General principles of PCB design PCB&PCBA board made by Tmpking technology Co.,LTD. Tel&Whatsapp&Wechat:008613418507839

General principles of PCB design

PCB&PCBA board made by Tmpking technology Co.,LTD. Tel&Whatsapp&Wechat:008613418507839

In order to obtain the best performance of electronic circuits, the layout of components and wiring is very important. In order to design good quality, low cost PCB. The following general principles should be followed:

PCB&PCBA board made by Tmpking technology Co.,LTD. Tel&Whatsapp&Wechat:008613418507839

1. Layout

First, consider the PCB size. When the PCB size is too large, the printed lines are long, the impedance is increased, the anti-noise capability is reduced, and the cost is also increased. After determining the PCB size. Then determine the location of the special components. Finally, according to the functional units of the circuit, all components of the circuit are laid out.

遵守 Observe the following principles when determining the location of special components:

(1) Shorten the connection between high-frequency components as much as possible, and try to reduce their distribution parameters and electromagnetic interference between them. Vulnerable components should not be placed too close to each other, and input and output components should be kept as far away as possible.

(2) There may be a high potential difference between some components or wires, and the distance between them should be increased to avoid accidental short circuit caused by discharge. Components with high voltage should be placed as hard as possible during the debugging.

(3) Components weighing more than 15g should be fixed with a bracket and then soldered. Those large, heavy, and heat-generating components should not be installed on printed boards. Instead, they should be installed on the chassis base of the entire machine, and heat dissipation should be considered. Keep the thermal element away from the heating element.

(4) For the layout of adjustable components such as potentiometers, adjustable inductors, variable capacitors, and micro-switches, the structural requirements of the whole machine should be considered. If it is adjusted inside the machine, it should be placed on a printed board where it is easy to adjust. If it is adjusted outside the machine, its position should be compatible with the position of the adjustment knob on the chassis panel.

(5) The position occupied by the printed positioning holes and the fixing bracket should be reserved.

According to the functional unit of the circuit. The layout of all components of the circuit must meet the following principles:

(1) Arrange the position of each functional circuit unit according to the flow of the circuit, make the layout convenient for signal circulation, and keep the signal in the same direction as possible.

(2) Centering on the core components of each functional circuit, layout around it. Components should be evenly, neatly and compactly arranged on the PCB. Minimize and shorten leads and connections between components.

(3) For circuits operating at high frequencies, the distribution parameters between components must be considered. Generally, the components should be arranged in parallel as much as possible. In this way, it is not only beautiful. And easy to install and weld. Easy to mass produce.

(4) The components located on the edge of the circuit board are generally not less than 2mm away from the edge of the circuit board. The optimal shape of the circuit board is rectangular. The aspect ratio is 3: 2 to 4: 3. When the circuit board size is greater than 200x150mm. Consideration should be given to the mechanical strength of the board.

2. wiring

The principle of wiring is as follows:

(1) The wires for the input and output terminals should be avoided as much as possible. It is best to add a ground wire between the wires to avoid feedback coupling.

(2) The minimum width of the printed wire is mainly determined by the adhesion strength between the wire and the insulating substrate and the value of the current flowing through them. When the thickness of copper foil is 0.05mm and the width is 1 ~ 15mm. With a current of 2A, the temperature will not be higher than 3 ° C. A wire width of 1.5mm is sufficient. For integrated circuits, especially digital circuits, a wire width of 0.02 to 0.3 mm is usually selected. Of course, as long as you can, use wide lines whenever possible. Especially the power and ground wires. The minimum spacing of the wires is mainly determined by the worst-case insulation resistance and breakdown voltage. For integrated circuits, especially digital circuits, as long as the process allows, the pitch can be as small as 5 ~ 8mm.

(3) The bend of the printed conductor is generally circular, and the right angle or the included angle will affect the electrical performance in high-frequency circuits. In addition, try to avoid using large area copper foil, otherwise. When heated for a long time, the copper foil swells and falls off easily. When a large area of copper foil must be used, it is best to use a grid. This is helpful to exclude the volatile gas generated by heating the adhesive between the copper foil and the substrate.

3. Pad

The center hole of the pad is slightly larger than the diameter of the device lead. Pads that are too large are prone to false soldering. The pad outer diameter D is generally not less than (d + 1.2) mm, where d is the lead hole diameter. For high-density digital circuits, the minimum pad diameter can be (d + 1.0) mm.

PCB and circuit anti-interference measures

The anti-interference design of the printed circuit board is closely related to the specific circuit. Here are just some explanations of several common measures for PCB anti-interference design.

1. Power cord design

According to the current of the printed circuit board, try to increase the width of the power line to reduce the loop resistance. At the same time, the direction of the power line and the ground line should be consistent with the direction of data transmission, which will help enhance the anti-noise capability.

