In many CAD systems, wiring is based on the decision of the network system. The grid is too dense, the pathway is increased, but the step is too small, the amount of data in the figure field is too large, which is bound to have higher requirements for the storage space of the equipment, but also the object of the computer class electronic products have a great impact on the speed of computing. And some of the pathway is invalid, such as the pad occupied by the component leg or by the installation hole, fixed their holes occupied by the. The grid is too sparse, too little access to the cloth through the rate of great impact. So there should be a reasonable grid system to support the wiring process.
The distance between the two legs of the standard components is 0.1 inch (2.54 mm), so the basis of the grid system is generally set at 0.1 inch (2.54 mm) or an integer multiple of less than 0.1 inch, such as: 0.05 inch, 0.025 inch, 0.02 inch, etc.
After the wiring design is completed, the wiring design needs to be carefully checked to see if it conforms to the rules set by the designer, and also to confirm whether the rules set meet the needs of the production process of the printed circuit board, generally checking the following aspects.
a. Line and line, line and component pads, line and through-hole, component pads and through-hole, the distance between through-hole and through-hole is reasonable, whether to meet the production requirements.
b. Whether the width of the power and ground lines are appropriate, and whether the power and ground lines are tightly coupled (low wave impedance)? Whether there are still places in the PCB where the ground line can be widened.
c. for the critical signal lines are taken the best measures, such as the shortest length, add protection lines, input and output lines are clearly separated.
d. Analog and digital circuit parts, whether there are their own independent ground.
e. After the addition of graphics in the PCB (such as icons, note labels) whether the signal will cause a short circuit.
f. Modification of some undesirable lineshapes.
g. Whether the PCB is added to the process line? Whether the solder resist meets the requirements of the production process, whether the size of the solder resist is appropriate, whether the character mark is pressed on the device pads, so as not to affect the quality of the electrical installation.
h. Multilayer board in the power ground layer of the outer frame edge is narrowed, such as the power ground layer of copper foil exposed outside the board is easy to cause a short circuit.
a. For high-frequency current, when the corner of the wire presents a right angle or even an acute angle, in the part near the corner, the magnetic flux density and electric field strength are higher, will radiate stronger electromagnetic waves, and the inductance here will be larger, the inductive resistance will also be larger than the obtuse or rounded corners.
b. For the bus wiring of digital circuits, the wiring corners show obtuse or rounded corners, and the area occupied by the wiring is relatively small. Under the same conditions of line spacing, the total width occupied by the line spacing is 0.3 times less than that of a right angle bend.
a. Clock, reset, more than 100M signals and some key bus signals and other signal line wiring must meet the 3W principle, the same layer and adjacent layers without longer flat walking line, and the link on the perforation as little as possible.
b. The number of high-speed signals through the hole problem, some device guidelines on the number of high-speed signals through the hole requirements are generally more stringent, consulting the principle of interconnection is in addition to the necessary pin fanout through the hole, it is strictly prohibited to play the extra hole in the inner layer, they have laid 8G PCIE 3.0 alignment, also played 4 holes, no problem.
c. The same layer clock and high-speed signal center distance to strictly meet the 3H (H for the alignment layer to the reflow plane spacing); adjacent layers of signals are strictly prohibited to overlap, it is recommended that also meet the principle of 3H, on the above crosstalk, there are tools to check the.
