Basic Design Process and Layout of PCB Design(3)

5. Main Principles When Wiring of PCB

Generally, the power line and ground line should be wired first to ensure the electrical performance of the circuit board. As far as the conditions allow, widen the width of the power and ground wires as much as possible, preferably the ground wire is wider than the power wire. Their relationship is ground wire> power wire> signal wire. Usually the signal wire width is 0.2 ~ 0.3mm . The smallest width can reach 0.05～0.07mm, and the power cord is generally 1.2～2.5mm. For the PCB of the digital circuit, a wide ground wire can be used to form a loop, that is, to form a ground net to use (the ground of the analog circuit cannot be used in this way).

Wire lines with strict requirements (such as high-frequency lines) in advance. And the edges of the input end and the output end should be avoided adjacent to parallel to avoid reflection interference. If necessary, ground wire should be added for isolation, and the wiring of two adjacent layers should be perpendicular to each other. Parasitic coupling is easy to occur in parallel.

The housing of the oscillator should be grounded, and the clock line should be as short as possible, and it should not be drawn everywhere. Under the clock oscillation circuit, the area of the special high-speed logic circuit should be enlarged, and other signal lines should not be used to make the surrounding electric field approach zero.

Use 45 degree polyline wiring as much as possible, and 90 degree polyline should not be used to reduce the radiation of high-frequency signals. (Highly demanding lines should also use double-curved lines)

Do not form loops on any signal lines. If it is unavoidable, loops should be as small as possible. Signal lines should have as few vias as possible.

The key line should be as short and thick as possible, and protective ground should be added on both sides.

When transmitting sensitive signals and noisy field band signals through flat cables, they should be led out in the way of “ground wire-signal-ground wire”.

Test points should be reserved for key signals to facilitate production and maintenance testing. After the schematic wiring is completed, the wiring should be optimized.

At the same time, after the initial network inspection and DRC inspection, the unwiring area is filled with ground wire, and a large area of copper layer is used as the ground wire, and the unused parts are connected to the ground as the ground wire on the printed board. Or it can be made into a multilayer board, and the power supply and ground wires occupy one layer each.