Cause the welding defects of flexible circuit boards
The factors that cause the welding defects of flexible circuit boards have the following three reasons：
I.The solderability of the flexible circuit board holes affects the soldering quality
The poor solderability of the holes of the flexible circuit board 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, resulting in the failure of the entire circuit function.
The main factors affecting the solderability of printed flexible circuit boards are:
1. The composition of the solder and the properties of the solder being soldered. 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, and the impurity content 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. Soldering temperature and cleanliness of the sheet metal surface also affect solderability.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 flexible circuit board and the solder.Soldering defects and contamination on the surface of the flexible circuit board will also affect the solderability and cause defects. These defects include solder balls, open circuits, and poor gloss.
II. Welding defects caused by warpage
Flexible circuit boards and components are warped during the welding process, and defects such as virtual welding and short circuits are generated due to stress deformation.Warpage is often caused by the unbalanced temperature of the upper and lower parts of the flexible circuit board.For large PCBs, warpage will also occur due to the weight of the board itself.Ordinary PBGA devices are about 0.5mm away from the printed flexible circuit board. If the device on the flexible circuit board is large, as the circuit board returns to its normal shape after cooling down, the solder joints will be under stress for a long time. If the device is raised by 0.1mm, the Enough to cause open soldering.
III.The design of the flexible circuit board affects the welding quality
In terms of layout, when the size of the flexible circuit board is too large, although the welding 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 will decrease, the welding will be difficult to control, and the adjacent lines will easily interfere with each other, such as the electromagnetic interference of the circuit board.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 suitable for mass production.
The flexible circuit board is best designed as a rectangle of 4:3. Do not change the wire width abruptly to avoid wiring discontinuities.
When the flexible 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.