 2. Ground wire design

The principle of ground wire design is:

(1) Digital ground is separated from analog ground. If there are both logic and linear circuits on the board, they should be separated as much as possible. The ground of low-frequency circuits should be grounded in a single point in parallel as much as possible. When the actual wiring is difficult, it can be partially connected in series and then grounded in parallel. High-frequency circuits should be grounded in multiple points in series. The ground wire should be short and leased. Use grid-like large-area ground foils around high-frequency components as much as possible.

(2) The grounding wire should be as thick as possible. If the ground wire is a very straight line, the ground potential changes with the change of current, which reduces the anti-noise performance. Therefore, the ground wire should be thickened so that it can pass three times the allowable current on the printed board. If possible, the ground wire should be above 2 ~ 3mm.

(3) The ground wire forms a closed loop. Most of the printed circuit board composed of digital circuits, the grounding circuit of the printed circuit can improve the anti-noise ability.

3. Back capacitor configuration

之一 One of the common practices in PCB design is to configure appropriate decoupling capacitors in various key parts of the printed board.

The general configuration principles of back 藕 capacitors are:

(1) The input terminal of the power supply is connected across the electrolytic capacitor of 10 ~ 100uf. If possible, it is better to connect above 100uF.

(2) In principle, each integrated circuit chip should be equipped with a 0.01pF ceramic capacitor. If the printed board has insufficient space, a 1 ~ 10pF capacitor can be arranged for every 4 ~ 8 chips.

(3) For devices with weak anti-noise capability and large power changes during shutdown, such as RAM and ROM storage devices, a decoupling capacitor should be directly connected between the chip's power line and ground line.

(4) The lead of the capacitor should not be too long, especially the high-frequency bypass capacitor should not have lead.

In addition, you should pay attention to the following two points:

(1) When there are contactors, relays, buttons and other components in the printed board. When operating them, a large spark discharge will occur, and the RC circuit shown in the figure must be used to absorb the discharge current. Generally R takes 1 ~ 2K, C takes 2.2 ~ 47UF.

(2) The input impedance of CMOS is very high, and it is susceptible to induction. Therefore, the unused terminal should be grounded or connected to a positive power source when in use.

PCB wiring

In PCB design, wiring is an important step to complete product design. It can be said that the previous preparations are done for it. In the entire PCB, the design process of wiring is the highest, the skills are the finest, and the workload is the largest. PCB wiring includes single-sided wiring, double-sided wiring, and multilayer wiring. There are also two types of wiring: automatic routing and interactive routing. Before automatic routing, you can use interactive to pre-route more stringent lines, and the edges of the input and output should be avoided to be adjacent and parallel to avoid reflection interference. When necessary, ground wire isolation should be added, and the wiring of two adjacent layers should be perpendicular to each other, and parasitic coupling is easy to occur in parallel.

The routing rate of automatic routing depends on a good layout. The routing rules can be set in advance, including the number of bends in the traces, the number of vias, and the number of steps. Generally, the exploratory warp thread is first connected to quickly connect the short lines, and then the labyrinth wiring is performed. The wiring to be routed is optimized for the global routing path. It can disconnect the routed wires as needed. And try rewiring to improve the overall effect.

The current high-density PCB design has felt that through-holes are not well adapted. It wastes many valuable wiring channels. To solve this contradiction, blind and buried hole technologies have emerged, which not only complete the role of vias. It also saves many wiring channels to make the wiring process more convenient, smoother and more complete. The PCB board design process is a complex and simple process. To master it well, it also requires extensive electronic engineering design. Only when people experience it can they get the true meaning of it.

1 Handling of power and ground

Even if the wiring in the entire PCB is completed very well, the interference caused by poor consideration of the power supply and the ground wire will reduce the performance of the product and sometimes even affect the success rate of the product. Therefore, the wiring of electricity and ground wires must be taken seriously to reduce the noise interference caused by electricity and ground wires to a minimum to ensure the quality of the product.

For every engineer who is engaged in the design of electronic products, he understands the cause of the noise between the ground wire and the power wire. Now only the reduced noise suppression is described:

(1) It is well known to add decoupling capacitors between power and ground.

(2) Widen the power and ground wires as much as possible. It is best that the ground wires are wider than the power wires. The relationship between them is: ground wire> power wire> signal wire, usually the signal wire width is 0.2 to 0.3 mm. Thin width up to 0.05 ～ 0.07mm, power cord is 1.2 ～ 2.5 mm

For the digital circuit PCB, a wide ground wire can be used to form a loop, that is, a ground network can be used (the ground of analog circuits cannot be used in this way)

(3) Use a large-area copper layer as the ground wire, and connect the unused places to the ground on the printed board as the ground wire. Or make a multilayer board, power supply, and ground each occupy one layer.

2 Common ground processing of digital and analog circuits

Many PCBs are no longer a single function circuit (digital or analog circuit), but are composed of a mixture of digital and analog circuits. Therefore, it is necessary to consider the mutual interference between them when wiring, especially the noise interference on the ground.

