In the design of pcb, we are pulling wires except the layout, so there are some awkward requirements for the outgoing line. It’s not just a random one. Let’s learn with Xiaobian!
1. In order to meet the production process capability requirements of domestic PCB factories, the conventional wiring line width is ≥4mil(0.1016mm) (3.5mil, i.e. 0.0889mm, can be used in special cases); Less than this value will greatly challenge the production capacity of the factory and increase the scrap rate.
2. PCB design wiring cannot be routed at any angle, which challenges the manufacturer’s production capacity. Many problems occur when copper wires are etched, so 45 or 135 wiring is recommended.
3. The same network should not be routed at 90 right angle or acute angle, which is generally required to be avoided in PCB wiring, and it has almost become one of the standards to measure the quality of wiring. Right-angle routing will change the line width of the transmission line, resulting in impedance discontinuity and signal reflection, and EMI at the tip will affect the line.
4. The shapes of pads are generally regular, such as BGA pads are round, QFP pads are oblong, CHIP pads are rectangular, etc. However, the actually made PCB has irregular pads, which can be said to be grotesque. Take the pad of R0402 resistor package as an example. After the regular pad goes out of line, the process deviation in production will turn into the actual pad. It is composed of a small rectangular pad on the basis of the original rectangular pad, which is irregular and leads out of the special-shaped pad.
In the pcb design, if the two pads of 0402 resistor package are routed diagonally, and the solder resist deviation caused by PCB production accuracy (one side of solder resist window is 0.1mm larger than the pad), the pad shown in the left figure of Figure 4 will be formed. In this case, when resistance welding, due to the surface tension of solder, bad rotation will occur.
Reasonable wiring method and fan-out method with axial symmetry about the long axis can effectively reduce the bad rotation of CHIP components after mounting. If the fan-out line of the pad is also symmetrical about the short axis, the drift of the CHIP component after mounting can also be reduced.
5. The adjacent pads are in the same network, so they cannot be directly connected. It is necessary to connect the pads at the joint first, and then connect them. The straight chain is easy to cause continuous welding during manual welding.
6. The connector discipline cable needs to be pulled out from the center of the pad and then go out. No other angles are allowed, so as to avoid tearing the cable when the connector is unplugged.
After learning these specifications, do you have a deeper understanding of PCB cables? Hurry and apply them to your own projects!
