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Via design of PCB

2021-12-28

Via design of PCB

The via design scheme of PCB is analyzed in detail

Multi layer PCB needs to be used in PCB design, and via is an important factor in multi-layer PCB design. The via in PCB is mainly composed of hole, pad area around the hole and power layer isolation area.

1. Influence of via in PCB

In a PCB multilayer board, when signals are transmitted from one layer of interconnect to another layer of interconnect, they need to be connected through vias. When the frequency is lower than 1GHz, the vias can play a good connection role, and their parasitic capacitance and inductance can be ignored. When the frequency is higher than 1 GHz, the influence of the parasitic effect of vias on signal integrity can not be ignored, At this time, the via appears as a breakpoint with discontinuous impedance on the transmission path, which will cause signal integrity problems such as signal reflection, delay and attenuation. When the signal is transmitted to another layer through the via, the reference layer of the signal line also serves as the return path of the via signal, and the return current will flow between the reference layers through capacitive coupling, causing problems such as ground elasticity.

2. Type of PCB via

Vias are generally divided into three categories: through holes, blind holes and buried holes.

Blind hole: it refers to a certain depth on the top and bottom surfaces of the printed circuit board, which is used for the connection between the surface circuit and the inner circuit below. The depth and aperture of the hole usually do not exceed a certain ratio.
Buried hole: refers to the connection hole located in the inner layer of the printed circuit board, which will not extend to the surface of the printed circuit board.
Through hole: this kind of hole passes through the whole circuit board and can be used for internal interconnection or as the installation and positioning hole of components. Because the through-hole is easier to realize in technology and lower in cost, it is generally used in printed circuit boards.

3. Via design in PCB

Vias in PCB design often bring great negative effects to circuit design. In order to reduce the adverse impact caused by the parasitic effect of vias, the following can be achieved as far as possible in the design:
(1) Select reasonable via size. For multi-layer PCB design with general density, select 0.25mm/0.51mm/0.91mm (drilling / pad / power isolation area) has better vias; for some high-density PCBs, vias of 0.20mm/0.46mm/0.86mm can also be used, or non through holes can be tried; for vias of power supply or ground wire, larger size can be considered to reduce impedance;
(2) The larger the power isolation area, the better. Considering the via density on the PCB, it is generally D1 = D2 + 0.41;
(3) The signal routing on PCB shall not change layers as far as possible, that is, vias shall be minimized;
(4) The use of thinner PCB is beneficial to reduce the two parasitic parameters of vias;
(5) The pins of power supply and ground shall be through holes nearby, and the shorter the lead between the via and pin, the better, because they will lead to the increase of inductance. At the same time, the lead of power supply and ground shall be as thick as possible to reduce impedance;
(6) Place some grounding vias near the vias of signal layer change to provide short-distance circuit for the signal.
In addition, the via length is also one of the main factors affecting the via inductance. For vias used for top and bottom conduction, the via length is equal to the PCB thickness. Due to the continuous increase of the number of PCB layers, the PCB thickness often reaches more than 5 mm. However, in the design of high-speed PCB, in order to reduce the problems caused by vias, the via length is generally controlled within 2.0mm. For vias with a via length greater than 2.0 mm, the via impedance continuity can be improved to a certain extent by increasing the via diameter. When the via length is 1.0 mm or less, the optimal via diameter is 0.20 mm - 0.30 mm