Brief Discussion on PCBs and Circuit Anti-Interference Measures

A Printed Circuit Board (PCB) is the support for circuit components and devices in electronic products. It provides electrical connections between circuit components and devices. With the rapid development of electronic technology, the density of PCBs has become increasingly high. The quality of PCB design has a significant impact on anti-interference capability. Therefore, when conducting PCB design, it is imperative to comply with the general principles of PCB design and meet the requirements of anti-interference design.

To achieve optimal performance in electronic circuits, the placement of components and the routing of conductors are critical. To design PCBs with high quality and low cost, the following general principles shall be followed:

First, consider the dimensions of the PCB. If the PCB size is too large, the printed traces will be longer, leading to increased impedance, reduced noise immunity, and higher cost. If the size is too small, heat dissipation will be poor, and adjacent traces will be prone to interference. After determining the PCB dimensions, define the positions of special components. Arrange all components of the circuit according to the functional units of the circuit.

The following principles shall be observed when determining the positions of special components: (1) Minimize the connection lengths between high-frequency components as much as possible, and strive to reduce their distributed parameters and mutual electromagnetic interference. Interference-susceptible components shall not be placed too close to each other, and input and output components shall be kept as far apart as possible. (2) Increase the distance between certain components or conductors that may have a high potential difference to prevent accidental short circuits caused by discharge. Components with high voltage shall be placed in positions that are not easily accessible by hand during debugging. (3) Components weighing more than 15g shall be fixed with brackets before soldering. Large, heavy, and high-heat-generating components are not suitable for mounting on the printed board; instead, they shall be installed on the chassis base of the complete machine, and heat dissipation shall be considered. Thermosensitive components shall be placed away from heat-generating components. (4) The placement of adjustable components such as potentiometers, adjustable inductor coils, variable capacitors, and micro switches shall consider the structural requirements of the complete machine. For internal adjustment, they shall be placed in easily accessible positions on the printed board; for external adjustment, their positions shall match the positions of the adjustment knobs on the chassis panel. (5) Reserve space for the positioning holes and fixing brackets of the printed board.

When arranging all components of the circuit according to its functional units, the following principles shall be met: (1) Arrange the positions of each functional circuit unit in accordance with the circuit flow to facilitate signal transmission and maintain a consistent signal direction as much as possible. (2) Take the core component of each functional circuit as the center and arrange components around it. Components shall be placed uniformly, neatly, and compactly on the PCB to minimize and shorten the leads and connections between components. (3) For circuits operating at high frequencies, consider the distributed parameters between components. For general circuits, arrange components in parallel as much as possible. This not only ensures an aesthetic appearance but also facilitates assembly, soldering, and mass production. (4) Components located at the edge of the circuit board shall generally be no less than 2mm away from the board edge. The circuit board shall be rectangular with an aspect ratio of 3:2 or 4:3. When the board size exceeds 200×150mm, consider the mechanical strength of the circuit board.

The principles of routing are as follows: (1) Conductors for input and output terminals shall avoid adjacent parallel routing as much as possible. Add ground wires between traces to prevent feedback coupling. (2) The width of printed conductors is mainly determined by the adhesion strength between conductors and the insulating substrate, as well as the current flowing through them. When the copper foil thickness is 0.05mm and the width is 1–15mm, a current of 2A can pass through without the temperature exceeding 3℃. Therefore, a conductor width of 1.5mm meets the requirements. For integrated circuits, especially digital circuits, a conductor width of 0.02–0.3mm is typically selected. Of course, use wider traces whenever possible, especially for power lines and ground lines. The spacing between conductors is mainly determined by the inter-line insulation resistance and breakdown voltage under worst-case conditions. For integrated circuits, especially digital circuits, the spacing can be reduced to 5–8mm as permitted by the process. (3) Printed conductors shall generally use arc-shaped corners; right-angle or angled corners will affect electrical performance in high-frequency circuits. In addition, avoid using large-area copper foils as much as possible; otherwise, the copper foil is prone to expansion and peeling after long-term heating. If large-area copper foil is necessary, use a grid pattern to facilitate the discharge of volatile gases generated by the adhesive between the copper foil and the substrate when heated.

The center hole of the pad shall be slightly larger than the lead diameter of the device. An excessively large pad is prone to cold solder joints. The outer diameter D of the pad is generally not less than (d+1.2)mm, where d is the lead aperture. For high-density digital circuits, the pad diameter can be (d+1.0)mm.

The anti-interference design of printed circuit boards is closely related to the specific circuit. This section only explains several common measures for PCB anti-interference design.

Widen the power line as much as possible according to the current of the printed circuit board to reduce loop resistance. At the same time, align the routing direction of power lines and ground lines with the direction of data transmission, which helps enhance noise immunity.

The principles of ground line design are: (1) Separate digital ground from analog ground. If the circuit board contains both logic circuits and linear circuits, keep them as separate as possible. For low-frequency circuits, adopt single-point parallel grounding as much as possible; if practical routing is difficult, partial series connection can be used before parallel grounding. For high-frequency circuits, multi-point series grounding is preferred. Ground lines shall be short and thick, and large-area grid-shaped ground foils shall be used around high-frequency components as much as possible. (2) Thicken the ground line as much as possible. If the ground line is very thin, the ground potential will fluctuate with current changes, reducing noise immunity. Therefore, thicken the ground line to allow three times the allowable current of the printed board. If possible, the ground line shall be more than 2–3mm wide. (3) Form a closed loop with the ground line. For printed boards composed only of digital circuits, arranging the grounding circuit into a closed loop can mostly improve noise immunity.

One of the conventional practices in PCB design is to configure appropriate decoupling capacitors at key positions on the printed board.

The general configuration principles for decoupling capacitors are: (1) Connect an electrolytic capacitor of 10–100μF across the power input terminal. If possible, use a capacitor above 100μF for better performance. (2) In principle, one 0.01pF ceramic capacitor shall be arranged for each integrated circuit chip. If space on the printed board is insufficient, one 1–10pF tantalum capacitor can be arranged for every 4–8 chips. (3) For devices with weak noise immunity and large power supply changes during turn-off, such as RAM and ROM memory devices, directly connect a decoupling capacitor between the power line and ground line of the chip. (4) Capacitor leads shall not be too long, especially high-frequency bypass capacitors shall have no leads.

In addition, pay attention to the following two points: (1) When components such as contactors, relays, and buttons are present on the printed board, their operation will generate large spark discharges. An RC circuit as shown in the attached figure must be used to absorb the discharge current. Generally, R is 1–2kΩ and C is 2.2–47μF. (2) CMOS has a high input impedance and is susceptible to induction. Therefore, unused terminals shall be grounded or connected to the positive power supply during use.

About Maxipcb
Maxipcb enables advanced electronic innovation. We deliver one-stop solutions including circuit design, simulation, testing, PCB fabrication, component sourcing and SMT&PCBA assembly, to boost R&D efficiency, speed up mass production and control full-cycle risks. We serve global sectors like communication, industrial automation, aerospace, automotive and semiconductor, jointly forging a safer, connected intelligent future.