The frequency of digital circuits is high, and the sensitivity of analog circuits is strong. For signal lines, high-frequency signal lines are as far away from sensitive analog circuit devices as possible. For ground lines, the whole PCB has only one node to the outside world, so Digital and analog ground must be handled inside the PCB, but the digital ground and analog ground are actually separated inside the board. They are not connected to each other, but at the interface where the PCB is connected to the outside world (such as plugs). The digital ground is slightly shorted to the analog ground. Please note that there is only one connection point. There are also non-common grounds on the PCB, which is determined by the system design.

3 The signal line is routed on the electrical (ground) layer

When multi-layer printed circuit boards are wired, because there are not enough wires left in the signal line layer, adding more layers will cause waste and add a certain amount of work to production. The cost will increase accordingly. To resolve this contradiction, you can consider wiring on the electrical (ground) layer. The power plane should be considered first, followed by the ground plane. Because it is best to preserve the integrity of the formation.

4 Handling of connecting legs in large-area conductors

In large-area grounding (electricity), the legs of commonly used components are connected to it. The treatment of the connection legs needs to be comprehensively considered. In terms of electrical performance, it is better that the pads of the component legs and the copper surface are fully connected. There are some hidden dangers in the welding assembly of components, such as: ① high power heaters are required for welding. ② Easy to cause false solder joints. Therefore, taking into account the electrical performance and process requirements, a cross-shaped pad is called a heat shield, commonly known as a thermal pad. In this way, the possibility of virtual solder joints due to cross-section overheating during welding can be achieved. Sex is greatly reduced. The handling of the ground (ground) leg of the multilayer board is the same.

5 The role of network systems in wiring

In many CAD systems, wiring is determined based on the network system. The grid is too dense, although the path has increased, but the step is too small, and the amount of data in the picture field is too large. This will inevitably have higher requirements for the storage space of the device, and also the computing speed of the target computer electronics. Great influence. And some paths are invalid, such as occupied by the pads of the component legs or occupied by mounting holes and fixed holes. The grid is too sparse, and too few channels have a great impact on the spread rate. Therefore, there must be a dense and reasonable grid system to support the wiring.

The distance between the legs of standard components is 0.1 inches (2.54mm), so the basis of the grid system is generally set to 0.1 inches (2.54 mm) or an integer multiple of less than 0.1 inches, such as: 0.05 inches, 0.025 inches, 0.02 Inches etc.

6 Design Rule Check (DRC)

After the wiring design is completed, it is necessary to carefully check whether the wiring design conforms to the rules formulated by the designer. At the same time, it is also necessary to confirm whether the formulated rules meet the requirements of the printed board production process. The general inspection has the following aspects:

(1) Whether the distance between the line and the line, the line and the component pad, the line and the through hole, the component pad and the through hole, the through hole and the through hole are reasonable and meet the production requirements.

(2) Is the width of the power and ground wires appropriate, and is there a tight coupling between the power and ground wires (low wave impedance)? Is there any place in the PCB to make the ground line wider?

(3) Whether the best measures have been taken for key signal lines, such as the shortest length, adding protection lines, and the input and output lines are clearly separated.

(4) Does the analog circuit and digital circuit have separate ground wires?

PCB layout

In design, layout is an important link. The quality of the layout results will directly affect the effect of the layout. Therefore, you can think that a reasonable layout is the first step in the success of the PCB design.

There are two types of layout methods, one is interactive layout and the other is automatic layout. Generally, the interactive layout is adjusted based on the automatic layout. During the layout, the gate circuit can be re-adjusted according to the routing situation. Allocation, the two gate circuits are swapped, making it the best layout for easy wiring. After the layout is completed, the design files and related information can be returned to the schematic diagram, so that the relevant information in the PCB board is consistent with the schematic diagram, so that future file creation and design changes can be synchronized, and the simulated Relevant information is updated to enable board-level verification of the electrical performance and functionality of the circuit.

-Consider the overall beauty

The success of a product depends on the internal quality and the overall aesthetics. Both are perfect in order to consider the product successful.

On a PCB board, the layout of components needs to be balanced, dense and orderly, and cannot be light or heavy.

--- Layout check

Does the size of the printed board match the size of the processing drawing? Can it meet the requirements of PCB manufacturing process? Is there an anchor?

Does the component conflict in two or three dimensions?

Is the component layout neat and orderly and neatly arranged? Is it all finished?

Can components that need to be replaced frequently be replaced easily? Is it easy to insert the card into the device?

Is there an appropriate distance between the thermal element and the heating element?

Is it easy to adjust the adjustable elements?

Where is the heat sink installed? Is the air flow clear?

Is the signal flow smooth and minimally interconnected?

Do plugs, sockets, etc. conflict with the mechanical design?

Is the interference problem of the line considered?