PCB Multilayer Circuit Board Impedance Wire Routing Skills

PCB Multilayer printed circuit board impedance wire routing skills

1. what is PCB impedance?

PCB impedance is the combination of capacitance and inductance of the circuit at high frequency operation, although it is also measured in Ω, but it is somewhat different from the resistance as a DC characteristic, the impedance is an AC characteristic, which means that it is related to the frequency, while the resistance is not.

2. the significance of impedance to PCB

PCB to achieve high-speed data transmission, impedance matching is a must.

Taking the mobile phone USB interface as an example, it can realize the synchronous transmission of data in two directions, with high speed and low cost. The two common types are type A, type B and type C.

The USB protocol defines (D+, D-), (TX+, TX-), (Rx+, RX-) differential signal lines to transmit digital signals. In order to ensure the stability of the transmitted signal, the PCB design differential line must be strictly in accordance with the routing rules of the differential signal.

3. some precautions for multi-layer PCB impedance wiring

(1). the component PAD as close as possible to arrange, shorten the distance between the PADS, so that the difference line as short as possible, as little as possible through the hole.

(2). the route should be parallel, symmetrical, not allowed to go 90° Angle line, should go 45° or arc line. Line spacing is controlled within 4mil.

(3). series resistance, capacitor, should be up and down or left and right alignment.

(4). the differential impedance line as far as possible, equidistant, to avoid timing deviation and common mode interference.

(5). due to pin distribution, through the hole, line space and other factors, the differential impedance line length is easy to mismatch. Once the line length does not match, the timing will shift, which reduces the signal quality. Therefore, it is necessary to compensate the mismatch of differential pairs to make the line length match. The length difference is usually controlled within 5MIL, and the compensation principle is to compensate the parts where the length difference occurs.