Sn-Pb-Ag. The impurity content must have a certain ratio control. In order to prevent the impurities produced by the oxide dissolved by flux. The function of the flux is to help the solder wet the surface of the circuit of the plate to be welded by transferring heat and removing rust. White rosin and isopropyl alcohol solvents are generally used.
(2) The soldering temperature and the cleanness of the metal plate surface also affect the solderability. If the temperature is too high, the diffusion rate of the solder is accelerated. At this time, it has a high activity, which rapidly oxidizes the circuit board and the molten surface of the solder, resulting in welding defects. The surface of the circuit board is contaminated, which also affects the solderability and thus causes defects. Including tin beads, solder balls, open circuits, and poor gloss.
2.welding defects caused by warping
Circuit boards and components are warped during the welding process, and defects such as cold welding and short circuits are generated due to stress deformation. Warpage is often caused by temperature imbalances in the upper and lower parts of the board. For large PCBs, warpage may also occur due to board weight falling. Ordinary PBGA device is 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. If the device is raised by 0.1mm, it will be enough to cause Welded open circuit.
3. the design of the circuit board affects the quality of welding
In the layout, when the circuit board size is too large, although the welding is easier to control, the printed lines are longer, the impedance is increased, the anti-noise ability is decreased, and the cost is increased; when the size is too small, the heat dissipation is decreased, the welding is not easy to control, and adjacent lines easily appear. Mutual interference, such as the electromagnetic interference of the circuit board. Therefore, PCB board design must be optimized:
(1) Shorten the connection between high-frequency components and reduce EMI interference.
(2) Heavy components (eg more than 20g) shall be fixed by brackets and then welded.
(3) The heat-generating component should consider the heat dissipation problem to prevent the component surface from having a large ΔT defect and rework, and the heat-sensitive component should be far away from the heat source.
(4) The arrangement of the components is as parallel as possible, which is not only aesthetic but also easy to solder, and mass production is preferred. The circuit board is designed to have a 4:3 rectangle best. Do not change the width of the wire to avoid wiring discontinuities. When the circuit board is heated for a long time, the copper foil is likely to swell and fall off. Therefore, large-area copper foil should be avoided.